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Enzyme

Large biological molecule that acts as a catalyst

"Biocatalyst" redirects here. For the use of natural catalysts in organic chemistry, see Biocatalysis.

Enzymes () are proteins that act as biologicalcatalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life.[1]: 8.1 Metabolic pathways depend upon enzymes to catalyze individual steps. The study of enzymes is called enzymology and the field of pseudoenzyme analysis recognizes that during evolution, some enzymes have lost the ability to carry out biological catalysis, which is often reflected in their amino acid sequences and unusual 'pseudocatalytic' properties.[2][3]

Enzymes are known to catalyze more than 5,000 biochemical reaction types.[4] Other biocatalysts are catalytic RNA molecules, called ribozymes. Enzymes' specificity comes from their unique three-dimensional structures.

Like all catalysts, enzymes increase the reaction rate by lowering its activation energy. Some enzymes can make their conversion of substrate to product occur many millions of times faster. An extreme example is orotidine 5'-phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds.[5][6] Chemically, enzymes are like any catalyst and are not consumed in chemical reactions, nor do they alter the equilibrium of a reaction. Enzymes differ from most other catalysts by being much more specific. Enzyme activity can be affected by other molecules: inhibitors are molecules that decrease enzyme activity, and activators are molecules that increase activity. Many therapeutic drugs and poisons are enzyme inhibitors. An enzyme's activity decreases markedly outside its optimal temperature and pH, and many enzymes are (permanently) denatured when exposed to excessive heat, losing their structure and catalytic properties.

Some enzymes are used commercially, for example, in the synthesis of antibiotics. Some household products use enzymes to speed up chemical reactions: enzymes in biological washing powders break down protein, starch or fat stains on clothes, and enzymes in meat tenderizer break down proteins into smaller molecules, making the meat easier to chew.

Etymology and history

Photograph of Eduard Buchner.

By the late 17th and early 18th centuries, the digestion of meat by stomach secretions[7] and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified.[8]

French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833.[9] A few decades later, when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."[10]

In 1877, German physiologist Wilhelm Kühne (1837–1900) first used the term enzyme, which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process.[11] The word enzyme was used later to refer to nonliving substances such as pepsin, and the word ferment was used to refer to chemical activity produced by living organisms.[12]

Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin, he found that sugar was fermented by yeast extracts even when there were no living yeast cells in the mixture.[13] He named the enzyme that brought about the fermentation of sucrose "zymase".[14] In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are usually named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate (e.g., lactase is the enzyme that cleaves lactose) or to the type of reaction (e.g., DNA polymerase forms DNA polymers).[15]

The biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others (such as Nobel laureate Richard Willstätter) argued that proteins were merely carriers for the true enzymes and that proteins per se were incapable of catalysis.[16] In 1926, James B. Sumner showed that the enzyme urease was a pure protein and crystallized it; he did likewise for the enzyme catalase in 1937. The conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley, who worked on the digestive enzymes pepsin (1930), trypsin and chymotrypsin. These three scientists were awarded the 1946 Nobel Prize in Chemistry.[17]

The discovery that enzymes could be crystallized eventually allowed their structures to be solved by x-ray crystallography. This was first done for lysozyme, an enzyme found in tears, saliva and egg whites that digests the coating of some bacteria; the structure was solved by a group led by David Chilton Phillips and published in 1965.[18] This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail.[19]

Classification and nomenclature

Enzymes can be classified by two main criteria: either amino acid sequence similarity (and thus evolutionary relationship) or enzymatic activity.

Enzyme activity. An enzyme's name is often derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase.[1]: 8.1.3  Examples are lactase, alcohol dehydrogenase and DNA polymerase. Different enzymes that catalyze the same chemical reaction are called isozymes.[1]: 10.3 

The International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers (for "Enzyme Commission"). Each enzyme is described by "EC" followed by a sequence of four numbers which represent the hierarchy of enzymatic activity (from very general to very specific). That is, the first number broadly classifies the enzyme based on its mechanism while the other digits add more and more specificity.[20]

The top-level classification is:

These sections are subdivided by other features such as the substrate, products, and chemical mechanism. An enzyme is fully specified by four numerical designations. For example, hexokinase (EC 2.7.1.1) is a transferase (EC 2) that adds a phosphate group (EC 2.7) to a hexose sugar, a molecule containing an alcohol group (EC 2.7.1).[21]

Sequence similarity. EC categories do not reflect sequence similarity. For instance, two ligases of the same EC number that catalyze exactly the same reaction can have completely different sequences. Independent of their function, enzymes, like any other proteins, have been classified by their sequence similarity into numerous families. These families have been documented in dozens of different protein and protein family databases such as Pfam.[22]

Structure

See also: Protein structure

Enzymes are generally globular proteins, acting alone or in larger complexes. The sequence of the amino acids specifies the structure which in turn determines the catalytic activity of the enzyme.[23] Although structure determines function, a novel enzymatic activity cannot yet be predicted from structure alone.[24] Enzyme structures unfold (denature) when heated or exposed to chemical denaturants and this disruption to the structure typically causes a loss of activity.[25] Enzyme denaturation is normally linked to temperatures above a species' normal level; as a result, enzymes from bacteria living in volcanic environments such as hot springs are prized by industrial users for their ability to function at high temperatures, allowing enzyme-catalysed reactions to be operated at a very high rate.

Enzymes are usually much larger than their substrates. Sizes range from just 62 amino acid residues, for the monomer of 4-oxalocrotonate tautomerase,[26] to over 2,500 residues in the animal fatty acid synthase.[27] Only a small portion of their structure (around 2–4 amino acids) is directly involved in catalysis: the catalytic site.[28] This catalytic site is located next to one or more binding sites where residues orient the substrates. The catalytic site and binding site together compose the enzyme's active site. The remaining majority of the enzyme structure serves to maintain the precise orientation and dynamics of the active site.[29]

In some enzymes, no amino acids are directly involved in catalysis; instead, the enzyme contains sites to bind and orient catalytic cofactors.[29] Enzyme structures may also contain allosteric sites where the binding of a small molecule causes a conformational change that increases or decreases activity.[30]

A small number of RNA-based biological catalysts called ribozymes exist, which again can act alone or in complex with proteins. The most common of these is the ribosome which is a complex of protein and catalytic RNA components.[1]: 2.2 

Mechanism

Substrate binding

Enzymes must bind their substrates before they can catalyse any chemical reaction. Enzymes are usually very specific as to what substrates they bind and then the chemical reaction catalysed. Specificity is achieved by binding pockets with complementary shape, charge and hydrophilic/hydrophobic characteristics to the substrates. Enzymes can therefore distinguish between very similar substrate molecules to be chemoselective, regioselective and stereospecific.[31]

Some of the enzymes showing the highest specificity and accuracy are involved in the copying and expression of the genome. Some of these enzymes have "proof-reading" mechanisms. Here, an enzyme such as DNA polymerase catalyzes a reaction in a first step and then checks that the product is correct in a second step.[32] This two-step process results in average error rates of less than 1 error in 100 million reactions in high-fidelity mammalian polymerases.[1]: 5.3.1  Similar proofreading mechanisms are also found in RNA polymerase,[33]aminoacyl tRNA synthetases[34] and ribosomes.[35]

Conversely, some enzymes display enzyme promiscuity, having broad specificity and acting on a range of different physiologically relevant substrates. Many enzymes possess small side activities which arose fortuitously (i.e. neutrally), which may be the starting point for the evolutionary selection of a new function.[36][37]

Hexokinase displayed as an opaque surface with a pronounced open binding cleft next to unbound substrate (top) and the same enzyme with more closed cleft that surrounds the bound substrate (bottom)
Enzyme changes shape by induced fit upon substrate binding to form enzyme-substrate complex. Hexokinasehas a large induced fit motion that closes over the substrates adenosine triphosphateand xylose. Binding sites in blue, substrates in black and Mg2+cofactor in yellow. (PDB: 2E2N​, 2E2Q​)

"Lock and key" model

To explain the observed specificity of enzymes, in 1894 Emil Fischer proposed that both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another.[38] This is often referred to as "the lock and key" model.[1]: 8.3.2  This early model explains enzyme specificity, but fails to explain the stabilization of the transition state that enzymes achieve.[39]

Induced fit model

In 1958, Daniel Koshland suggested a modification to the lock and key model: since enzymes are rather flexible structures, the active site is continuously reshaped by interactions with the substrate as the substrate interacts with the enzyme.[40] As a result, the substrate does not simply bind to a rigid active site; the amino acid side-chains that make up the active site are molded into the precise positions that enable the enzyme to perform its catalytic function. In some cases, such as glycosidases, the substrate molecule also changes shape slightly as it enters the active site.[41] The active site continues to change until the substrate is completely bound, at which point the final shape and charge distribution is determined.[42] Induced fit may enhance the fidelity of molecular recognition in the presence of competition and noise via the conformational proofreading mechanism.[43]

Catalysis

See also: Enzyme catalysis and Transition state theory

Enzymes can accelerate reactions in several ways, all of which lower the activation energy (ΔG, Gibbs free energy)[44]

  1. By stabilizing the transition state:
    • Creating an environment with a charge distribution complementary to that of the transition state to lower its energy[45]
  2. By providing an alternative reaction pathway:
    • Temporarily reacting with the substrate, forming a covalent intermediate to provide a lower energy transition state[46]
  3. By destabilising the substrate ground state:
    • Distorting bound substrate(s) into their transition state form to reduce the energy required to reach the transition state[47]
    • By orienting the substrates into a productive arrangement to reduce the reaction entropy change[48] (the contribution of this mechanism to catalysis is relatively small)[49]

Enzymes may use several of these mechanisms simultaneously. For example, proteases such as trypsin perform covalent catalysis using a catalytic triad, stabilise charge build-up on the transition states using an oxyanion hole, complete hydrolysis using an oriented water substrate.[50]

Dynamics

See also: Protein dynamics

Enzymes are not rigid, static structures; instead they have complex internal dynamic motions – that is, movements of parts of the enzyme's structure such as individual amino acid residues, groups of residues forming a protein loop or unit of secondary structure, or even an entire protein domain. These motions give rise to a conformational ensemble of slightly different structures that interconvert with one another at equilibrium. Different states within this ensemble may be associated with different aspects of an enzyme's function. For example, different conformations of the enzyme dihydrofolate reductase are associated with the substrate binding, catalysis, cofactor release, and product release steps of the catalytic cycle,[51] consistent with catalytic resonance theory.

Substrate presentation

Substrate presentation is a process where the enzyme is sequestered away from its substrate. Enzymes can be sequestered to the plasma membrane away from a substrate in the nucleus or cytosol. Or within the membrane, an enzyme can be sequestered into lipid rafts away from its substrate in the disordered region. When the enzyme is released it mixes with its substrate. Alternatively, the enzyme can be sequestered near its substrate to activate the enzyme. For example, the enzyme can be soluble and upon activation bind to a lipid in the plasma membrane and then act upon molecules in the plasma membrane.

Allosteric modulation

Main article: Allosteric regulation

Allosteric sites are pockets on the enzyme, distinct from the active site, that bind to molecules in the cellular environment. These molecules then cause a change in the conformation or dynamics of the enzyme that is transduced to the active site and thus affects the reaction rate of the enzyme.[52] In this way, allosteric interactions can either inhibit or activate enzymes. Allosteric interactions with metabolites upstream or downstream in an enzyme's metabolic pathway cause feedback regulation, altering the activity of the enzyme according to the flux through the rest of the pathway.[53]

Cofactors

Main article: Cofactor (biochemistry)

Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity.[54] Cofactors can be either inorganic (e.g., metal ions and iron-sulfur clusters) or organic compounds (e.g., flavin and heme). These cofactors serve many purposes; for instance, metal ions can help in stabilizing nucleophilic species within the active site.[55] Organic cofactors can be either coenzymes, which are released from the enzyme's active site during the reaction, or prosthetic groups, which are tightly bound to an enzyme. Organic prosthetic groups can be covalently bound (e.g., biotin in enzymes such as pyruvate carboxylase).[56]

An example of an enzyme that contains a cofactor is carbonic anhydrase, which uses a zinc cofactor bound as part of its active site.[57] These tightly bound ions or molecules are usually found in the active site and are involved in catalysis.[1]: 8.1.1  For example, flavin and heme cofactors are often involved in redox reactions.[1]: 17 

Enzymes that require a cofactor but do not have one bound are called apoenzymes or apoproteins. An enzyme together with the cofactor(s) required for activity is called a holoenzyme (or haloenzyme). The term holoenzyme can also be applied to enzymes that contain multiple protein subunits, such as the DNA polymerases; here the holoenzyme is the complete complex containing all the subunits needed for activity.[1]: 8.1.1 

Coenzymes

Coenzymes are small organic molecules that can be loosely or tightly bound to an enzyme. Coenzymes transport chemical groups from one enzyme to another.[58] Examples include NADH, NADPH and adenosine triphosphate (ATP). Some coenzymes, such as flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), thiamine pyrophosphate (TPP), and tetrahydrofolate (THF), are derived from vitamins. These coenzymes cannot be synthesized by the body de novo and closely related compounds (vitamins) must be acquired from the diet. The chemical groups carried include:

Since coenzymes are chemically changed as a consequence of enzyme action, it is useful to consider coenzymes to be a special class of substrates, or second substrates, which are common to many different enzymes. For example, about 1000 enzymes are known to use the coenzyme NADH.[59]

Coenzymes are usually continuously regenerated and their concentrations maintained at a steady level inside the cell. For example, NADPH is regenerated through the pentose phosphate pathway and S-adenosylmethionine by methionine adenosyltransferase. This continuous regeneration means that small amounts of coenzymes can be used very intensively. For example, the human body turns over its own weight in ATP each day.[60]

Thermodynamics

A two dimensional plot of reaction coordinate (x-axis) vs. energy (y-axis) for catalyzed and uncatalyzed reactions. The energy of the system steadily increases from reactants (x = 0) until a maximum is reached at the transition state (x = 0.5), and steadily decreases to the products (x = 1). However, in an enzyme catalysed reaction, binding generates an enzyme-substrate complex (with slightly reduced energy) then increases up to a transition state with a smaller maximum than the uncatalysed reaction.
The energies of the stages of a chemical reaction. Uncatalysed (dashed line), substrates need a lot of activation energyto reach a transition state, which then decays into lower-energy products. When enzyme catalysed (solid line), the enzyme binds the substrates (ES), then stabilizes the transition state (ES) to reduce the activation energy required to produce products (EP) which are finally released.

Main articles: Activation energy, Thermodynamic equilibrium, and Chemical equilibrium

As with all catalysts, enzymes do not alter the position of the chemical equilibrium of the reaction. In the presence of an enzyme, the reaction runs in the same direction as it would without the enzyme, just more quickly.[1]: 8.2.3  For example, carbonic anhydrase catalyzes its reaction in either direction depending on the concentration of its reactants:[61]

{\displaystyle {\ce {CO2{}+H2O->[{\text{Carbonic anhydrase}}]H2CO3}}}(in tissues; high CO2concentration)

 

 

 

 

(1)

{\displaystyle {\ce {CO2{}+H2O<-[{\text{Carbonic anhydrase}}]H2CO3}}}(in lungs; low CO2concentration)

 

 

 

 

(2)

The rate of a reaction is dependent on the activation energy needed to form the transition state which then decays into products. Enzymes increase reaction rates by lowering the energy of the transition state. First, binding forms a low energy enzyme-substrate complex (ES). Second, the enzyme stabilises the transition state such that it requires less energy to achieve compared to the uncatalyzed reaction (ES). Finally the enzyme-product complex (EP) dissociates to release the products.[1]: 8.3 

Enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive" a thermodynamically unfavourable one so that the combined energy of the products is lower than the substrates. For example, the hydrolysis of ATP is often used to drive other chemical reactions.[62]

Kinetics

Main article: Enzyme kinetics

Enzyme kinetics is the investigation of how enzymes bind substrates and turn them into products.[63] The rate data used in kinetic analyses are commonly obtained from enzyme assays. In 1913 Leonor Michaelis and Maud Leonora Menten proposed a quantitative theory of enzyme kinetics, which is referred to as Michaelis–Menten kinetics.[64] The major contribution of Michaelis and Menten was to think of enzyme reactions in two stages. In the first, the substrate binds reversibly to the enzyme, forming the enzyme-substrate complex. This is sometimes called the Michaelis–Menten complex in their honor. The enzyme then catalyzes the chemical step in the reaction and releases the product. This work was further developed by G. E. Briggs and J. B. S. Haldane, who derived kinetic equations that are still widely used today.[65]

Enzyme rates depend on solution conditions and substrate concentration. To find the maximum speed of an enzymatic reaction, the substrate concentration is increased until a constant rate of product formation is seen. This is shown in the saturation curve on the right. Saturation happens because, as substrate concentration increases, more and more of the free enzyme is converted into the substrate-bound ES complex. At the maximum reaction rate (Vmax) of the enzyme, all the enzyme active sites are bound to substrate, and the amount of ES complex is the same as the total amount of enzyme.[1]: 8.4 

Vmax is only one of several important kinetic parameters. The amount of substrate needed to achieve a given rate of reaction is also important. This is given by the Michaelis–Menten constant (Km), which is the substrate concentration required for an enzyme to reach one-half its maximum reaction rate; generally, each enzyme has a characteristic KM for a given substrate. Another useful constant is kcat, also called the turnover number, which is the number of substrate molecules handled by one active site per second.[1]: 8.4 

The efficiency of an enzyme can be expressed in terms of kcat/Km. This is also called the specificity constant and incorporates the rate constants for all steps in the reaction up to and including the first irreversible step. Because the specificity constant reflects both affinity and catalytic ability, it is useful for comparing different enzymes against each other, or the same enzyme with different substrates. The theoretical maximum for the specificity constant is called the diffusion limit and is about 108 to 109 (M−1 s−1). At this point every collision of the enzyme with its substrate will result in catalysis, and the rate of product formation is not limited by the reaction rate but by the diffusion rate. Enzymes with this property are called catalytically perfect or kinetically perfect. Example of such enzymes are triose-phosphate isomerase, carbonic anhydrase, acetylcholinesterase, catalase, fumarase, β-lactamase, and superoxide dismutase.[1]: 8.4.2  The turnover of such enzymes can reach several million reactions per second.[1]: 9.2  But most enzymes are far from perfect: the average values of {\displaystyle k_{\rm {cat}}/K_{\rm {m}}} and {\displaystyle k_{\rm {cat}}} are about {\displaystyle 10^{5}{\rm {s}}^{-1}{\rm {M}}^{-1}} and {\displaystyle 10{\rm {s}}^{-1}}, respectively.[66]

Michaelis–Menten kinetics relies on the law of mass action, which is derived from the assumptions of free diffusion and thermodynamically driven random collision. Many biochemical or cellular processes deviate significantly from these conditions, because of macromolecular crowding and constrained molecular movement.[67] More recent, complex extensions of the model attempt to correct for these effects.[68]

Inhibition

Two dimensional representations of the chemical structure of folic acid and methotrexate highlighting the differences between these two substances (amidation of pyrimidone and methylation of secondary amine).

The coenzyme folic acid (left) and the anti-cancer drug methotrexate (right) are very similar in structure (differences show in green). As a result, methotrexate is a competitive inhibitor of many enzymes that use folates.

Main article: Enzyme inhibitor

Enzyme reaction rates can be decreased by various types of enzyme inhibitors.[69]: 73–74 

Types of inhibition

Competitive

A competitive inhibitor and substrate cannot bind to the enzyme at the same time.[70] Often competitive inhibitors strongly resemble the real substrate of the enzyme. For example, the drug methotrexate is a competitive inhibitor of the enzyme dihydrofolate reductase, which catalyzes the reduction of dihydrofolate to tetrahydrofolate.[71] The similarity between the structures of dihydrofolate and this drug are shown in the accompanying figure. This type of inhibition can be overcome with high substrate concentration. In some cases, the inhibitor can bind to a site other than the binding-site of the usual substrate and exert an allosteric effect to change the shape of the usual binding-site.[72]

Non-competitive

A non-competitive inhibitor binds to a site other than where the substrate binds. The substrate still binds with its usual affinity and hence Km remains the same. However the inhibitor reduces the catalytic efficiency of the enzyme so that Vmax is reduced. In contrast to competitive inhibition, non-competitive inhibition cannot be overcome with high substrate concentration.[69]: 76–78 

Uncompetitive

An uncompetitive inhibitor cannot bind to the free enzyme, only to the enzyme-substrate complex; hence, these types of inhibitors are most effective at high substrate concentration. In the presence of the inhibitor, the enzyme-substrate complex is inactive.[69]: 78  This type of inhibition is rare.[73]

Mixed

A mixed inhibitor binds to an allosteric site and the binding of the substrate and the inhibitor affect each other. The enzyme's function is reduced but not eliminated when bound to the inhibitor. This type of inhibitor does not follow the Michaelis–Menten equation.[69]: 76–78 

Irreversible

An irreversible inhibitor permanently inactivates the enzyme, usually by forming a covalent bond to the protein.[74]Penicillin[75] and aspirin[76] are common drugs that act in this manner.

Functions of inhibitors

In many organisms, inhibitors may act as part of a feedback mechanism. If an enzyme produces too much of one substance in the organism, that substance may act as an inhibitor for the enzyme at the beginning of the pathway that produces it, causing production of the substance to slow down or stop when there is sufficient amount. This is a form of negative feedback. Major metabolic pathways such as the citric acid cycle make use of this mechanism.[1]: 17.2.2 

Since inhibitors modulate the function of enzymes they are often used as drugs. Many such drugs are reversible competitive inhibitors that resemble the enzyme's native substrate, similar to methotrexate above; other well-known examples include statins used to treat high cholesterol,[77] and protease inhibitors used to treat retroviral infections such as HIV.[78] A common example of an irreversible inhibitor that is used as a drug is aspirin, which inhibits the COX-1 and COX-2 enzymes that produce the inflammation messenger prostaglandin.[76] Other enzyme inhibitors are poisons. For example, the poison cyanide is an irreversible enzyme inhibitor that combines with the copper and iron in the active site of the enzyme cytochrome c oxidase and blocks cellular respiration.[79]

Factors affecting enzyme activity

As enzymes are made up of proteins, their actions are sensitive to change in many physio chemical factors such as pH, temperature, substrate concentration, etc.

The following table shows pH optima for various enzymes.[80]

Enzyme Optimum pH pH description
Pepsin 1.5–1.6 Highly acidic
Invertase 4.5 Acidic
Lipase (stomach) 4.0–5.0 Acidic
Lipase (castor oil) 4.7 Acidic
Lipase (pancreas) 8.0 Alkaline
Amylase (malt) 4.6–5.2 Acidic
Amylase (pancreas) 6.7–7.0 Acidic-neutral
Cellobiase 5.0 Acidic
Maltase 6.1–6.8 Acidic
Sucrase 6.2 Acidic
Catalase 7.0 Neutral
Urease 7.0 Neutral
Cholinesterase 7.0 Neutral
Ribonuclease 7.0–7.5 Neutral
Fumarase 7.8 Alkaline
Trypsin 7.8–8.7 Alkaline
Adenosine triphosphate 9.0 Alkaline
Arginase 10.0 Highly alkaline

Biological function

Enzymes serve a wide variety of functions inside living organisms. They are indispensable for signal transduction and cell regulation, often via kinases and phosphatases.[81] They also generate movement, with myosin hydrolyzing ATP to generate muscle contraction, and also transport cargo around the cell as part of the cytoskeleton.[82] Other ATPases in the cell membrane are ion pumps involved in active transport. Enzymes are also involved in more exotic functions, such as luciferase generating light in fireflies.[83]Viruses can also contain enzymes for infecting cells, such as the HIV integrase and reverse transcriptase, or for viral release from cells, like the influenza virus neuraminidase.[84]

An important function of enzymes is in the digestive systems of animals. Enzymes such as amylases and proteases break down large molecules (starch or proteins, respectively) into smaller ones, so they can be absorbed by the intestines. Starch molecules, for example, are too large to be absorbed from the intestine, but enzymes hydrolyze the starch chains into smaller molecules such as maltose and eventually glucose, which can then be absorbed. Different enzymes digest different food substances. In ruminants, which have herbivorous diets, microorganisms in the gut produce another enzyme, cellulase, to break down the cellulose cell walls of plant fiber.[85]

Metabolism

Several enzymes can work together in a specific order, creating metabolic pathways.[1]: 30.1  In a metabolic pathway, one enzyme takes the product of another enzyme as a substrate. After the catalytic reaction, the product is then passed on to another enzyme. Sometimes more than one enzyme can catalyze the same reaction in parallel; this can allow more complex regulation: with, for example, a low constant activity provided by one enzyme but an inducible high activity from a second enzyme.[86]

Enzymes determine what steps occur in these pathways. Without enzymes, metabolism would neither progress through the same steps and could not be regulated to serve the needs of the cell. Most central metabolic pathways are regulated at a few key steps, typically through enzymes whose activity involves the hydrolysis of ATP. Because this reaction releases so much energy, other reactions that are thermodynamically unfavorable can be coupled to ATP hydrolysis, driving the overall series of linked metabolic reactions.[1]: 30.1 

Control of activity

There are five main ways that enzyme activity is controlled in the cell.[1]: 30.1.1 

Regulation

Enzymes can be either activated or inhibited by other molecules. For example, the end product(s) of a metabolic pathway are often inhibitors for one of the first enzymes of the pathway (usually the first irreversible step, called committed step), thus regulating the amount of end product made by the pathways. Such a regulatory mechanism is called a negative feedback mechanism, because the amount of the end product produced is regulated by its own concentration.[87]: 141–48  Negative feedback mechanism can effectively adjust the rate of synthesis of intermediate metabolites according to the demands of the cells. This helps with effective allocations of materials and energy economy, and it prevents the excess manufacture of end products. Like other homeostatic devices, the control of enzymatic action helps to maintain a stable internal environment in living organisms.[87]: 141 

Post-translational modification

Examples of post-translational modification include phosphorylation, myristoylation and glycosylation.[87]: 149–69  For example, in the response to insulin, the phosphorylation of multiple enzymes, including glycogen synthase, helps control the synthesis or degradation of glycogen and allows the cell to respond to changes in blood sugar.[88] Another example of post-translational modification is the cleavage of the polypeptide chain. Chymotrypsin, a digestive protease, is produced in inactive form as chymotrypsinogen in the pancreas and transported in this form to the stomach where it is activated. This stops the enzyme from digesting the pancreas or other tissues before it enters the gut. This type of inactive precursor to an enzyme is known as a zymogen[87]: 149–53  or proenzyme.

Quantity

Enzyme production (transcription and translation of enzyme genes) can be enhanced or diminished by a cell in response to changes in the cell's environment. This form of gene regulation is called enzyme induction. For example, bacteria may become resistant to antibiotics such as penicillin because enzymes called beta-lactamases are induced that hydrolyse the crucial beta-lactam ring within the penicillin molecule.[89] Another example comes from enzymes in the liver called cytochrome P450 oxidases, which are important in drug metabolism. Induction or inhibition of these enzymes can cause drug interactions.[90] Enzyme levels can also be regulated by changing the rate of enzyme degradation.[1]: 30.1.1  The opposite of enzyme induction is enzyme repression.

Subcellular distribution

Enzymes can be compartmentalized, with different metabolic pathways occurring in different cellular compartments. For example, fatty acids are synthesized by one set of enzymes in the cytosol, endoplasmic reticulum and Golgi and used by a different set of enzymes as a source of energy in the mitochondrion, through β-oxidation.[91] In addition, trafficking of the enzyme to different compartments may change the degree of protonation (e.g., the neutral cytoplasm and the acidic lysosome) or oxidative state (e.g., oxidizing periplasm or reducing cytoplasm) which in turn affects enzyme activity.[92] In contrast to partitioning into membrane bound organelles, enzyme subcellular localisation may also be altered through polymerisation of enzymes into macromolecular cytoplasmic filaments.[93][94]

Organ specialization

In multicellulareukaryotes, cells in different organs and tissues have different patterns of gene expression and therefore have different sets of enzymes (known as isozymes) available for metabolic reactions. This provides a mechanism for regulating the overall metabolism of the organism. For example, hexokinase, the first enzyme in the glycolysis pathway, has a specialized form called glucokinase expressed in the liver and pancreas that has a lower affinity for glucose yet is more sensitive to glucose concentration.[95] This enzyme is involved in sensing blood sugar and regulating insulin production.[96]

Involvement in disease

See also: Genetic disorder

Since the tight control of enzyme activity is essential for homeostasis, any malfunction (mutation, overproduction, underproduction or deletion) of a single critical enzyme can lead to a genetic disease. The malfunction of just one type of enzyme out of the thousands of types present in the human body can be fatal. An example of a fatal genetic disease due to enzyme insufficiency is Tay–Sachs disease, in which patients lack the enzyme hexosaminidase.[97][98]

One example of enzyme deficiency is the most common type of phenylketonuria. Many different single amino acid mutations in the enzyme phenylalanine hydroxylase, which catalyzes the first step in the degradation of phenylalanine, result in build-up of phenylalanine and related products. Some mutations are in the active site, directly disrupting binding and catalysis, but many are far from the active site and reduce activity by destabilising the protein structure, or affecting correct oligomerisation.[99][100] This can lead to intellectual disability if the disease is untreated.[101] Another example is pseudocholinesterase deficiency, in which the body's ability to break down choline ester drugs is impaired.[102] Oral administration of enzymes can be used to treat some functional enzyme deficiencies, such as pancreatic insufficiency[103] and lactose intolerance.[104]

Another way enzyme malfunctions can cause disease comes from germline mutations in genes coding for DNA repair enzymes. Defects in these enzymes cause cancer because cells are less able to repair mutations in their genomes. This causes a slow accumulation of mutations and results in the development of cancers. An example of such a hereditary cancer syndrome is xeroderma pigmentosum, which causes the development of skin cancers in response to even minimal exposure to ultraviolet light.[105][106]

Evolution

Similar to any other protein, enzymes change over time through mutations and sequence divergence. Given their central role in metabolism, enzyme evolution plays a critical role in adaptation. A key question is therefore whether and how enzymes can change their enzymatic activities alongside. It is generally accepted that many new enzyme activities have evolved through gene duplication and mutation of the duplicate copies although evolution can also happen without duplication. One example of an enzyme that has changed its activity is the ancestor of methionyl amino peptidase (MAP) and creatine amidinohydrolase (creatinase) which are clearly homologous but catalyze very different reactions (MAP removes the amino-terminal methionine in new proteins while creatinase hydrolyses creatine to sarcosine and urea). In addition, MAP is metal-ion dependent while creatinase is not, hence this property was also lost over time.[107] Small changes of enzymatic activity are extremely common among enzymes. In particular, substrate binding specificity (see above) can easily and quickly change with single amino acid changes in their substrate binding pockets. This is frequently seen in the main enzyme classes such as kinases.[108]

Artificial (in vitro) evolution is now commonly used to modify enzyme activity or specificity for industrial applications (see below).

Industrial applications

Main article: Industrial enzymes

Enzymes are used in the chemical industry and other industrial applications when extremely specific catalysts are required. Enzymes in general are limited in the number of reactions they have evolved to catalyze and also by their lack of stability in organic solvents and at high temperatures. As a consequence, protein engineering is an active area of research and involves attempts to create new enzymes with novel properties, either through rational design or in vitro evolution.[109][110] These efforts have begun to be successful, and a few enzymes have now been designed "from scratch" to catalyze reactions that do not occur in nature.[111]

Application Enzymes used Uses
Biofuel industryCellulasesBreak down cellulose into sugars that can be fermented to produce cellulosic ethanol.[112]
LigninasesPretreatment of biomass for biofuel production.[112]
Biological detergentProteases, amylases, lipasesRemove protein, starch, and fat or oil stains from laundry and dishware.[113]
MannanasesRemove food stains from the common food additive guar gum.[113]
Brewing industryAmylase, glucanases, proteasesSplit polysaccharides and proteins in the malt.[114]: 150–9 
BetaglucanasesImprove the wort and beer filtration characteristics.[114]: 545 
Amyloglucosidase and pullulanasesMake low-calorie beer and adjust fermentability.[114]: 575 
Acetolactate decarboxylase (ALDC) Increase fermentation efficiency by reducing diacetyl formation.[115]
Culinary usesPapainTenderize meat for cooking.[116]
Dairy industryRenninHydrolyze protein in the manufacture of cheese.[117]
LipasesProduce Camembert cheese and blue cheeses such as Roquefort.[118]
Food processingAmylasesProduce sugars from starch, such as in making high-fructose corn syrup.[119]
ProteasesLower the protein level of flour, as in biscuit-making.[120]
TrypsinManufacture hypoallergenic baby foods.[120]
Cellulases, pectinasesClarify fruit juices.[121]
Molecular biologyNucleases, DNA ligase and polymerasesUse restriction digestion and the polymerase chain reaction to create recombinant DNA.[1]: 6.2 
Paper industryXylanases, hemicellulases and lignin peroxidasesRemove lignin from kraft pulp.[122]
Personal careProteasesRemove proteins on contact lenses to prevent infections.[123]
Starch industryAmylasesConvert starch into glucose and various syrups.[124]

See also

Enzyme databases

References

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  35. ^Rodnina MV, Wintermeyer W (2001). "Fidelity of aminoacyl-tRNA selection on the ribosome: kinetic and structural mechanisms". Annual Review of Biochemistry. 70: 415–35. doi:10.1146/annurev.biochem.70.1.415. PMID 11395413.
  36. ^Khersonsky O, Tawfik DS (2010). "Enzyme promiscuity: a mechanistic and evolutionary perspective". Annual Review of Biochemistry. 79: 471–505. doi:10.1146/annurev-biochem-030409-143718. PMID 20235827.
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Источник: https://en.wikipedia.org/wiki/Enzyme

Sharing 98% of our genes

What are the main threats?

The main threats to the chimpanzee are habitat loss and hunting for bushmeat. The relative severity of these threats differs from region to region, but the two are linked. 

Habitat loss and degradation

Degradation of forests through logging, mining, farming, and other forms of land development is contributing to the decline of primate species throughout tropical Africa. Remaining habitat patches are often small and unconnected, leaving chimpanzee populations isolated.

Deforestation is most advanced in West Africa, where only remnant tracts of primary rainforest remain. The small populations of western, Nigerian, and eastern chimpanzees are primarily located in remnant forest reserves and national parks.

In many such "protected areas", poaching for meat and live infants is common, as is unauthorized logging, mining and farming. Logging activities improve access to formerly remote forest areas, leading to increased hunting pressure.

Bushmeat
'Bushmeat' has always been a primary source of dietary protein in Central and West African countries. However in recent years, hunting for bushmeat, once a subsistence activity, has become heavily commercialized and much of the meat goes to urban residents who can afford to pay premium prices for it.

The effect of the bushmeat trade on chimp populations has yet to be evaluated, but a study in Congo showed that offtake was 5-7%, surpassing annual population increase. In addition, apes are often injured or killed in snares set for other animals. Infant chimpanzees are frequently taken alive and sold in the cities as pets.

Many conservationists believe that the bushmeat trade is now the greatest threat to forest biodiversity in West and Central Africa.

Disease- the Ebola crisis
In late 2002 an outbreak of Ebola hemorrhagic fever in humans was reported in the north of the Republic of Congo on the border with Gabon. The human infections coincided with a large-scale die-off of great apes in the region.

Two great apes are found in Central Africa, the area currently affected by Ebola: western lowland gorilla and the central chimpanzee. Both have been severely affected by the virus, which has drastically reduced populations. The disease reportedly had a great impact in Odzala National Park, a site known to have the highest density of great apes in Africa.
Источник: https://wwf.panda.org/discover/knowledge_hub/endangered_species/great_apes/chimpanzees/

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Cyber threats to a control system refer to persons who attempt unauthorized access to a control system device and/or network using a data communications pathway. This access can be directed from within an organization by trusted users or from remote locations by unknown persons using the Internet. Threats to control systems can come from numerous sources, including hostile governments, terrorist groups, disgruntled employees, and malicious intruders. To protect against these threats, it is necessary to create a secure cyber-barrier around the Industrial Control System (ICS). Though other threats exist, including natural disasters, environmental, mechanical failure, and inadvertent actions of an authorized user, this discussion will focus on the deliberate threats mentioned above.

For the purpose of this discussion, deliberate threats will be categorized consistent with the remarks in the Statement for the Record to the Joint Economic Committee by Lawrence K. Gershwin, the Central Intelligence Agency's National Intelligence Officer for Science and Technology, 21 June 2001. These include: national governments, terrorists, industrial spies, organized crime groups, hacktivists, and hackers. Activities could include espionage, hacking, identity theft, crime, and terrorism.


National Governments

National cyber warfare programs are unique in posing a threat along the entire spectrum of objectives that might harm US interests. These threats range from propaganda and low-level nuisance web page defacements to espionage and serious disruption with loss of life and extensive infrastructure disruption. Among the array of cyber threats, as seen today, only government-sponsored programs are developing capabilities with the future prospect of causing widespread, long-duration damage to U.S. critical infrastructures.

The tradecraft needed to effectively employ technology and tools remains an important limiting factor, particularly against more difficult targets such as classified networks or critical infrastructures. For the next 5 to 10 years, only nation states appear to have the discipline, commitment, and resources to fully develop capabilities to attack critical infrastructures.

Their goal is to weaken, disrupt or destroy the U.S. Their sub-goals include espionage for attack purposes, espionage for technology advancement, disruption of infrastructure to attack the US economy, full scale attack of the infrastructure when attacked by the U.S. to damage the ability of the US to continue its attacks.

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Terrorists

Traditional terrorist adversaries of the U.S., despite their intentions to damage U.S. interests, are less developed in their computer network capabilities and propensity to pursue cyber means than are other types of adversaries. They are likely, therefore, to pose only a limited cyber threat. Since bombs still work better than bytes, terrorists are likely to stay focused on traditional attack methods in the near term. We anticipate more substantial cyber threats are possible in the future as a more technically competent generation enters the ranks.

Their goal is to spread terror throughout the U.S. civilian population. Their sub-goals include: attacks to cause 50,000 or more casualties within the U.S. and attacks to weaken the U.S. economy to detract from the Global War on Terror.

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Industrial Spies and Organized Crime Groups

International corporate spies and organized crime organizations pose a medium-level threat to the US through their ability to conduct industrial espionage and large-scale monetary theft as well as their ability to hire or develop hacker talent.

Their goals are profit based. Their sub-goals include attacks on infrastructure for profit to competitors or other groups listed above, theft of trade secrets, and gain access and blackmail affected industry using potential public exposure as a threat.

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Hacktivists

Hacktivists form a small, foreign population of politically active hackers that includes individuals and groups with anti-U.S. motives. They pose a medium-level threat of carrying out an isolated but damaging attack. Most international hacktivist groups appear bent on propaganda rather than damage to critical infrastructures. Their goal is to support their political agenda. Their sub-goals are propaganda and causing damage to achieve notoriety for their cause.

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Hackers

Although the most numerous and publicized cyber intrusions and other incidents are ascribed to lone computer-hacking hobbyists, such hackers pose a negligible threat of widespread, long-duration damage to national-level infrastructures. The large majority of hackers do not have the requisite tradecraft to threaten difficult targets such as critical U.S. networks and even fewer would have a motive to do so. Nevertheless, the large worldwide population of hackers poses a relatively high threat of an isolated or brief disruption causing serious damage, including extensive property damage or loss of life. As the hacker population grows, so does the likelihood of an exceptionally skilled and malicious hacker attempting and succeeding in such an attack.

In addition, the huge worldwide volume of relatively less skilled hacking activity raises the possibility of inadvertent disruption of a critical infrastructure.

For the purposes of this discussion, hackers are subdivided as follows:

  • Sub-communities of hackers
  • Script kiddies are unskilled attackers who do NOT have the ability to discover new vulnerabilities or write exploit code, and are dependent on the research and tools from others. Their goal is achievement. Their sub-goals are to gain access and deface web pages.
  • Worm and virus writers are attackers who write the propagation code used in the worms and viruses but not typically the exploit code used to penetrate the systems infected. Their goal is notoriety. Their sub-goals are to cause disruption of networks and attached computer systems.
  • Security researcher and white hat have two sub-categories; bug hunters and exploit coders. Their goal is profit. Their sub-goals are to improve security, earn money, and achieve recognition with an exploit.
  • Professional hacker-black hat who gets paid to write exploits or actually penetrate networks; also falls into the two sub-categories-bug hunters and exploit coders. Their goal is profit.

Nature of the Computer Security Community

Hackers and researchers interact with each other to discuss common interests, regardless of color of hat. Hackers and researchers specialize in one or two areas of expertise and depend on the exchange of ideas and tools to boost their capabilities in other areas. Information regarding computer security research flows slowly from the inner circle of the best researchers and hackers to the general IT security world, in a ripple-like pattern.

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GAO Threat Table

The following table is an excerpt from NIST 800-82, "Guide to Supervisory Control and Data Acquisition (SCADA) and Industrial Control System Security (SME draft), provides a description of various threats to CS networks:

ThreatDescription
Bot-network operatorsBot-network operators are hackers; however, instead of breaking into systems for the challenge or bragging rights, they take over multiple systems in order to coordinate attacks and to distribute phishing schemes, spam, and malware attacks. The services of these networks are sometimes made available in underground markets (e.g., purchasing a denial-of-service attack, servers to relay spam, or phishing attacks, etc.).
Criminal groups Criminal groups seek to attack systems for monetary gain. Specifically, organized crime groups are using spam, phishing, and spyware/malware to commit identity theft and online fraud. International corporate spies and organized crime organizations also pose a threat to the United States through their ability to conduct industrial espionage and large-scale monetary theft and to hire or develop hacker talent.
Foreign intelligence services Foreign intelligence services use cyber tools as part of their information-gathering and espionage activities. In addition, several nations are aggressively working to develop information warfare doctrine, programs, and capabilities. Such capabilities enable a single entity to have a significant and serious impact by disrupting the supply, communications, and economic infrastructures that support military power - impacts that could affect the daily lives of U.S. citizens across the country.
HackersHackers break into networks for the thrill of the challenge or for bragging rights in the hacker community. While remote cracking once required a fair amount of skill or computer knowledge, hackers can now download attack scripts and protocols from the Internet and launch them against victim sites. Thus while attack tools have become more sophisticated, they have also become easier to use. According to the Central Intelligence Agency, the large majority of hackers do not have the requisite expertise to threaten difficult targets such as critical U.S. networks. Nevertheless, the worldwide population of hackers poses a relatively high threat of an isolated or brief disruption causing serious damage.
InsidersThe disgruntled organization insider is a principal source of computer crime. Insiders may not need a great deal of knowledge about computer intrusions because their knowledge of a target system often allows them to gain unrestricted access to cause damage to the system or to steal system data. The insider threat also includes outsourcing vendors as well as employees who accidentally introduce malware into systems.
PhishersIndividuals, or small groups, who execute phishing schemes in an attempt to steal identities or information for monetary gain. Phishers may also use spam and spyware/malware to accomplish their objectives.
SpammersIndividuals or organizations who distribute unsolicited e-mail with hidden or false information in order to sell products, conduct phishing schemes, distribute spyware/malware, or attack organizations (i.e., denial of service).
Spyware/malware authorsIndividuals or organizations with malicious intent carry out attacks against users by producing and distributing spyware and malware. Several destructive computer viruses and worms have harmed files and hard drives, including the Melissa Macro Virus, the Explore.Zip worm, the CIH (Chernobyl) Virus, Nimda, Code Red, Slammer, and Blaster.
TerroristsTerrorists seek to destroy, incapacitate, or exploit critical infrastructures in order to threaten national security, cause mass casualties, weaken the U.S. economy, and damage public morale and confidence. Terrorists may use phishing schemes or spyware/malware in order to generate funds or gather sensitive information.

Source: Government Accountability Office (GAO), Department of Homeland Security's (DHS's) Role in Critical Infrastructure Protection (CIP) Cybersecurity, GAO-05-434 (Washington, D.C.: May, 2005).

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Источник: https://us-cert.cisa.gov/ics/content/cyber-threat-source-descriptions

Trend Micro Endpoint Security with Apex One

Advantages:

Maximum XGen™ security

Infuses high-fidelity machine learning with other detection techniques for the broadest protection against ransomware and advanced attacks

OfficScan
  • Progressively filters out threats using the most efficient technique for maximum detection without false positives.
  • Blends signature-less techniques including high-fidelity machine learning, behavioral analysis, variant protection, census check, application control, exploit prevention, and good-file check with other techniques like file reputation, web reputation, and command and control (C&C) blocking.
  • Trend Micro is the first to infuse high-fidelity machine learning which uniquely analyzes files not only before execution but also during runtime for more accurate detection.
  • Noise cancellation techniques like census and whitelist checking at each layer reduce false positives.
  • Instantly shares information on suspicious network activity and files with other security layers to stop subsequent attacks.
  • Advanced ransomware protection monitors for suspicious file encryption activities at the endpoint, terminates malicious activities, and even recovers lost files if necessary.

Minimum Impact:

Reduce user impact and management costs.

  • Trend Micro Apex One™ as a Service (only available from Smart Protection Suites) allows you to deploy and manage Apex One from our cloud-based service and offers full-feature parity with the on-premises option.
  • This lightweight and optimized agent uses the right detection technique at the right time to ensure minimal impact on devices and networks.
  • Comprehensive central view of endpoint status lets you quickly get visibility to security risks.
  • Automatic sharing of threat intelligence across security layers enables protection from emerging threats across the whole organization.
  • Enable off-premises compliance and protection with the Edge Relay that enables employees to work outside the corporate network and still connect to Apex One without a VPN.
  • Customizable dashboards to fit different administration responsibilities.
  • 24x7 support means that if a problem arises, Trend Micro is there to resolve it quickly.

Proven Security Partner

Trend Micro has a history of constant innovation to provide the most effective and efficient security technologies. We are always looking ahead to develop the technology needed to fight tomorrow’s ever changing threats.

  • Over 25 years of security innovation.
  • Protects over 155 million endpoints.
  • Trusted by 45 of the top 50 global corporations.
  • Trend Micro placed Highest and Furthest in the Leaders quadrant for its Ability to Execute and Completeness of Vision in the 2017 Gartner Magic Quadrant for Endpoint Protection Platforms.

Customization:

Apex One is a critical component of Trend Micro™ Smart Protection™ Suites, allowing you to add security and investigation capabilities to fit your organizations unique security needs.

Endpoint Sensor

  • Context-aware investigation and response (EDR/XDR), recording, and reporting of system-level activities to rapidly assess attacks across email, endpoint, and server*.
  • Server side sweeping for indicators of compromise (IoCs) using multi-level criteria like OpenIOC and YARA.
  • Detailed root cause analysis (RCA) shows source and spread of attacks.
  • Threat hunting tools leveraging indicators of attack (IoA) and behavioral analysis rules.
  • Detects and analyzes advanced threat indicators such as fileless attacks.
  • Rapidly respond to threats to limit the scope of impact and protect sensitive data before it is lost.
*Note: Endpoint Sensor investigation capabilities across email and server are only available on the SaaS model. Endpoint Sensor on-premises provides investigation and response functions across endpoints.

Vulnerability Protection

  • Backed by world-class vulnerability research (DVLabs and ZDI).
  • Eliminates risk exposure due to missing patches and allows patching on your own timelines.
  • Delivers critical patches to legacy operating systems no longer being patched by the vendor.
  • Reduces downtime for recovery with incremental protection against zero-day attacks.
  • Lowers potential legal exposure by improving data security compliance.
  • Enhances firewall protection for remote and mobile enterprise endpoints.
  • Protection with minimal impact on network throughput, performance, or user productivity.

Trend Micro Apex One™ Application Control™

  • Prevents damage from unwanted/unknown applications (executables, DLLs, and other PE files).
  • Flexible, dynamic policies and whitelisting/blacklisting capabilities to reduce attack exposure.
  • Allows users to install applications based on reputation-based variables (prevalence, usage, and maturity).
  • Provides global and local real-time threat intelligence based on good file reputation data.
  • Categorizes applications and provides updates via our Trend Micro Certified Safe Software Service.
  • Coverage of pre-categorized applications that can be selected from our application catalog.
  • Visibility and policy management via Trend Micro Apex Central™.
  • Interconnects with additional layers of security to better correlate data and stop threats more often.

Data Loss Prevention (DLP)

  • Provides visibility and control of data and prevents data loss via USB, email, cloud storage, etc.
  • Protection for your data at rest, in motion, for less cost than traditional DLP solutions.
  • Simplifies deployment and complies with guidelines and regulations.
  • Enables cloud storage with DLP file encryption and SaaS application with DLP for Microsoft® Office 365®.
  • Detects improper data use based on keywords, regular expressions, and file attributes.
  • Educates on corporate data usage policies through alerts, blocking or soft-blocking, and reporting.
  • Single endpoint security, device control, and content DLP reduces resource and performance impact.
  • Integrated with Apex Central to consolidate policy, events, and reporting across DLP solutions.

Trend Micro™ Endpoint Encryption

  • Ensures data privacy by encrypting data stored on your endpoints.
  • Provides full-disk encryption, folder and file encryption, and removable media encryption.
  • Automates data management with self-encrypting hard drives.
  • Encrypts data in specific files, shared folders, and removable media.
  • Sets granular policies for device control and data management.
  • Manages Microsoft BitLocker and Apple FileVault.

*Note: Trend Micro Endpoint Encryption is available on-premises only and as a separate agent from Apex One single agent.
Источник: https://www.trenddefense.com/Apex-one-endpoint.asp

Trend Micro Endpoint Security with Apex One

Advantages:

Maximum XGen™ security

Infuses high-fidelity machine learning with other detection techniques for the broadest protection against ransomware and advanced attacks

OfficScan
  • Progressively filters out threats using the most efficient technique for maximum detection without false positives.
  • Blends signature-less techniques including high-fidelity machine learning, behavioral analysis, variant protection, census check, application control, exploit prevention, and good-file check with other techniques like file reputation, web reputation, and command and control (C&C) blocking.
  • Trend Micro is the first to infuse high-fidelity machine learning which uniquely analyzes files not only before execution but also during runtime for more accurate detection.
  • Noise cancellation techniques like census and whitelist checking at each layer reduce false positives.
  • Instantly shares information on suspicious network activity and files with other security layers to stop subsequent attacks.
  • Advanced ransomware protection monitors for suspicious file encryption activities at the endpoint, terminates malicious activities, and even recovers lost files if necessary.

Minimum Impact:

Reduce user impact and management costs.

  • Trend Micro Apex One™ as a Service (only available from Smart Protection Suites) allows you to deploy and manage Apex One from our cloud-based service and offers full-feature parity with the on-premises option.
  • This lightweight and optimized agent uses the right detection technique at the right time to ensure minimal impact on devices and networks.
  • Comprehensive central view of endpoint status lets you quickly get visibility to security risks.
  • Automatic sharing of threat intelligence across security layers enables protection from emerging threats across the whole organization.
  • Enable off-premises compliance and protection with the Edge Relay that enables employees to work outside the corporate network and still connect to Apex One without a VPN.
  • Customizable dashboards to fit different administration responsibilities.
  • 24x7 support means that if a problem arises, Trend Micro is there to resolve it quickly.

Proven Security Partner

Trend Micro has a history of constant innovation to provide the most effective and efficient security technologies. We are always looking ahead to develop the technology needed to fight tomorrow’s ever changing threats.

  • Over 25 years of security innovation.
  • Protects over 155 million endpoints.
  • Trusted by 45 of the top 50 global corporations.
  • Trend Micro placed Highest and Furthest in the Leaders quadrant for its Ability to Execute and Completeness of Vision in the 2017 Gartner Magic Quadrant for Endpoint Protection Platforms.

Customization:

Apex One is a critical component of Trend Micro™ Smart Protection™ Suites, allowing you to add security and investigation capabilities to fit your organizations unique security needs.

Endpoint Sensor

  • Context-aware investigation and response (EDR/XDR), recording, and reporting of system-level activities to rapidly assess attacks across email, endpoint, and server*.
  • Server side sweeping for indicators of compromise (IoCs) using multi-level criteria like OpenIOC and YARA.
  • Detailed root cause analysis (RCA) shows source and spread of attacks.
  • Threat hunting tools leveraging indicators of attack (IoA) and behavioral analysis rules.
  • Detects and analyzes advanced threat indicators such as fileless attacks.
  • Rapidly respond to threats to limit the scope of impact and protect sensitive data before it is lost.
*Note: Endpoint Sensor investigation capabilities across email and server are only available on the SaaS model. Endpoint Sensor on-premises provides investigation and response functions across endpoints.

Vulnerability Protection

  • Backed by world-class vulnerability research (DVLabs and ZDI).
  • Eliminates risk exposure due to missing patches and allows patching on your own timelines.
  • Delivers critical patches to legacy operating systems no longer being patched by the vendor.
  • Reduces downtime for recovery with incremental protection against zero-day attacks.
  • Lowers potential legal exposure by improving data security compliance.
  • Enhances firewall protection for remote and mobile enterprise endpoints.
  • Protection with minimal impact on network throughput, performance, or user productivity.

Trend Micro Apex One™ Application Control™

  • Prevents damage from unwanted/unknown applications (executables, DLLs, and other PE files).
  • Flexible, dynamic policies and whitelisting/blacklisting capabilities to reduce attack exposure.
  • Allows users to install applications based on reputation-based variables (prevalence, usage, and maturity).
  • Provides global and local real-time threat intelligence based on good file reputation data.
  • Categorizes applications and provides updates via our Trend Micro Certified Safe Software Service.
  • Coverage of pre-categorized applications that can be selected from our application catalog.
  • Visibility and policy management via Trend Micro Apex Central™.
  • Interconnects with additional layers of security to better correlate data and stop threats more often.

Data Loss Prevention (DLP)

  • Provides visibility and control of data and prevents data loss via USB, email, cloud storage, etc.
  • Protection for your data at rest, in motion, for less cost than traditional DLP solutions.
  • Simplifies deployment and complies with guidelines and regulations.
  • Enables cloud storage with DLP file encryption and SaaS application with DLP for Microsoft® Office 365®.
  • Detects improper data use based on keywords, regular expressions, and file attributes.
  • Educates on corporate data usage policies through alerts, blocking or soft-blocking, and reporting.
  • Single endpoint security, device control, and content DLP reduces resource and performance impact.
  • Integrated with Apex Central to consolidate policy, events, and reporting across DLP solutions.

Trend Micro™ Endpoint Encryption

  • Ensures data privacy by encrypting data stored on your endpoints.
  • Provides full-disk encryption, folder and file encryption, and removable media encryption.
  • Automates data management with self-encrypting hard drives.
  • Encrypts data in specific files, shared folders, and removable media.
  • Sets granular policies for device control and data management.
  • Manages Microsoft BitLocker and Apple FileVault.

*Note: Trend Micro Endpoint Encryption is available on-premises only and as a separate agent from Apex One single agent.
Источник: https://www.trenddefense.com/Apex-one-endpoint.asp

Enzyme

Large biological molecule that acts as a catalyst

"Biocatalyst" redirects here. For the use of natural catalysts in organic chemistry, see Biocatalysis.

Enzymes () are proteins that act as biologicalcatalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life.[1]: 8.1 Metabolic pathways depend upon enzymes to catalyze individual steps. The study of enzymes is called enzymology and the field of pseudoenzyme analysis recognizes that during evolution, some enzymes have lost the ability to carry out biological catalysis, which is often reflected in their amino acid sequences and unusual 'pseudocatalytic' properties.[2][3]

Enzymes are known to catalyze more than 5,000 biochemical reaction types.[4] Other biocatalysts are catalytic RNA molecules, called ribozymes. Enzymes' specificity comes from their unique three-dimensional structures.

Like all catalysts, enzymes increase the reaction rate by lowering its activation energy. Some enzymes can make their conversion of substrate to product occur many millions of times faster. An extreme example is orotidine 5'-phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds.[5][6] Chemically, enzymes are like any catalyst and are not consumed in chemical reactions, nor do they alter the equilibrium of a reaction. Enzymes differ from most other catalysts by being much more specific. Enzyme activity can be affected by other molecules: inhibitors are molecules that decrease enzyme activity, and activators are molecules that increase activity. Many therapeutic drugs and poisons are enzyme inhibitors. An enzyme's activity decreases markedly outside its optimal temperature and pH, and many enzymes are (permanently) denatured when exposed to excessive heat, losing their structure and catalytic properties.

Some enzymes are used commercially, for example, in the synthesis of antibiotics. Some household products use enzymes to speed up chemical reactions: enzymes in biological washing powders break down protein, starch or fat stains on clothes, and enzymes in meat tenderizer break down proteins into smaller molecules, making the meat easier to chew.

Etymology and history

Photograph of Eduard Buchner.

By the late 17th and early 18th centuries, the digestion of meat by stomach secretions[7] and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified.[8]

French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833.[9] A few decades later, when studying the fermentation malware hunter census - Crack Key For U sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."[10]

In 1877, German physiologist Wilhelm Kühne (1837–1900) first used the term enzyme, which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process.[11] The word enzyme was used later to refer to nonliving substances such as pepsin, and the word ferment was used to refer to chemical activity produced by living organisms.[12]

Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin, he found that sugar was fermented by yeast extracts even when there were no living yeast cells in the mixture.[13] He named the enzyme that brought about the fermentation of sucrose "zymase".[14] In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are usually named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate (e.g., lactase is the enzyme that cleaves lactose) or to the type of reaction (e.g., DNA polymerase forms DNA polymers).[15]

The biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others (such as Nobel laureate Richard Willstätter) argued that proteins were merely carriers for the true enzymes and that proteins per se were incapable of catalysis.[16] In 1926, James B. Sumner showed that the enzyme urease was a pure protein and crystallized it; he did likewise for the enzyme catalase in 1937. The conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley, who worked on the digestive enzymes pepsin (1930), trypsin and chymotrypsin. These three scientists were awarded the 1946 Nobel Prize in Chemistry.[17]

The discovery that enzymes could be crystallized eventually allowed their structures to be solved by x-ray crystallography. This was first done for lysozyme, an enzyme found in tears, saliva and egg whites that digests the coating of some bacteria; the structure was solved by a group led by David Chilton Phillips and published in 1965.[18] This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail.[19]

Classification and nomenclature

Enzymes can be classified by two main criteria: either amino acid sequence similarity (and thus evolutionary relationship) or enzymatic activity.

Enzyme activity. An enzyme's name is often derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase.[1]: 8.1.3  Examples are lactase, alcohol dehydrogenase and DNA polymerase. Different enzymes that catalyze the same chemical reaction are called isozymes.[1]: 10.3 

The International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers (for "Enzyme Commission"). Each enzyme is described by "EC" followed by a sequence of four numbers which represent the hierarchy of enzymatic activity (from very general to very specific). That is, the first number broadly classifies the enzyme based on its mechanism while the other digits add more and more specificity.[20]

The top-level classification is:

These sections are subdivided by other features such as the substrate, products, and chemical mechanism. An enzyme is fully specified by four numerical designations. For example, hexokinase (EC 2.7.1.1) is a transferase (EC 2) that adds a phosphate group (EC 2.7) to a hexose sugar, a molecule containing an alcohol group (EC 2.7.1).[21]

Sequence similarity. EC categories do not reflect sequence similarity. For instance, two ligases of the same EC number that catalyze exactly the same reaction can have completely different sequences. Independent of their function, enzymes, like any other proteins, have been classified by their sequence similarity into numerous families. These families have been documented in dozens of different protein and protein family databases such as Pfam.[22]

Structure

See also: Protein structure

Enzymes are generally globular proteins, acting alone or in larger complexes. The sequence of the amino acids specifies the structure which in turn determines the catalytic activity of the enzyme.[23] Although structure determines function, a novel enzymatic activity cannot yet be predicted from structure alone.[24] Enzyme structures unfold (denature) when heated or exposed to chemical denaturants and this disruption to the structure typically causes a loss of activity.[25] Enzyme denaturation is normally linked to temperatures above a species' normal level; as a result, enzymes from bacteria living in volcanic environments such as hot springs are prized by industrial users for their ability to function at high temperatures, allowing enzyme-catalysed reactions to be operated at a very high rate.

Enzymes are usually much larger than their substrates. Sizes range from just 62 amino acid residues, for the monomer of 4-oxalocrotonate tautomerase,[26] to over 2,500 residues in the animal fatty acid synthase.[27] Only a small portion of their structure (around 2–4 amino acids) is directly involved in catalysis: the catalytic malware hunter census - Crack Key For U This catalytic site is located next to one or more binding sites where residues orient the substrates. The catalytic site and binding site together compose the enzyme's active site. The remaining majority of the enzyme structure serves to maintain the precise orientation and dynamics of the active site.[29]

In some enzymes, no amino acids are directly involved in catalysis; instead, the enzyme contains sites to bind and orient catalytic cofactors.[29] Enzyme structures may also contain allosteric sites where the binding of a small molecule causes a conformational change that increases or decreases activity.[30]

A small number of RNA-based biological catalysts called ribozymes exist, which again can act alone or in complex with proteins. The most common of these is the ribosome which is a complex of protein and catalytic RNA components.[1]: 2.2 

Mechanism

Substrate binding

Enzymes must bind their substrates before they can catalyse any chemical reaction. Enzymes are usually very specific as to what substrates they bind and then the chemical reaction catalysed. Specificity is achieved by binding pockets with complementary shape, charge and hydrophilic/hydrophobic characteristics to the substrates. Enzymes can therefore distinguish between very similar substrate molecules to be chemoselective, regioselective and stereospecific.[31]

Some of the enzymes showing the highest specificity and accuracy are involved in the copying and expression of the genome. Some of these enzymes have "proof-reading" mechanisms. Here, an enzyme such as DNA polymerase catalyzes a reaction in a first step and then checks that the product is correct in a second step.[32] This two-step process results in average error rates of less than 1 error in 100 million reactions in high-fidelity mammalian polymerases.[1]: 5.3.1  Similar proofreading mechanisms are also found in RNA polymerase,[33]aminoacyl tRNA synthetases[34] and ribosomes.[35]

Conversely, some enzymes display enzyme promiscuity, having broad specificity and acting on a range of different physiologically relevant substrates. Many enzymes possess small side activities which arose fortuitously (i.e. neutrally), which may be the starting point for the evolutionary selection of a new function.[36][37]

Hexokinase displayed as an opaque surface with a pronounced open binding cleft next to unbound substrate (top) and the same enzyme with more closed cleft that surrounds the bound substrate (bottom)
Enzyme changes shape by induced fit upon substrate binding to form enzyme-substrate complex. Hexokinasehas a large induced fit motion that closes over the substrates adenosine triphosphateand xylose. Binding sites in blue, substrates in black and Mg2+cofactor in yellow. (PDB: 2E2N​, 2E2Q​)

"Lock and key" model

To explain the observed specificity of enzymes, in malware hunter census - Crack Key For U Emil Fischer proposed that both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another.[38] This is often referred to as "the lock and key" model.[1]: 8.3.2  This early model explains enzyme specificity, but fails to explain the stabilization of the transition state that enzymes achieve.[39]

Induced fit model

In 1958, Daniel Koshland suggested a modification to the lock and key model: since enzymes are rather flexible structures, the active site is continuously reshaped by interactions with the substrate as the substrate interacts with the enzyme.[40] As a result, the substrate does not simply bind to a rigid active site; the amino acid side-chains that make up the active site are molded into the precise positions that enable the enzyme to perform its catalytic function. In some cases, such as glycosidases, the substrate molecule also changes shape slightly as it enters the active site.[41] The active site continues to change until the substrate is completely bound, at which point the final shape and charge distribution is determined.[42] Induced fit may enhance the fidelity of molecular recognition in the presence of competition and noise via the conformational proofreading mechanism.[43]

Catalysis

See also: Enzyme catalysis and Transition state theory

Enzymes can accelerate reactions in several ways, all of which lower the activation energy (ΔG, Gibbs free energy)[44]

  1. By stabilizing the transition state:
    • Creating an environment with a charge distribution complementary to that of the transition state to lower its energy[45]
  2. By providing an alternative reaction pathway:
    • Temporarily reacting with the substrate, forming a covalent intermediate to provide a lower energy transition state[46]
  3. By destabilising the substrate ground state:
    • Distorting bound substrate(s) into their transition state form to reduce the energy required to reach the transition state[47]
    • By orienting the substrates into a productive arrangement to reduce the reaction entropy change[48] (the contribution of this mechanism to catalysis is relatively small)[49]

Enzymes may use several of these mechanisms simultaneously. For example, proteases such as trypsin perform covalent catalysis using a catalytic triad, stabilise charge build-up on the transition states using an oxyanion hole, complete hydrolysis using an oriented water substrate.[50]

Dynamics

See also: Protein dynamics

Enzymes are not rigid, static structures; instead they have complex internal dynamic motions – that is, movements of parts of the enzyme's structure such as individual amino acid residues, groups of residues forming a protein loop or unit of secondary structure, or even an entire protein domain. These motions give rise to a conformational ensemble of slightly different structures that interconvert with one another at equilibrium. Different states within this ensemble may be associated with different aspects of an enzyme's function. For example, different conformations of the enzyme dihydrofolate reductase are associated with the substrate binding, catalysis, cofactor release, and product release steps of the catalytic cycle,[51] consistent with catalytic resonance theory.

Substrate presentation

Substrate presentation is a process where the enzyme is sequestered away from its substrate. Enzymes can be sequestered to the plasma membrane away from a substrate in the nucleus or cytosol. Or within the membrane, an enzyme can be sequestered into lipid rafts away from its substrate in the disordered region. When the enzyme is released it mixes with its substrate. Alternatively, the enzyme can be sequestered near its substrate to activate the enzyme. For example, the enzyme can be soluble and upon activation bind to a lipid in the plasma membrane and then act upon molecules in the plasma membrane.

Allosteric modulation

Main article: Allosteric regulation

Allosteric sites are pockets on the enzyme, distinct from the active site, that bind to molecules in the cellular environment. These molecules then cause a change in the conformation or dynamics of the enzyme that is transduced to the active site and thus affects the reaction rate of the enzyme.[52] In this way, allosteric interactions can either inhibit or activate enzymes. Allosteric interactions with metabolites upstream or downstream in an enzyme's metabolic pathway cause feedback regulation, altering the activity of the enzyme according to the flux through the rest of the pathway.[53]

Cofactors

Main article: Cofactor (biochemistry)

Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity.[54] Cofactors can be either inorganic (e.g., metal ions and iron-sulfur clusters) or organic compounds (e.g., flavin and heme). These cofactors serve many purposes; for instance, metal ions can help in stabilizing nucleophilic species within the active site.[55] Organic cofactors can be either coenzymes, which are released from the enzyme's active site during the reaction, or prosthetic groups, which are tightly bound to an enzyme. Organic prosthetic groups can be covalently bound (e.g., biotin in enzymes such as pyruvate carboxylase).[56]

An example of an enzyme that contains a cofactor is carbonic anhydrase, which uses a zinc cofactor bound as part of its active site.[57] These tightly bound ions or molecules are usually found in the active site and are involved in catalysis.[1]: 8.1.1  For example, flavin and heme cofactors are often involved in redox reactions.[1]: 17 

Enzymes that require a cofactor but do not have one bound are called apoenzymes or apoproteins. An enzyme together with the cofactor(s) required for activity is called a holoenzyme (or haloenzyme). The term holoenzyme can also be applied to enzymes that contain multiple protein subunits, such as the DNA polymerases; here the holoenzyme is the complete complex containing all the subunits needed for activity.[1]: 8.1.1 

Coenzymes

Coenzymes are small organic molecules that can be loosely or tightly bound to an enzyme. Coenzymes transport chemical groups from one enzyme to another.[58] Examples include NADH, NADPH and adenosine triphosphate (ATP). Some coenzymes, such as flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), thiamine pyrophosphate (TPP), and tetrahydrofolate (THF), are derived from vitamins. These coenzymes cannot be synthesized by the body de novo and closely related compounds (vitamins) must be acquired from the diet. The chemical groups carried include:

Since coenzymes are chemically changed as a consequence of enzyme action, it is useful to consider coenzymes to be a special class of substrates, or second substrates, which are common to many different enzymes. For example, about 1000 enzymes are known to use the coenzyme NADH.[59]

Coenzymes are usually continuously regenerated and their concentrations maintained at a steady level inside the cell. For example, NADPH is regenerated through the pentose phosphate pathway and S-adenosylmethionine by methionine adenosyltransferase. This continuous regeneration means that small amounts of coenzymes can be used very intensively. For example, the human body turns over its own weight in ATP each day.[60]

Thermodynamics

A two dimensional plot of reaction coordinate (x-axis) vs. energy (y-axis) for catalyzed and uncatalyzed reactions. The energy of the system steadily increases from reactants (x = 0) until a maximum is reached at the transition state (x = 0.5), and steadily decreases to the products (x = 1). However, in an enzyme catalysed reaction, binding generates an enzyme-substrate complex (with slightly reduced energy) then increases up to a transition state with a smaller maximum than the uncatalysed reaction.
The energies of the stages of a chemical reaction. Uncatalysed (dashed line), substrates need a lot of activation energyto reach a transition state, which then decays into lower-energy products. When enzyme catalysed (solid line), the enzyme binds the substrates (ES), then stabilizes the transition state (ES) to reduce the activation energy required to produce products (EP) which are finally released.

Main articles: Activation energy, Thermodynamic equilibrium, and Chemical equilibrium

As with all catalysts, enzymes do not alter the position of the chemical equilibrium of the reaction. In the presence of an enzyme, the reaction runs in the same direction as it would without the enzyme, just more quickly.[1]: 8.2.3  For example, carbonic anhydrase catalyzes its reaction in either direction depending on the concentration of its reactants:[61]

{\displaystyle {\ce {CO2{}+H2O->[{\text{Carbonic anhydrase}}]H2CO3}}}(in tissues; high CO2concentration)

 

 

 

 

(1)

{\displaystyle {\ce {CO2{}+H2O<-[{\text{Carbonic anhydrase}}]H2CO3}}}(in lungs; low CO2concentration)

 

 

 

 

(2)

The rate of a reaction is dependent on the activation energy needed to form the transition state which then decays into products. Enzymes increase reaction rates by lowering the energy of the transition state. First, binding forms a low energy enzyme-substrate complex (ES). Second, the enzyme stabilises the transition state such that it requires less energy to achieve compared to the uncatalyzed reaction (ES). Finally the enzyme-product complex (EP) dissociates to release the products.[1]: 8.3 

Enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive" a thermodynamically unfavourable one so that the combined energy of the products is lower than the substrates. For example, the hydrolysis of ATP is often used to drive other chemical reactions.[62]

Kinetics

Main article: Enzyme kinetics

Enzyme kinetics is the investigation of how enzymes bind substrates and turn them into products.[63] The rate data used in kinetic analyses are commonly obtained from enzyme assays. In 1913 Leonor Michaelis and Maud Leonora Menten proposed a quantitative theory of enzyme kinetics, which is referred to as Michaelis–Menten kinetics.[64] The major contribution of Michaelis and Menten was to think of enzyme reactions in two stages. In the first, the substrate binds reversibly to the enzyme, forming the enzyme-substrate complex. This is sometimes called the Michaelis–Menten complex in their honor. The enzyme then catalyzes the chemical step in the reaction and releases the product. This work was further developed by G. E. Briggs and J. B. S. Haldane, who derived kinetic equations that are still widely used today.[65]

Enzyme rates depend on solution conditions and substrate concentration. To find the maximum speed of an enzymatic reaction, the substrate concentration is increased until a constant rate of product formation is seen. This is shown in the saturation curve on the right. Saturation happens because, as substrate concentration increases, more and more of the free enzyme is converted into the substrate-bound ES complex. At the maximum reaction rate (Vmax) of the enzyme, all the enzyme active sites are bound to substrate, and the amount of ES complex is the same as the total amount of enzyme.[1]: 8.4 

Vmax is only one of several important kinetic parameters. The amount of substrate needed to achieve a given rate of reaction is also important. This is given by the Michaelis–Menten constant (Km), which is the substrate concentration required for an enzyme to reach one-half its maximum reaction rate; generally, each enzyme has a characteristic KM for a given substrate. Another useful constant is kcat, also called the turnover number, which is the number of substrate molecules handled by one active site per second.[1]: 8.4 

The efficiency of an enzyme can be expressed in terms of kcat/Km. This is also called the specificity constant and incorporates the rate constants for all steps in the reaction up to and including the first irreversible step. Because the specificity constant reflects both affinity and catalytic ability, it is useful for comparing different enzymes against each other, or the same enzyme with different substrates. The theoretical maximum for the specificity constant is called the diffusion limit and is about 108 to 109 (M−1 s−1). At this point every collision of the enzyme with its substrate will result in catalysis, and the rate of product formation is not limited by the reaction rate but by the diffusion rate. Enzymes with this property are called catalytically perfect or kinetically perfect. Example of such enzymes are triose-phosphate isomerase, carbonic anhydrase, acetylcholinesterase, catalase, fumarase, β-lactamase, and superoxide dismutase.[1]: 8.4.2  The turnover of such enzymes can reach several million reactions per second.[1]: 9.2  But most enzymes are far from perfect: the average values of {\displaystyle k_{\rm {cat}}/K_{\rm {m}}} and {\displaystyle k_{\rm {cat}}} are about {\displaystyle 10^{5}{\rm {s}}^{-1}{\rm {M}}^{-1}} and {\displaystyle 10{\rm {s}}^{-1}}, respectively.[66]

Michaelis–Menten kinetics relies on the law of mass action, which is derived from the assumptions of free diffusion and thermodynamically driven random collision. Many biochemical or cellular processes deviate significantly from these conditions, because of macromolecular crowding and constrained molecular movement.[67] More recent, complex extensions of the model attempt to correct for these effects.[68]

Inhibition

Two dimensional representations of the chemical structure of folic acid and methotrexate highlighting the differences between these two substances (amidation of pyrimidone and methylation of secondary amine).

The coenzyme folic acid (left) and the anti-cancer drug methotrexate (right) are very similar in structure (differences show in green). As a result, methotrexate is a competitive inhibitor of many enzymes that use folates.

Main article: Enzyme inhibitor

Enzyme reaction rates can be decreased by various types of enzyme inhibitors.[69]: 73–74 

Types of inhibition

Competitive

A competitive inhibitor and substrate cannot bind to the enzyme at the same time.[70] Often competitive inhibitors strongly resemble the real substrate of the enzyme. For example, the drug methotrexate is a competitive inhibitor of the enzyme dihydrofolate reductase, which catalyzes the reduction of dihydrofolate to tetrahydrofolate.[71] The similarity between the structures of dihydrofolate and this drug are shown in the accompanying figure. This type of inhibition can be overcome with high substrate concentration. In some cases, the inhibitor can bind to a site other than the binding-site of the usual substrate and exert an allosteric effect to change the shape of the usual binding-site.[72]

Non-competitive

A non-competitive inhibitor binds to a site other than where the substrate binds. The substrate still binds with its usual affinity and hence Km remains the same. However the inhibitor reduces the catalytic efficiency of the enzyme so that Vmax is reduced. In contrast to competitive inhibition, non-competitive inhibition cannot be overcome with high substrate concentration.[69]: 76–78 

Uncompetitive

An uncompetitive inhibitor cannot bind to the free enzyme, only to the enzyme-substrate complex; hence, these types of inhibitors are most effective at high substrate concentration. In the presence of the inhibitor, the enzyme-substrate complex is inactive.[69]: 78  This type of inhibition is rare.[73]

Mixed

A mixed inhibitor binds to an allosteric site and the binding of the substrate and the inhibitor affect each other. The enzyme's function is reduced but not eliminated when bound to the inhibitor. This type of inhibitor does not follow the Michaelis–Menten equation.[69]: 76–78 

Irreversible

An irreversible inhibitor permanently inactivates the enzyme, usually by forming a covalent bond to the protein.[74]Penicillin[75] and aspirin[76] are common drugs that act in this manner.

Functions of inhibitors

In many organisms, inhibitors may act as part of a feedback mechanism. If an enzyme produces too much of one substance in the organism, that substance may act as an inhibitor for the enzyme at the beginning of the pathway that produces it, causing production of the substance to slow down or stop when there is sufficient amount. This is a form of negative feedback. Major metabolic pathways such as the citric acid cycle make use of this mechanism.[1]: 17.2.2 

Since inhibitors modulate the function of enzymes they are often used as drugs. Many such drugs are reversible competitive inhibitors that resemble the enzyme's native substrate, similar to methotrexate above; other well-known examples include statins used to treat high cholesterol,[77] and protease inhibitors used to treat retroviral infections such as HIV.[78] A common example of an irreversible inhibitor that is used as a drug is aspirin, which inhibits the COX-1 and COX-2 enzymes that produce the inflammation messenger prostaglandin.[76] Other enzyme inhibitors are poisons. For example, the poison cyanide is an irreversible enzyme inhibitor that combines with the copper and iron in the active site of the enzyme cytochrome c oxidase and blocks cellular respiration.[79]

Factors affecting enzyme activity

As enzymes are made up of proteins, their actions are sensitive to change in many physio chemical factors such as pH, temperature, substrate concentration, etc.

The following table shows pH optima for various enzymes.[80]

Enzyme Optimum pH pH description
Pepsin 1.5–1.6 Highly acidic
Invertase 4.5 Acidic
Lipase (stomach) 4.0–5.0 Acidic
Lipase (castor oil) 4.7 Acidic
Lipase (pancreas) 8.0 Alkaline
Amylase (malt) 4.6–5.2 Acidic
Amylase (pancreas) 6.7–7.0 Acidic-neutral
Cellobiase 5.0 Acidic
Maltase 6.1–6.8 Acidic
Sucrase 6.2 Acidic
Catalase 7.0 Neutral
Urease 7.0 Neutral
Cholinesterase 7.0 Neutral
Ribonuclease 7.0–7.5 Neutral
Fumarase 7.8 Alkaline
Trypsin 7.8–8.7 Alkaline
Adenosine triphosphate 9.0 Alkaline
Arginase 10.0 Highly alkaline

Biological function

Enzymes serve a wide variety of functions inside living organisms. They are indispensable for signal transduction and cell regulation, often via kinases and phosphatases.[81] They also generate movement, with myosin hydrolyzing ATP to generate muscle contraction, and also transport cargo around the cell as part of the cytoskeleton.[82] Other ATPases in the cell membrane are ion pumps involved in active transport. Enzymes are also involved in more exotic functions, such as luciferase generating light in fireflies.[83]Viruses can also contain enzymes for infecting cells, such as the HIV integrase and reverse transcriptase, or for viral release from cells, like the influenza virus neuraminidase.[84]

An important function of enzymes is in the digestive systems of animals. Enzymes such as amylases and proteases break down large molecules (starch or proteins, respectively) into smaller ones, so they can be absorbed by the intestines. Starch molecules, for example, are too large to be absorbed from the intestine, but enzymes hydrolyze the starch chains into smaller molecules such as maltose and eventually glucose, which can then be absorbed. Different enzymes digest different food substances. In ruminants, which have herbivorous diets, microorganisms in the gut produce another enzyme, cellulase, to break down the cellulose cell walls of plant fiber.[85]

Metabolism

Several enzymes can work together in a specific order, creating metabolic pathways.[1]: 30.1  In a metabolic pathway, one enzyme takes the product of another enzyme as a substrate. After the catalytic reaction, the product is then passed on to another enzyme. Sometimes more than one enzyme can catalyze the same reaction in parallel; this can allow more complex regulation: with, for example, a low constant activity provided by one enzyme but an inducible high activity from a second enzyme.[86]

Enzymes determine what steps occur in these pathways. Without enzymes, metabolism would neither progress through the same steps and could not be regulated to serve the needs of the cell. Most central metabolic pathways are regulated at a few key steps, typically through enzymes whose activity involves the hydrolysis of ATP. Because this reaction releases so much energy, other reactions that are thermodynamically unfavorable can be coupled to ATP hydrolysis, driving the overall series of linked metabolic reactions.[1]: 30.1 

Control of activity

There are five main ways that enzyme activity is controlled in the cell.[1]: 30.1.1 

Regulation

Enzymes can be either activated or inhibited by other molecules. For example, the end product(s) of a metabolic pathway are often inhibitors for one of the first enzymes of the pathway (usually the first irreversible step, called committed step), thus regulating the amount of end product made by the pathways. Such a regulatory mechanism is called a negative feedback mechanism, because the amount of the end product produced is regulated by its own concentration.[87]: 141–48  Negative feedback mechanism can effectively adjust the rate of synthesis of intermediate metabolites according to the demands of the cells. This helps with effective allocations of materials and energy economy, and it prevents the excess manufacture of end products. Like other homeostatic devices, the control of enzymatic action helps to maintain a stable internal environment in living organisms.[87]: 141 

Post-translational modification

Examples of post-translational modification include phosphorylation, myristoylation and glycosylation.[87]: 149–69  For example, in the response to insulin, the phosphorylation of multiple enzymes, including glycogen synthase, helps control the synthesis or degradation of glycogen and allows the cell to respond to changes in blood sugar.[88] Another example of post-translational modification is the cleavage of the polypeptide chain. Chymotrypsin, a digestive protease, is produced in inactive form as chymotrypsinogen in the pancreas and transported in this form to the stomach where it is activated. This stops the enzyme from digesting the pancreas or other tissues before it enters the gut. This type of inactive precursor to an enzyme is known as a zymogen[87]: 149–53  or proenzyme.

Quantity

Enzyme production (transcription and translation of enzyme genes) can be enhanced or diminished by a cell in response to changes in the cell's environment. This form of gene regulation is called enzyme induction. For example, bacteria may become resistant to antibiotics such as penicillin because enzymes called beta-lactamases are induced that hydrolyse the crucial beta-lactam ring within the penicillin molecule.[89] Another example comes from enzymes in the liver called cytochrome P450 oxidases, which are important in drug metabolism. Induction or inhibition of these enzymes can cause drug interactions.[90] Enzyme levels can also be regulated by changing the rate of enzyme degradation.[1]: 30.1.1  The opposite of enzyme induction is enzyme repression.

Subcellular distribution

Enzymes can be compartmentalized, with different metabolic pathways occurring in different cellular compartments. For example, fatty acids are synthesized by one set of enzymes in the cytosol, endoplasmic reticulum and Golgi and used by a different set of enzymes as a source of energy in the mitochondrion, through β-oxidation.[91] In addition, trafficking of the enzyme to different compartments may change the degree of protonation (e.g., the neutral cytoplasm and the acidic lysosome) or oxidative state (e.g., oxidizing periplasm or reducing cytoplasm) which in turn affects enzyme activity.[92] In contrast to partitioning into membrane bound organelles, enzyme subcellular localisation may also be altered through polymerisation of enzymes into macromolecular cytoplasmic filaments.[93][94]

Organ specialization

In multicellulareukaryotes, cells in different organs and tissues have different patterns of gene expression and therefore have different sets of enzymes (known as isozymes) available for metabolic reactions. This provides a mechanism for regulating the overall metabolism of the organism. For example, hexokinase, the first enzyme in the glycolysis pathway, has a specialized form called glucokinase expressed in the liver and pancreas that has a lower affinity for glucose yet is more sensitive to glucose concentration.[95] This enzyme is involved in sensing blood sugar and regulating insulin production.[96]

Involvement in disease

See also: Genetic disorder

Since the tight control of enzyme activity is essential for homeostasis, any malfunction (mutation, overproduction, underproduction or deletion) of a single critical enzyme can lead to a genetic disease. The malfunction of just one type of enzyme out of the thousands of types present in the human body can be fatal. An example of a fatal genetic disease due to enzyme insufficiency is Tay–Sachs disease, in which patients lack the enzyme hexosaminidase.[97][98]

One example of enzyme deficiency is the most common type of phenylketonuria. Many different single amino acid mutations in the enzyme phenylalanine hydroxylase, which catalyzes the first step in the degradation of phenylalanine, result in build-up of phenylalanine and related products. Some mutations are in the active site, directly disrupting binding and catalysis, but many are far from the active site and reduce activity by destabilising the protein structure, or affecting correct oligomerisation.[99][100] This can lead to intellectual disability if the disease is untreated.[101] Another example is pseudocholinesterase deficiency, in which the body's ability to break down choline ester drugs is impaired.[102] Oral administration of enzymes can be used to treat some functional enzyme deficiencies, such as pancreatic insufficiency[103] and lactose intolerance.[104]

Another way enzyme malfunctions can cause disease comes from germline mutations in genes coding for DNA repair enzymes. Defects in these enzymes cause cancer because cells are less able to repair mutations in their genomes. This causes a slow accumulation of mutations and results in the development of cancers. An example of such a hereditary cancer syndrome is xeroderma pigmentosum, which causes the development of skin cancers in response to even minimal exposure to ultraviolet light.[105][106]

Evolution

Similar to any other protein, enzymes change over time through mutations and sequence divergence. Given their central role in metabolism, enzyme evolution plays a critical role in adaptation. A key question is therefore whether and how enzymes can change their enzymatic activities alongside. It is generally accepted that many new enzyme activities have evolved through gene duplication and mutation of the duplicate copies although evolution can also happen without duplication. One example of an enzyme that has changed its activity is the ancestor of methionyl amino peptidase (MAP) and creatine amidinohydrolase (creatinase) which are clearly homologous but catalyze very different reactions (MAP removes the amino-terminal methionine in new proteins while creatinase hydrolyses creatine to sarcosine and urea). In addition, MAP is metal-ion dependent while creatinase is not, hence this property was also lost over time.[107] Small changes of enzymatic activity are extremely common among enzymes. In particular, substrate binding specificity (see above) can easily and quickly change with single amino acid changes in their substrate binding pockets. This is frequently seen in the main enzyme classes such as kinases.[108]

Artificial (in vitro) evolution is now commonly used to modify enzyme activity or specificity for industrial applications (see below).

Industrial applications

Main article: Industrial enzymes

Enzymes are used in the chemical industry and other industrial applications when extremely specific catalysts are required. Enzymes in general are limited in the number of reactions they have evolved to catalyze and also by their lack of stability in organic solvents and at high temperatures. As a consequence, protein engineering is an active area of research and involves attempts to create new enzymes with novel properties, either through rational design or in vitro evolution.[109][110] These efforts have begun to be successful, and a few enzymes have now been designed "from scratch" to catalyze reactions that do not occur in nature.[111]

Application Enzymes used Uses
malware hunter census - Crack Key For U industryCellulasesBreak down cellulose into sugars that can be fermented to produce cellulosic ethanol.[112]
LigninasesPretreatment of biomass for biofuel production.[112]
Biological detergentProteases, amylases, lipasesRemove protein, starch, and fat or oil stains from laundry and dishware.[113]
MannanasesRemove food stains from the common food additive guar gum.[113]
Brewing industryAmylase, glucanases, proteasesSplit polysaccharides and proteins in the malt.[114]: 150–9 
BetaglucanasesImprove the wort and beer filtration characteristics.[114]: 545 
Amyloglucosidase and pullulanasesMake low-calorie beer and adjust fermentability.[114]: 575 
Acetolactate decarboxylase (ALDC) Increase fermentation efficiency by reducing diacetyl formation.[115]
Culinary usesPapainTenderize meat for cooking.[116]
Dairy industryRenninHydrolyze protein in the manufacture of cheese.[117]
LipasesProduce Camembert cheese and blue cheeses such as Roquefort.[118]
Food processingAmylasesProduce sugars from starch, such as in making high-fructose corn syrup.[119]
ProteasesLower the protein level of flour, as in biscuit-making.[120]
TrypsinManufacture hypoallergenic baby foods.[120]
Cellulases, pectinasesClarify fruit juices.[121]
Molecular biologyNucleases, DNA ligase and polymerasesUse restriction digestion and the polymerase chain reaction to create recombinant DNA.[1]: 6.2 
Paper industryXylanases, hemicellulases and lignin peroxidasesRemove lignin from kraft pulp.[122]
Personal careProteasesRemove proteins on contact lenses to prevent infections.[123]
Starch industryAmylasesConvert starch into glucose and various syrups.[124]

See also

Enzyme databases

References

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Источник: https://en.wikipedia.org/wiki/Enzyme

The 100 Best Simpsons Episodes to Stream

This story was originally published in October 2014. We're republishing it because The Simpsons is now streaming on Disney+.

Photo: FOX

After three decades on the air and more than 650 episodes, Homer, Marge, Bart, Lisa, and Maggie have officially moved into the Mouse House. Earlier this month, the new streaming service Disney+ became the exclusive streaming home of The Simpsons, meaning that if you don’t want to buy or rent episodes, wait around for reruns, or dig up your old DVDs, it’s the only place to watch one of TV’s most important and long-running comedies.

Back in 2014, we assembled this list of the 100 most essential Simpsons episodes as a guide for fans both young and old, and for those who have yet to even see a single episode. The list reflects the show’s continued greatness over 30 years, meaning it includes the first episode and also some of the more recent ones. (Note: It doesn’t include the Michael Jackson episode, “Stark Raving Dad,” which was pulled from circulation earlier this year.) These 100 episodes best paint a picture of all the things the series does best. Happy streaming!

“The Day the Earth Stood Cool” and “Homerpalooza” are cousins of sorts. Both episodes deal with the alternative culture of the day, as a way of showing Homer facing the fact that he has aged out of relevance. Airing in late 2012, “The Day the Earth Stood Cool” is the show’s hipster episode. Homer convinces a hip family (voiced by Portlandia’s Fred Armisen and Carrie Brownstein) to move from Portland to Springfield, effectively transforming the city, with the Tire Fire becoming a farmer’s market, Android’s Dungeon becoming a Taschen, and the Kwik-E-Mart turning into Apu’s House of Spats.

Hipster jokes didn’t have the best track record before this episode aired, but The Simpsons was able to breathe life into them. First, it helps that the jokes were rooted in fleshed-out characters and real insecurities Homer and Marge have. Second, the jokes are spot-on. In a scene in which Flanders is trying to mediate the conflict between Homer and Armisen’s character, Homer goes, “Can we at least agree to both hate Flanders?” To which Armisen replies, “I like him. He talks in rhyme and owns a whimsical store in a failing mall: He’s like the dad in a Wes Anderson movie.” Over 25 years, The Simpsons takes the job of culture critic very seriously. Nothing gets by it.

The Simpsons is essentially a sitcom, but over time, it has taken on a role not unlike that of Saturday Night Live — an institution that’s expected to comment on the state of culture. That’s what “Homerpalooza” is. It’s the Gen-X, Lollapalooza world, filtered through the lens of the show. Homer becomes a part of traveling music festival, Hullabalooza … as a guy who takes a cannonball to his stomach.

The episode succeeds at both telling the story of Homer’s desire to still be cool and making fun of what was cool at that time. Take one scene: Homer gets onstage and a shirtless member of the audience comments to his buddy in the de rigueur ironic tone, “Here comes that cannonball guy. He’s cool.” His buddy asks, “Are you being sarcastic, dude?” He responds, “I don’t even know anymore.” The ’90s!

It’s not often Bart cares about something, but when he does, he’s all in. In “A Totally Fun Thing That Bart Will Never Do Again,” Bart sells everything he owns so the entire family can go on a cruise. He falls well short of his goal, but Marge and Lisa chip in, and all of a sudden, the Simpsons are ON A BOAT. (Don’t you ever forget.) Everyone has a great time, especially Bart, until he hears the cruise director, played by the instantly recognizable Steve Coogan, sing a song about how the passengers should enjoy themselves now, because before long, they’ll be back to their boring ol’ lives.

This terrifies Bart, so with some help from Treat Williams, he convinces everyone on board that a fatal virus has spread on the mainland and it’s up to them to keep humanity alive. It’s Bart at his most puckish: He’s pulling a prank, but only because he doesn’t want to go back to the reality of everyday life. It would seem childish if we didn’t think that, under different circumstances, Homer wouldn’t do the same thing.

This Halloween trilogy features a monster in an attic, a political satire, and a riff on The Incredible Shrinking Man, Fantastic Voyage, and other adventures in miniaturization. The attic story is actually a twin story: Apparently, the creature rattling around upstairs is Bart’s conjoined twin Hugo, an id creature à la The Dark Half who’s chained up for everyone’s protection and subsists on fish heads. The miniaturization story finds Bart and Lisa accidentally creating a race of tiny creatures in a science experiment, then messing with them, inadvertently and on purpose. The finale finds aliens Kang and Kodos running for president (against each other) by assuming the identities of 1996 presidential candidates Bill Clinton and Bob Dole.

The final story is an exercise in pure silliness that plays like an animated “Shouts and Murmurs” column, but it has a sting: Even after Homer crashes a spaceship into the Capitol and tells the world that “Bob Dole” and “Bill Clinton” are actually tentacled space aliens, the public is told that it has to choose one of them anyway because it’s a two-party system. (Man in crowd: “Well, I believe I’ll vote for a third-party candidate.” Kang: “Go ahead! Throw your vote away!”) The first two stories, though, have an imaginative depth that qualifies them as real science fiction (gags and all). The attic tale has the creepy power of the earlier myths and novels that it draws on, while the miniaturization story is a Frankenstein variant about the moral and philosophical implications of playing God with manufactured creatures. (Lisa: “Oh my God! I’ve created life!” Marge: “Lisa, breakfast! We’re having waffles!” Lisa: “Ooh, waffles!”)

The Simpsons has always thrived on pop culture, whether it’s parodying a film or writing in relevant guest stars, and that continues to be a strength (and sometimes a weakness) to this day. In “The Haw-Hawed Couple,” which in itself is a pun on The Odd Couple, there are references to Goodfellas, Brokeback Mountain, and even an entire subplot devoted to a faux–Harry Potter book series.

Bart and Nelson become unlikely best friends after Bart is the only kid who attends Nelson’s birthday party (complete with a fake Spider-Man and Doctor Octopus). Even though Bart doesn’t want to be BFFs, he plays along for protection and other benefits, only to have it all washed away after Nelson realizes how one-sided the affair actually is. It’s a brief friendship, but any time we get to see Nelson’s sensitive side is a winning one. Plus, the Brokeback Mountain kicker where Bart hugs his Nelson vest while Nelson shouts, “Haw haw, I touched your heart!” nails The Simpsons’ mix of bite and heart with one joke.

For basically every Simpsons episode, Lisa is the only genuinely smart person in Springfield. “Lisa’s Rival” shows what happens when she meets her match. Based on an idea Conan O’Brien had before he left for Late Night, “Lisa’s Rival” introduces us to Allison Taylor (wonderfully voiced by Winona Ryder). Not only does Allison also know the answers to questions in class and play saxophone like Lisa, she was actually skipped ahead a grade and is better at the instrument. This causes a new side of Lisa to come out. A jealous side. A darker side.

Of the show’s many achievements, one has to be its creation of a highly relatable and specific portrait of intelligence. The Simpsons is partly about how the intelligent view the world, and Lisa often is the writers’ proxy. What makes “Lisa’s Rival” so pivotal is that it drives home that Lisa is not a super genius, incapable of relating to a city of peons; she’s just a very smart young lady who wants to meet even an average person. Few Simpsons episode do a better job at capturing Lisa’s fundamental desire for connection.

Homer puts his money where his mouth is when he becomes the newest food critic for the Springfield Shopper. Eager glutton that he is, he gives everything a stellar review, except for Marge’s pork chops, which receive his lowest rating ever: seven thumbs up. Although Homer knows exactly what he loves — food, and lots of it — he’s unable to put his admiration into words that aren’t “SCREW FLANDERS,” so he gets Lisa to translate his drool into something more “groin-grabbingly transcendent.”

The other critics at the Shopper balk at his unanimous admiration and peer-pressure him into writing some negative reviews, which in turn upsets the local restaurant owners and chefs. It’s hard to believe that it took 11 seasons for the writers to get around to making “Guess Who’s Coming to Criticize Dinner?,” But it was worth the wait: The Lisa/Homer conflict is the glue that holds the plot together, and the only thing that makes more sense than Homer as a food critic is having Ed Asner play his editor.

The Simpsons may live in America’s Crud Bucket, but it’s THEIR crud bucket. After Bart gets caught writing his name in wet cement, Marge scolds her son and tells him that he should show more respect for his town. After all, it’s a part of us all, a part of us all, a part of us all. With his Springfield pride swelling, Bart goes to visit the town’s malware hunter census - Crack Key For U resource: a lemon tree. Except it’s gone, having been stolen by a gang of no-good kids from Shelbyville. Bart, Milhouse, Nelson, Martin, Todd, and Database sneak into their neighboring city on a rescue mission, only to find their beloved tree in an impound lot.

Shelbyville was oft-mentioned on The Simpsons before “Lemon of Troy,” but this was our first good look at it, expanding its world that much further. Essentially, it’s the same as Springfield, only with yellow fire hydrants instead of red ones. But that’s the thing: Most small towns are exactly the same; it’s your personal connection to them that makes the difference. As much as this show makes fun of the sometimes small-mindedness of Springfield, it has created a town that feels alive, and viewers have become as loyal to it as its residents prove to be here.

It’s a testament to how great “In Marge We Trust” is that the episode’s B plot is the more memorable one. While Marge is busy taking over as “The Listen Lady” at church, overshadowing a depressed Reverend Lovejoy, Homer, Bart, and Lisa go on a mission to find out why Homer’s head is on a Japanese box.

Turns out the dish-detergent product Mr. Sparkle’s logo was actually just a coincidental combination of a fish and a lightbulb (or “fish-bulb,” as Bart calls Homer), but its preposterous story line has since created one of the show’s most iconic images, next to the “I Choo-Choo-Choose You” Valentine and the Inanimate Carbon Rod.

Moe’s Tavern is a regular spot on the show, and Homer is often drunk. He is often drunk, he drives drunk, he does lots of things drunk. But “Duffless” finally calls him on the carpet for it. After going on a tour of the Duff Brewery and subsequently failing a Breathalyzer test, Homer is arrested and has his driver’s license revoked. Marge asks him to give up drinking for a month, which he agrees to, not realizing how difficult it is. Homer grows so desperate for a single sip of Duff that he admits in an Alcoholics Anonymous meeting that he “snuck into the football stadium and ate the dirt under the bleachers.”

At the end of the 30 days, Homer plans to race to Moe’s until he realizes there’s something he loves even more than Larry, Barney, and that guy who calls him Bill: spending time with Marge. It’s a sweet ending to a sincere episode, one that’s based on something real: We can’t imagine what it’s like to go into space, but we know what it’s like to worry for our loved ones. That reliability keeps “Duffless” grounded, despite an appearance from the Duff Blimp.

A parody of The Da Vinci Code and National Treasure, “Gone Maggie Gone” uses the puzzle-solving, conspiracy-mystery format to create a wholly unique and wonderful episode. The episode stars Lisa as its Robert Langdon, as she searches for an ancient gem as a way of finding Maggie.

Over its many seasons, The Simpsons has been able to take culture and filter it so thoroughly through its lens that the source material become less essential than the episode. “Gone Maggie Gone” is a classic example of this, where it’s still able to feel fresh and super funny, despite the reference text being a bit dated.

The most politically loaded episode of the entire series, “Sideshow Bob Roberts” is a masterful examination of how easy it is to convince a lazy, willing nation of, well, pretty much anything. Rush Limbaugh fill-in Birch Barlow pressures Mayor Quimby into releasing Sideshow Bob from prison. With the public on his side, Bob decides to run for mayor of Springfield, despite being an ex-con who recently tried to murder a young boy.

“Bob Roberts” (named after the Tim Robbins movie) is packed with decades Driver Talent Pro 8.0.2.10 Crack Free Activation Key 2021 references — including one to the Kennedy vs. Nixon debate, with Bob ably filling in for JFK — yet it’s never bogged down by dated satire. The jokes fly fast and freely, and it’s always a treat when Bart and Lisa work together, as they do here, exposing Bob, who wins the election, as a colossal fraud. If anything, “Sideshow Bob Roberts,” which mocks both “cold-hearted” Republicans and “guilty” Democrats equally, has only gotten better with time. And it was pretty great already.

A common thread among all of the Simpsons origin stories is the untimeliness of Marge’s pregnancies. This was demonstrated efficiently in “And Maggie Makes Three,” in a brief montage in which Homer rips out some of his hair upon finding out Marge was pregnant with Bart, and then more of it with Lisa, and then the last of it with Maggie. But “I Married Marge” focuses solely on Marge and Homer’s firstborn.

This flashback episode is prompted by an unclear pregnancy test Marge takes (don’t trust Barnacle Bill), which causes Homer to tell the kids the story of how he and Marge got married and how he got his job at the nuclear power-plant. What “I Married Marge” does best is create a heart-filled backstory that not only explains how Homer went from being a boy to a husband and father so quickly, but also reveals the undivided love between the two characters. Even if there are various other origins floating around the Simpsons universe, you can’t deny that Homer proposing to Marge with an onion ring fits his character perfectly.

After discovering her passion for filming documentaries, Lisa is encouraged by a suddenly industry-savvy Principal Skinner to create a feature on her family (her main source of drama, he notes). What ensues, obviously, is a meta-episode that somehow loops in guest appearances from Jim Jarmusch and John C. Reilly, a great montage using Spoon’s “I Turn My Camera On,” and a clever nod to The Simpsons Movie. (“No more Simpsons movies! One was plenty.”)

“Any Given Sundance” may bear a resemblance to season six’s A Star Is Burns, but it focuses much more on Lisa’s vision and her negative perception of her parents and brother. It’s a viewpoint that we’ve always gotten a glance at throughout the seasons, but this episode truly exposes Lisa’s inner judgements while also serving as a spot-on critique of the film-festival scene. (“I liked this movie more than the one by that little girl because I saw it today.”)

The Simpsons’ mythology has been rejiggered so many times that it’s hard to tell what really happened in the years leading up to Bart’s birth. Springfield Up offers yet another take on Homer and Marge’s history by putting it through the lens of a documentary called Springfield Up (which parodies the U.K.’s Up series), and it proves to be one of the more successful rewrites in recent seasons.

Filmmaker Declan Desmond (voiced by Monty Python’s Eric Idle) checks in on Springfield residents every eight years to track their growth from youth (apparently everyone in Springfield went to school together) to adulthood. This episode may not be an accurate retelling of The Simpsons’ history, but it is an amusing alternative narrative that would’ve just been as plausible and convincing. The show is never afraid to spin more contradicting details to its biography, and that’s half the fun of keeping up with the many seasons of The Simpsons.

“All’s Fair in Oven War” features two very funny Simpsons stories. Primarily, it’s a story of Marge cheating to win a baking contest because she wants to be known as more than “the wife of a husband who doesn’t go to work.” The second plotline is a ridiculous arc in which, after Marge cuts all the naked pictures out of Homer’s Playdudes (The Simpsons’ Playboys), Bart finds them and starts enacting a Hugh Hefner–like lifestyle, just without the sex.

Unlike most sitcoms, there is no one structure for a I episode: An episode could have one story, two, three, 22. “All’s Fair in Oven War” demonstrates the show’s consistently incredible ability to tell two very different yet completely satisfying tales.

The Simpsons goes sex-positive. Though it’s always implied that Homer and Marge have a solid sex life (or “snuggle life,” as Marge would likely call it), “Natural Born Kissers” foregrounds that assumption when the couple reignites their lost spark by realizing they like to do it in public.

As a non-linear show, The Simpsons succeeds at creating a portrait of a marriage through a series of snapshots. Kudos to the show for not pulling any punches here. Censors were taken aback by the frankness of the sex in the episode, but the producers thankfully fought back. Side note: The episode features a hilarious B story, in which Bart and Lisa discover an alternate ending to Casablanca.

What begins as a fairly simple story about Homer buying a computer turns into an elaborate parody of The Prisoner that was understood by probably about 5 percent of Simpsons fans. But it leaves you feeling like you’ve been gassed (in a good way). When Homer gets online following his purchase, he sets up a website, Mr. X, where he anonymously reveals local gossip. After the rumor well dries up, Homer begins making up stories, including one about how the government is controlling our minds through flu shots. Soon after the story goes live, Homer is kidnapped and wakes up on the Island, a mysterious land where people who know too much are kept.

From here on out, “Menace Shoes” is a twisty tale about drugs, doppelgängers, and guest-star Patrick McGoohan. It should work about as well as an anti-escape orb that’s been punctured with a knife, but this one’s a trippy kick.

“The Principal and the Pauper” is the WORST. EPISODE. EVER. for many Simpsons fans who care too much about canon. That’s a shame, because it’s actually an interesting and vastly underrated exploration of a character we thought we knew for years. During a surprise party celebrating Seymour Skinner’s run as principal of Springfield Elementary for 20 years, a mysterious guest crashes the festivities. He claims he’s the REAL Seymour Skinner, and our Seymour is actually an impostor named Armin Tamzarian, a claim that’s later verified. They served together in Vietnam, but when Skinner goes missing, Tamzarian takes on his identity.

It’s a pre–Mad Men plot twist that’s only successful because it’s done out of love: Tamzarian becomes Skinner because he can’t let Seymour’s mother, Agnes, know that her son is gone. The episode also works as a meta-commentary on Simpsons fans themselves and their reluctance to accept change. After the original Skinner is shipped out of town and Tamzarian becomes “Spanky” again, Judge Snyder decries that no one is allowed to mention what happened ever again, under penalty of torture. Ironically, “Pauper” is one of the MOST DISCUSSED. EPISODES. EVER.

This episode is a testament to The Simpsons’ ability not only to build a universe filled with unique and hilarious characters, but also to give them specific internal psychologies and motivations. The episode focuses on the courtship between Principal Skinner and Ms. Krabappel, who become romantically involved at the appropriately depressing place that is Martin’s birthday party. The only problem is that Bart saw it all happen, so the couple bribes him to keep his mouth shut and to serve as a go-between.

But this impressively adult episode is not about Bart; it’s about the two grown-ups, who bond over their mutual sadness. Sure, it’s filled with classic bits (namely, Homer realizing he was saying Krabappel’s name wrong, Ralph explaining seeing the couple kissing, and “Sex Cauldron”), but what makes the episode classic is how much you end up caring about two characters that no fan would ever rank in the top 20 — two characters who usually just exist to react to Bart’s pranks.

Yet another of Homer’s dream jobs. This time, after paying off all his debts, he quits the power plant to work at a bowling alley. It’s simple work, but he likes it and he does it well. Then Marge gets pregnant with Maggie, and he finds himself having to balance his dreams with his responsibilities as a father. The end of this episode packs one of the series’ strongest emotional wallops. We’re going to describe it here for those who have seen it to remember — if you haven’t, skip ahead and come back later.

After Homer has to beg for his power-plant job back, Mr. Burns installs him back at his console in front of a sign that reads, “Don’t forget, you’re here forever.” It’s mean and existentially horrible. The final moment returns to that sign, now surrounded by pictures of Maggie. Homer has used some of them to block certain letters of the sign, which now reads, “Do It for Her.” It’s the simplest of images and it has the greatest of meanings — sometimes life involves doing things you don’t want to do for those you love. And sometimes it’s worth it.

Moe is easily The Simpsons’ darkest character, so a lot of comedy and heart can be gotten out of him in the rare instances when something good happens. In classic Moe fashion, the main plot of “Moe Baby Blues” starts with a suicide attempt, where, before he can jump from a bridge, he catches Maggie (who had been flying through the air after a fender-bender). From that point on, the two form an adorable yet undeniable, weird (not that sort of weird) bond.

“It’s so nice to be with a person who can’t understand the horrible things I say,” remarks Moe after seeing a copy of Alice in Wonderland and presuming it’s a take on that movie he just saw, Alice in Underpants. It’s this sort of balance of dark and light that makes for such a compelling episode. Because The Simpsons’ universe essentially resets every episode, Moe is living a life not unlike the sequence in Groundhog Day where Bill Murray keeps on trying to find new ways to kill himself. Though we know Moe’s life is going to reset, it’s nice to see him happy.

What happens when the kids of Springfield Elementary are trapped in the school with Principal Skinner and Groundskeeper Willie because of a devastating blizzard? Utter chaos, that’s what. In order to keep his “Cal Ripken-like streak of school openage” intact, Skinner ignores the punishing weather and makes his students come in on the last day before Christmas break. Using his Army training, Skinner tries to keep law and order, but Bart and Nelson, among others, revolt and stuff their principal into a burlap sack.

“Skinner’s Sense of Snow” turns every kid’s nightmare — attending the only school in your district to remain open during a storm — into a riotous dream. The Simpsons has addressed almost every grown-up topic under the sun, but this one’s pure wonderful throwback.

On its surface, “The Saga of Carl” seems like just another episode where the show goes to a foreign locale and makes fun of the place. But it wouldn’t be on this list if that were simply the case. While the plot follows Homer, Moe, and Lenny as they chase down Carl after he skips off to Iceland with their lotto winnings, the episode is truly a perceptive look at male friendship.

Two-thirds of the way through the episode, Homer says, “Guys, do you think Carl was right? What if we’re not real friends? Maybe we are just lonely guys who do guy stuff.” What follows is a shockingly moving, 20-second montage (aided by a Sigur Rós score) that features Homer, Moe, Lenny, and Carl sitting and standing in a row with their faces fixed and emotionless as the setting around them changes. It perfectly captures the idea of men having “side-by-side” friendship, in which, instead of talking to each other, they simply take part in the same activities next to each other. “The Saga of Carl” again shows that The Simpsons is willing to travel a long way to tell a small, personal story.

Ned Flanders is a perpetual winner in the world of The Simpsons. Whereas other characters are defined by their flaws — Moe’s lack of a love life, Skinner’s crippling attachment to his mother, and Homer’s general stupidity — Flanders is always relatively free of problems. “When Flanders Failed” reverses the dynamics between Homer and Flanders, though, and for the first time, we see Flanders spiraling downwards instead of his erratic neighborino.

When Homer wins a wishbone and hopes for Flanders’s new Leftorium to go bankrupt (at least it’s better than death), we see the same troubles that normally befall Homer happen to Flanders. The results are decidedly sadder, making this episode more somber than most, but Homer redeems himself in a triumphant moment where he rallies up friends to help revive Flanders’s business à la It’s a Wonderful Life.

Building on the Krusty-centric episode “Like Father, Like Clown,” “Today I Am a Clown” delves even deeper into the clown’s Jewish roots, starting with Krusty strolling along Springfield’s Jewish Walk of Fame with his new puppy (acquired via Lisa, part of a litter spawned when Santa’s Little Helper impregnated Dr. Hibbert’s dog, Rosa Barks) and continuing with an alternately goofy and heartfelt journey into a sepia-toned past modeled on The Jazz Singer. Krusty’s quest begins when he learns he’s ineligible for the Walk of Fame because he never had a bar mitzvah. Bart and Lisa investigate and learn that Krusty’s dad, Rabbi Hyman Krustofsky, didn’t want to perform the ceremony because he was afraid Krusty (a.k.a. Herschel Krustofsky) wouldn’t take it seriously.

This is a sentimental episode, but also a serious one, treating the conflict between generational sensibilities at least as seriously as the two versions of The Jazz Singer that it spoofs. Some of the jokes are so knowing that they verge on Woody Allen-style cultural-insiderism; when Bart asks Lisa why she knows so much about Judaism, she replies, “I have a Jewish imaginary friend. Her name is Rachel Cohen and she just got into Brandeis.”

To prove that there’s no such thing as a soul, Bart sells his for the low, low price of $5 — enough money to buy a disappointing pack of expandable water toys. It means nothing to Bart, but weird things begin to happen: Electronic doors won’t open, and he’s even unable to find Itchy & Scratchy funny anymore. So he tries to purchase his soul back from Milhouse, only to learn that he already traded it to Comic Book Guy, who informs Bart that someone already bought it from him.

“Bart Sells His Soul” is a unique episode — Bart is humbled to learn that a soul is something you can lose, which Lisa, usually the skeptic, already knows to be true. There’s also a legitimately terrifying nightmare sequence with Bart as the only child in Springfield who doesn’t have a soul. But that mix of spiritual belief and painful loss, along with a dash of Uncle Moe’s Family Feedbag, is why Matt Groening named this one of his favorite episodes.

This episode answers the obvious question: Who needs the Kwik-E-Mart? Apu, of course. An essential character who orbits the Simpsons universe on a regular basis, Apu’s world is generally contained within the walls of the convenience store he works in. However, when Apu gets busted for selling expired meats — primarily to Homer — and is subsequently fired, he is suddenly thrust outside those confines. Naturally, this leads him into the home of the man who ratted him out.

What makes “Homer and Apu” a classic is the way in which it places a side character directly in the middle of the main quintet’s world while still using Homer as the catalyst for chaos. While viewers get a better insight into Apu’s life and commitment to his work, it is ultimately Homer who incites the adventures. From stomping on the tiny camera inside Kent Brockman’s oversize novelty hat to essentially wasting a trip to the top of a mountain in India, this episode is a highlight for both characters.

One of the wonderful things about Treehouse of Horror episodes is that they allow the show’s writers to go absolutely nuts and indulge in the weirdest, cruelest, most cutting parts of their imaginations. The first segment in this sixth entry — “Attack of the 50-Foot Eyesores” — is the latter. An atmospheric storm brings giant fast-food and other capitalist mascots to life, and they go on a destructive tear through town. At story’s end, Kent Brockman intones portentously, “Even as I speak, the scourge of advertising could be headed towards your town!” Cut to commercial.

“Nightmare on Evergreen Terrace” is the cruel one, effectively riffing on Wes Craven’s “you dream, you die” conceit. When Martin dies in class after getting frightened in a dream, the sheet is accidentally pulled off of his gurney to reveal his death rictus. Everyone screams. Then the gurney is accidentally wheeled into the kindergarten class. The final segment was memorable at the time for expanding into computer animation. Like in that one Twilight Zone episode, Homer disappears through an invisible portal in his wall and finds himself in another dimension. There, he’s … 3-D! It feels a little tame now, so advanced are the animation graphics of today, but there’s something wonderfully bizarre about the episode’s kicker, which involves erotic cakes.

If “The Cartridge Family” ended after the first act, with the Simpsons and the rest of Springfield attending a soccer match only to realize just how boring the beautiful game is, it would still be on this list. Luckily, the second half is pretty great, too. Homer buys a gun to protect his family from intruders and the King of England, much to Marge’s dismay.

“The Cartridge Family” doesn’t so much criticize owning a gun as it does condemn gun nuts; there’s a difference between the two. To quote something showrunner Mike Scully once said in an interview, “Guns in the hands of people like Homer Simpson are bad.” But an episode about it? That’s great.

One of the most remarkable things about The Simpsons is the number and quality of guest stars it’s been able to book over its quarter century on the air. This episode shows that it’s also the best TV show at figuring out how to use them. Homer’s shuffled through hundreds of jobs, but if he could settle on just one, he’d probably go with world-famous musician. He seems perfectly content living the sex, drugs, and rock ’n’ roll lifestyle, minus the sex and drugs, in “How I Spent My Strummer Vacation.” After receiving a one-week crash course in rock from Mick Jagger, Keith Richards, Tom Petty, Lenny Kravitz, Elvis Costello, and Brian Setzer, Homer scores a gig opening for the assembled musicians at a benefit show.

Or so he thinks: Homer’s actually just a roadie, but once he sees his family cheering for him in the audience, he starts acting like a headliner. It’s a twist that works so well because of how “Strummer” treats the actual rock gods. Camp counselors Mick and Keith are captured as guys who are more interested in using cheaper oatmeal and putting up storm windows than letting it bleed.

“$pringfield (Or, How I Learned to Stop Worrying and Love Legalized Gambling)” is kind of like a morality play, examining the many ways in which gambling can affect a small town. Homer becomes a blackjack dealer, Marge a gambling addict, Bart opens his own casino, Lisa suffers from neglect, and Mr. Burns suffers from complete madness.

“$pringfield” is one of the earliest episodes to involve so many Springfield residents. It’s reminiscent of The Simpsons Movie in how expansive it feels and how many characters are involved. Still, “$pringfield” is best known as the episode in which Mr. Burns goes full Howard Hughes, in what is seriously one of the show’s craziest, silliest arcs ever.

As the title suggests, this episode brings together two of Springfield’s eternal outsiders: Moe and Lisa. The two bond when Lisa arranges Moe’s grimy, suicidal ramblings, or his “brain goo,” as he calls it, into a poem. The success of that verse leads the pair to the prestigious Wordloaf Literary Conference (a play on the Bread Loaf Conference) and results in a hilarious takedown of the literary world.

Never one to shy away from a reference, only The Simpsons could pull off an episode malware hunter census - Crack Key For U this following an NFL game. Whether it is Gore Vidal revealing he got the title for Burr by seeing it on an advertisement for Eskimo Pie, Thomas Wolfe requesting everyone’s leftover garlic mashed potatoes, or the violent rivalry between Michael Chabon and Jonathan Franzen (all four authors provided their own voices), the show succeeds at somehow turning these literary heavyweights into buffoons. It’s the highbrow, medium-brow, lowbrow mixture that we look for in the best episodes.

Ned is so often the calm voice of reason in a town filled with LOUD OPINIONS, so what would it take to make him snap? Short answer: Homer. Long answer: a hurricane destroying his home, which his friends try to rebuild, only it’s about as sturdy as you’d expect a “house that love built” to be. Once the whole thing collapses, Ned cracks and checks himself into Calmwood Mental Hospital. While there, Ned meets his childhood psychiatrist, and through a flashback, we learn that Ned used to be quite the little hell-raiser, until eight months of continual spanking took care of that.

It’s the most unlikely of origin stories: How Ned Learned to Love the Doodily. “Hurricane Neddy” is a dangerously funny episode, but it’s also an intense character study. Suddenly, all the nonsensical ramblings begin to make sense. Ned’s been repressed since childhood, and all the built-up anger came out at once. The Simpsons is an animated show, but its psychological game runs deep.

It’s all in the title. A riff on the Frank Capra/Jimmy Stewart movie, this episode is an early example of how extreme and committed Lisa could get when she glommed onto something she believed in. Here, of course, the “something” is nothing less than America itself, the compromises and corruptions of which she is forced to confront after going to the nation’s capital following her victory in an essay contest. There she sees her local congressman take a bribe to raze a forest in Springfield.

“Mr. Lisa” is one in a long line of “The Simpsons go to another city/country and interact with the local sights” episodes — “Bart, get out of the Spirit of St. Louis” — and is precedent for many, many “Lisa against the world” (“Lisa vs. Malibu Stacy,” “Lisa the Vegetarian) stories to come.

In her video will, the recently deceased Aunt Gladys implores Marge’s sisters, Patty and Selma, not to die alone, so Selma begins looking for a suitable life mate. She’s unsuccessful, but is given the chance to show off her motherly skills when she takes Bart and Lisa to Duff Gardens. It’s a disaster.

He gets stuck on a roller coaster; she thinks she’s the Lizard Queen. In defeat, she realizes something: The “little version of me I could hold in my arms” doesn’t have to be a child, it can also be a lizard named Jub-Jub. Patty and Selma are everything Marge is not: cynical, mean, un-beehived. But “Selma’s Choice” does a remarkable job of making us empathize with one of the Gruesome Twosome. It’s proof that the extended Simpsons family can be as complex as the core group.

On a seemingly innocuous nature walk to some local caverns, Homer stupidly touches a giant stalactite and opens a hole in the ground through which the family falls. Stuck deep down below (especially Homer, who is literally stuck in a tunnel because of his girth), Lisa begins to tell him a story. Then, in that story, Mr. Burns starts telling a story. Then, in Burns’s story, Moe starts telling a story … and so on.

It’s all fairly ridiculous, ending in a Good, Bad and the Ugly Mexican standoff, but it’s extremely lively and includes a genuine spit-out- your-water moment involving an aggressive goat and a bracelet. It’s one of the best jokes The Simpsons has done in its last decade. This is an episode that rests on its complexity, and the structure is simultaneously silly and beautiful.

You could probably organize a wickedly entertaining marathon consisting only of episodes in which Krusty interacts with Bart and/or Lisa. This one follows up on season one’s “Krusty Gets Busted,” in which Bart saves Krusty from prison by revealing that it was Sideshow Bob who actually robbed the Kwik-E-Mart. (A key episode, it seems, as it would also kick off the Sideshow Bob story line that would result in our No. 1 pick, “Cape Feare.”) Here, after missing several dinners at the Simpsons’ house, promised to Bart after he got Krusty out of his jam, Krusty reveals that he is Jewish and that he is estranged from his rabbi father.

Jackie Mason (natch) plays Rabbi Hyman Krustofsky, and the episode goes HAM on the Jewish jokes. Sensitively, of course. We see the Lower East Side of Springfield, a young Krusty performing at a Catskills rabbi convention, and it’s all one big nod to The Jazz Singer. The Simpsons is an animated family sitcom, and the animated family is the Simpsons. Yet “Like Father Like Clown” showed early on that it could train its eye on other character histories and find humor, pathos, and universality.

There’s not a wasted line in “Homer Badman.” It’s one classic scene after another. The episode begins with a trip to a candy convention, where Homer steals a rare gummy. From there, Homer is accused of sexual harassment by the babysitter he hired to watch the kids, even though he was only trying to grab the gummy from her “sweet-sweet can.” He’s labeled as a monster by the press (and in the made-for-TV movie, Homer S.: Portrait of an Ass-Grabber).

“Homer Badman” isn’t an easy episode to pull off: For most of its running time, our ostensible hero is labeled a pervert. But even though the rest of his family has doubts, we know Homer’s innocent — how could any man who wants to live under the sea do something so terrible?

One of The Simpsons’ more assured dips into pure misery, “Kamp Krusty” finds Bart and Lisa in despair at conditions in the titular summer camp. Krusty has subcontracted everything in this lakeside hellpit to bureaucratic underling Mr. Black, who, in turn, delegates to thuggish enforcers Jimbo, Dolph, and Kearney. (Bart: “Don’t we get to roast marshmallows?” Dolph: “Shut up and eat yer pinecone!”) As the kids eat Krusty Brand imitation gruel (“nine out of ten orphans can’t tell the difference”), endure forced marches, and manufacture wallets in a sweatshop, Bart gets fed up and finally leads a rebellion.

“This punishment is not boring and pointless,” Bart writes on the chalkboard at the start of “Kamp Krusty,” foreshadowing miseries to come. Like many classic early episodes, this focuses ruthlessly on its A story and keeps a tight leash on its tone, which might’ve gone from brutally farcical to brutally unpleasant in an instant had a single gag been miscalculated. None are. Lisa: “I feel like I’m gonna die, Bart.” Bart: “We’re all gonna die, Lise.” Lisa: “I meant soon.” Bart: “So did I.”

Springfield’s mob mentality is what makes the town so united yet so, so blind to strange traditions such as Whacking Day (which was inspired by a real annual event in a town in Texas). Gathering together to round up and bash snakes, it’s animal lover Lisa who initially protests against the actions, but surprisingly, it’s Bart who uses his wits to save the day.

Bart gets expelled from school after driving a tractor into Superintendent Chalmers’s butt and is homeschooled by Marge, but becomes a bookworm after getting into a novel about a boy who goes to war. When Bart realizes that Whacking Day is actually a sham, he helps Lisa lure the town’s snake population into the their home by inviting Barry White, who is equally appalled by the holiday (“You people make me sick!”), to sing and use his baritone voice to create vibrations that attract the snakes. It sounds hilariously absurd, but that’s the kind of plot The Simpsons thrives on.

In one of the series’ most ambitious, structurally daring episodes, “Trilogy of Error” is split into Homer’s Day, Lisa’s Day, and Bart’s Day, and they all intersect. Think Go and Run Lola Run, but with poor grammar, fake addresses, thumbless buffoons, and hillbillies picking up hitchhikers.

It should fall apart right around the time Lisa, who misses her school bus because she’s preoccupied fixing her linguistics robot Linguo, gets a ride from Krusty and Teeny to the previously unmentioned West Springfield Elementary School, but it never does — the episode continually builds momentum toward a satisfying finale. “Trilogy of Error” is a one-of-a-kind marvel.

Anyone who claims The Simpsons hasn’t been good in years clearly hasn’t seen “Steal This Episode,” the 539th episode of the show, and one that feels as vital and fresh as episode No. 50. Homer grows sick of paying a ridiculous amount of money to see a movie only to be bombarded by 20 minutes of deceptive commercials and trailers before the film even begins, so he begins downloading them illegally online.

It doesn’t last long, and Homer eventually turns himself in. His trial is attended by the likes of Judd Apatow, Seth Rogen, and other Hollywood bigwigs who claim Homer’s robbing them creatively and financially. What follows is an impassioned speech from Homer about piracy and how, to quote Lisa, everyone is “trying to steal as much money as they can.” “Steal This Episode” never takes a side in the pirates-versus-producers war — it doesn’t have to. The most shocking thing about it (in a good way) is how funny the show can still be this late in the game.

The Simpsons’ writers came across an issue of Time that theorized what would happen if a comet were to hit Earth. Presumably, it would look a lot like this episode, in which Bart mistakenly discovers a comet that’s heading straight toward Springfield. All 22 minutes are very good, but the episode becomes “all-time great” near the end, when the Simpsons and at least two dozen other recurring characters squeeze into Ned’s bomb shelter. It’s like the cabin scene from A Night at the Opera, but with everyone involved trying to prove they’re worth saving because the door won’t close and someone needs to be booted.

The world needs laughter, so Krusty makes the cut, as do religious gossips the Lovejoys. It’s decided that Flanders is the “only useless person here,” and he’s kicked out. With Ned mournfully singing “Que Será, Será (Whatever Will Be, Will Be)” in the background, things devolve into chaos, and Homer follows his neighbor out of the shelter. The rest soon join. The lesson here is that it’s better to die in peace than to live in disharmony. Springfield is a true community — part of the show’s joy is the variety and familiarity of its characters. “Bart’s Comet” is a sterling example of the humor that arises when you bounce them all off each other.

When you’ve been on the air for as long as The Simpsons has been and covered as much territory as The Simpsons has and influenced as much as The Simpsons influenced, it’s hard to find something that feels unique. So, in the later seasons, sure they aren’t pumping out classic after classic, but the special ones are as special as the show ever has been. “Halloween of Horror” is one of those special episodes.

That’s right: “Halloween of Horror,” not “Treehouse.” After 27 seasons, the show decided to do an actual Halloween episode. And because of the “Treehouse of Horror” tradition, there was just so much material for the writers to pull from in their first foray in joking about the actual holiday. When we talk about The Simpsons at its best, we talk about maximizing the space of television, including as many jokes, ideas, and parodies as the space can hold, and this episode achieves that sort of density, covering eight different types of Halloween. All while this is happening, “Halloween of Horror” tells two really lovely stories about the relationship between kids and Halloween and how parents try to protect their children from the realities of adulthood. The writing and directing effectively show the world both from Bart and Lisa’s eyes, as well as Homer and Marge’s, Smart Defrag 6.1 Full Free - Crack Key For U the same episode. No show has ever done big and small storytelling concurrently as well as The Simpsons and this is a masterful example.

“Eternal Moonshine of the Simpsons Mind” won the 2008 Emmy for Outstanding Animated Program (for Programming Less Than One Hour), but it’s Outstanding Comedy Series-worthy. It’s that good. Homer is unable to recall the events of the Driver toolkit 8.5 Crack + License Key Full Free Download day due to taking the powerful “Forget-Me-Shot” at Moe’s, so in Eternal Sunshine of the Spotless Mind and Memento fashion (with a hint of “El Viaje Misterioso de Nuestro Jomer,” too), he has to recall a painful memory he literally drank away. Got that?

It makes perfect sense in the actual episode, which includes a visually rich parody of those “Take a Photo of Yourself Once a Day for Five Years” videos. Although the multi-layered, mind-bending “Eternal Moonshine” is all about forgetting, it’s an episode whose delightful structure makes it a cinch to remember.

Tonally, “The Last Temptation of Homer” is one the show’s greatest achievements. The show had dealt with infidelity in the past — namely, “Life on the Fast Lane” and “Colonel Homer” — but “The Last Temptation of Homer” surpasses them both. In the episode, Homer is introduced to Mindy, a new female co-worker who happens to be his perfect match.

The show does a particularly special job with Mindy’s character (perfectly voiced by Michelle Pfeiffer). Instead of making her a flirty, seductress, the writers make her very real — she’s caught off guard by the situation just as much as Homer is. Rather than being lustful or playful, there’s a nervousness and a discomfort about her. Beyond Mindy, the episode succeeds in not selling Homer out, by letting him honestly feel the temptation. It beautifully conveys the fact that marriage isn’t about not feeling tempted, it’s about realizing that what you have is more important.

For the generation of viewers who first caught The Simpsons when they were youngest, a million cultural referents were planted in their brains, just waiting to burst forth. What 10-year-old saw this episode and knew that the rope bridge moment was referring to William Friedkin’s Sorcerer, or that the flashback to pre-alcohol Barney grabs its slow-dissolve montage from Dr. Jekyll Virtual DJ Studio Crack Mr. Hyde, or that Bart’s felling by snowball was a nod to the end of Bonnie and Clyde? Of course, though, the reason everyone remembers this episode is because of the song.

When Homer buys a giant plow following the Springfield Auto Show, he finds himself making money hand over plow by clearing snow from driveways and streets. Then Barney buys a bigger truck, and the two become rivals. The situation’s pretty straightforward and a few jokes are self-referential (after paying for a super-late-night commercial to advertise his services, Homer says, “It may be on a lousy channel, but The Simpsons are on TV!). But, well, “I’m Mr. Plow, that’s my name. That name again is Mr. Plow.” It’s simple, vaguely palindromic, and catchy enough for children to carry with them all their lives.

The year is 2010. To viewers in 1995, when the flash-forward episode “Lisa’s Wedding” first aired, that must have seemed like the impossibly distant future, a future in which The Simpsons probably wasn’t even on TV anymore. (Ha!) Not much has changed, though: Homer and Marge are still together; Maggie still isn’t talking; Bart’s still wrecking things, except now it’s his job; and Lisa’s still the smart one. She’s a college student at Eastern University, which is where she meets Hugh (Mandy Patinkin). He’s smart, handsome, and understands the magic of Jim Carrey, so when he asks Lisa to marry him, she immediately replies yes.

Lisa and Hugh travel to Springfield, where the wedding’s to be held, as happy as can be, but the similar-seeming couple quickly learns just how different they really are. “Lisa’s Wedding” is packed with clever allusions to the, um, future, but it’s more concerned with how much one little girl loves her father, no matter how “trying” he is.

When a Lego episode was first announced, it was hard not to be skeptical, given how much it felt like a shameless promotional tie-in. Fortunately, those suspicions couldn’t have been further from the truth. “Brick Like Me” is heartfelt and ambitious (both in terms of production and storytelling). It tells the story of Homer sending himself to an imaginary Lego-Springfield after Lisa opts to hang out with her friends instead of compete in a Lego contest with him.

Homer and Lisa stories routinely end up being the most touching — just look at this list, which is packed with them — and in this case, “Brick Like Me” does a wonderful job of grounding this highest of concepts. Behind all the Lego bells and Lego whistles is an episode of television about the fundamental experience of being a parent and watching your child grow up.

It’s easy to forget how big of a deal the “Who Shot Mr. Burns?” series was when it aired. Meant to be a parody of Dallas’s “Who shot J.R.?” story arc, “Who Shot Mr. Burns?” was arguably just as momentous. The first half, the finale of season six, effectively set up the fact that literally every resident of Springfield would have a motive to shoot Mr. Burns, who crossed the line between everyday villainy to cartoonish supervillainy by stealing oil from the elementary school and then blocking out the sun.

In the season-seven premiere, Lisa worked with Chief Wiggum to find the culprit, parodying Twin Peaks along the way. The show wanted to create an event around its season-six cliff-hanger, and it did just that. Fans spent the summer debating this question, trying to win the contest for guessing the correct person and logging on to the newly created Springfield.com (this episode is considered one of the first successful internet tie-ins by a TV show). It was a surprisingly good mystery with a perfectly Simpsonian ending.

Origin stories are not new. But here, at least, was a meaningful one. If early Simpsons episode were tales about a family (whereas later ones were about a family and a town), “The Way We Was” was the first step in the long road to Bart, Lisa, and Maggie.

It’s high school, and Marge gets sent to detention for the first time ever. Homer, a regular there, sees her walk in and falls in love at first sight. One French tutoring session, some deception, and a disastrous prom night later (on account of the memorable know-it-all Artie Ziff, voiced by Jon Lovitz), Homer ends XnView Licenses key uttering one of the most romantic things he’s said on the show to date — “I got a problem. Once you stop this car, I’m gonna hug you and kiss you. And then I’ll never be able to let you go.”

Mr. Burns isn’t a complete monster. He can occasionally be a decent guy, so long as he has his Bobo. In “Rosebud,” we see Burns at his best and worst. The worst: Burns learns that Maggie is in possession of his cherished childhood teddy bear (à la the main character’s sled in Citizen Kane) and is willing to give the Simpsons “a million dollars and three Hawaiian islands. The good ones, not the leper ones” for him. As much as it pains him to reject the offer, Homer has to, because he can see how much his daughter loves the stuffed animal.

After a series of unsuccessful robberies, Burns reroutes all the beer trucks heading to Springfield, which will remain “dry as a bone” until Homer gives up the bear. The best: a pitiful Burns levels with Maggie and lets her keep Bobo, although not before giving her some advice: “Don’t make the same mistake I made.” It’s a rare moment of good will for an otherwise bad man. The Simpsons is a generous show — allowing viewers the opportunity to empathize with so many characters. Here, Burns gets his day.

When a heat wave descends upon Springfield, the Simpsons buy a pool, immediately making Bart and Lisa the most popular kids in town. Then Bart gets injured (Nelson: “Ha-ha.” Milhouse: “Hey, Nelson, he’s really hurt. I think he broke his leg.” Nelson: “I said ha-ha.”), necessitating a cast that will keep him out of the pool for the rest of the summer. He retreats to his room and Lisa gives him her telescope with which to pass the time. He observes a neighbor commit a terrible act.

An homage to Rear Window — there are two moments when Bart literally sees a similarly injured Jimmy Stewart staring back at him through his telescope — “Bart of Darkness”’s two plots work hand in hand. The pool story shows Lisa (and poor pantsless Martin) how fickle popularity can be, while the Bart plot delivers its own unrelated humor, including a incredible sight gag (Bart’s leg in a trashcan, in a patch of grass, etc.). There’s also a moment when Bart calls a Moviefone-like phone service that demonstrates how often Simpsons gags work on two wavelengths at one; no one knows what Moviefone is anymore. It hasn’t been a thing for a while. But layer the word regicide on top of that reference, and it remains funny for the rest of days.

At the annual “Bi-Mon-Sci-Fi-Con,” Homer saves Mark Hamill’s life by carrying him away from a pack of ravenous nerds. Mayor Quimby is impressed by the act of heroism, and he hires Homer as his personal bodyguard. It’s a pairing the show doesn’t do much, but it works very well here: They’re both slaves to their desires, be they women (Quimby) or free stuff (Homer). Their relationship nearly falls apart, though, when Homer learns that Quimby’s working with Fat Tony to sell rat “malk” to the children of Springfield Elementary. He agrees to continue working with the mayor, but only if he’ll arrest the mobster.

Quimby does, but Fat Tony is quickly freed, and he tries to kill the man who sent him away … at a dinner-theater production of Guys and Dolls starring Mark Hamill. It’s an ingenious way of connecting the first act to the third, making “Mayored to the Mob” feel more conceptually complete than other “Homer gets a new job” episodes.

Itchy & Scratchy Land: where nothing can possi-blye go wrong. Except for that. And the killer robots. Marge wants to go on a nice, peaceful vacation with the family, but Bart and Lisa have other bird-sanctuary-less ideas. Eventually, they badger their parents into taking them to the most violent place on Earth, Itchy & Scratchy Land.

Things start off well enough, but the Simpsons gene eventually kicks in, and Bart and Homer are arrested for various indiscretions, and a gang of Itchy and Scratchy robots are ready to kill. The episode is absolutely packed with film references — Jurassic Park, Westworld, The Birds, Walt Disney — but it also has one of the series best ever punch lines: “No, my son is also named Bort!”

Our list of 100 probably gives Marge the shortest shrift, but in this episode, we encounter her neuroses head-on. After finding a Chanel suit that’s marked down from $2,800 to $90, Marge begins to wear it constantly, including a trip to the Kwik-E-Mart, where she runs into a friend from high school, Evelyn, who’s as rich as she is snotty. Evelyn’s also a fan of Marge’s keen fashion sense, and she invites the entire Simpsons family to the Springfield Country Club.

They’re obvious outsiders, but Marge wants to be on the inside, so she keeps tailoring her suit, afraid that she won’t be accepted if she wears her usual attire.You can’t help but sympathize with Marge’s desperation, even if the country club is a “hotbed of exclusionist snobs and status-seeking social climbers.” Ultimately, Marge unknowingly gets what she wants — Evelyn sponsors the Simpsons’ membership — but at a cost. She snaps at Lisa, and Homer sadly suggests, “You kids should thank your mother. Now that she’s a better person, we can see how awful we really are.” It’s one of the more cutting lines from the show’s first decade.

Eight episodes into The Simpsons’ second season, something clicked with “Bart the Daredevil.” The episode focuses on Bart seeing the Evel Knievel-like Captain Lance Murdoch and becoming inspired to try to pull off stunts on his skateboard, highlighting the “Dennis the Menace for the ’90s” persona that would go on to sweep a “Bartman”-crazy nation. It’s also about so much more, namely Homer and Bart’s relationship, and the bizarre lengths to which Homer will go to in order to teach a fatherly lesson.

An episode that Matt Groening once called his favorite, “Bart the Daredevil” is maybe best known for the joke that changed everything. (If you haven’t yet seen this episode, find it, watch it, and come back here.) After Homer stops Bart from trying to jump over Springfield Gorge on his skateboard, he accidentally finds himself zooming down the ramp instead. This doesn’t end well. Homer falls for a comically long, violent time. Finally, he is airlifted and safely placed inside an ambulance, only for the ambulance to instantly crash into a tree and for his gurney to roll out the back. Homer tumbles down the Gorge the same way, with the same painful sounds. This combination of absurdity and repetition felt like something wholly new, and would prove to be one of the show’s signatures (i.e., “Cape Feare”’s Sideshow Bob vs. Rakes).

We’re in the middle of fall premiere season, when networks roll out a dozen or so new shows. And the first time we realized that most of a TV series’ early episodes were pointless was when Lisa told her father, “The first episode was just a pilot, Dad. Producers fiddle with shows all the time. They change characters, drop others, and push some into the background.”

It’s midseason on TV, and there’s only one show that catches Homer’s eye: Police Cops, starring none other than … Homer Simpson. Except unlike our Homer, this other Homer is cool, calm, and not just a cop, but a POLICE cop. Homer struts around Springfield for the next week, taking advantage of his unearned fame, but in the next episode, Cool Homer is turned into Bumbling Sidekick Homer. His catchphrase: “Uh-oh, SpaghettiOs.” Where once Homer adored his friends mistaking fiction for fact, he now rejects it completely by changing his name to Max Power. “Homer to the Max” is undoubtedly assisted by its meta humor (Homer has gone through various levels of intelligence since season one), but it also works as yet another funny episode about how ridiculous TV can be.

“Homer at the Bat” is a transition episode. Coming towards the end of season three (though it was written much earlier than that — it just took an especially long time to produce), it is arguably the most absurdist episode of the show’s first era (seasons 1–4, when all the original writers were still onboard). Though it starts with the simple concept of Homer playing on the power plant’s softball team, it descends (or ascends) into madness when Mr. Burns brings in nine professional baseball players to act as ringers.

Terry Cashman’s song “Talkin’ Softball,” which plays over the end credits (a reworked version of his 1981 song “Talkin’ Baseball”) is one of the show’s most iconic, if only because it famously lists all the tragedies that befall the ringers. Increasingly ridiculous — from Roger Clemens thinking he’s a chicken to Ken Griffey Jr.’s gigantism to Ozzie Smith falling into space for all eternity — the misfortunes offered a specific weirdness that would come to define the show after season five.

“El Viaje Mistierioso” is the one in which homer eats a super chili “grown deep in the jungle by the inmates of a Guatemalan insane asylum.” But replace the chili with a bowl of peyote, or take a bowl of peyote yourself, and you’d get to the same place — a clever and gorgeously drawn episode in which Homer hallucinates a talking coyote (voiced by Johnny Cash) and a Marge with no face.

It’s psychedelic, but underneath it all is Homer’s fear that Marge might not be his soul mate following a misunderstanding. This episode is a Trojan horse — hiding an emotional story inside a visually sumptuous package. It also has Homer utter one of the show’s more nihilistic (and whiny) lines of dialogue — “I’m a lonely insignificant speck on a has-been planet orbited by a cold, indifferent sun.”

It’s not that the residents of Springfield are particularly mean or hateful; they’re just easily agitated and quick to riot. Everyone in town is terrified of bears roaming the streets, but their glee once again turns into hostility when their taxes are raised to pay for the Bear Patrol that’s keeping them safe. To protect himself, Mayor Quimby passes the blame along to someone else: illegal immigrants. Leading the charge is Homer, until he learns that Apu isn’t from this country and he’s not supposed to be here. “Much Apu About Nothing” shows what happens when you put a face to your ignorance.

Homer is ready to ship all the foreigners out of the country because he has to pay an extra $5 per month, but once he realizes this would include a friend, he begins to fight Proposition 24. Meanwhile, Apu does whatever he can to stay, including receiving a horrible education from Homer, who takes living in America for granted, unlike Apu, who loves “this country more than I love a cold beer on a hot Christmas morning.”

In 2000, Patrick Stewart told the BBC, “I think my appearance in The Simpsons and an appearance that I did on Sesame Street … were perhaps the two most distinguished bits of work that I’ve done in the U.S.” Do we agree? We do.

Stewart lends his unmistakable gravitas to Number One, the leader of the Freemason-like secret society the Stonecutters, until it’s revealed that Homer is the Chosen One. From then on, everything goes great for Homer until he realizes something: He wants to lose. Or, in the words of Lisa, “You’re experiencing spiritual emptiness because your power has isolated you from other human beings.” Homer tries to do some good in the world by opening a Stonecutters Daycare Center, among other life-affirming activities, but his brothers quickly grow tired of their leader’s newfound virtue, and everyone leaves to form the No Homers’ Club. “Homer the Great” is all about the man in the episode title and how he shows remarkable unselfishness by asking more of himself and others. By episode’s end, he’s back with the Simpsons, the best club in town.

Periodically, especially in later seasons, The Simpsons takes on an issue of the day and nails it. “Homer’s Phobia” and “Much Apu Nothing” are maybe the most classic examples. In “Homer’s Phobia,” the Simpsons befriend John (impeccably voiced by John Waters), the owner of a kitschy collectables store. Homer, like the rest of the family, loves John, until he learns he’s gay. He then spends the rest of the episode trying to prevent Bart from becoming gay in the most hilariously inept ways. (Most memorably, he takes Bart to a very gay steel mill.)

The reason the episode works so well is because Homer makes the perfect patsy, of sorts. Homer isn’t particularly political; he’s just reactionary and, well, stupid. He takes incorrect stances to such extremes that they reveal the absurdity of the position even in the most minor sense. And then, because he takes it to such an extreme, when he realizes the error of his ways, it is that much more compelling. It’s easy to watch “Homer’s Phobia” now and think the stance the show takes is obvious, but it’s important to remember the time and the audience, many of whom were likely getting their first exposure to gay rights through the show.

“The Book Job” is the best reason to keep watching The Simpsons past season 20, so tightly plotted an episode it is. Lisa is shocked to discover that her favorite children’s-book author (shades of J.K. Rowling) is a fake — the woman on the cover is an actress, and a team of marketing hacks put the book together. Her disgust is Homer and Bart’s get-rich-quick scheme, and before long, they’ve drafted Skinner, Patty, Moe, Frink, and even special guest-star Neil Gaiman to write the Perfect YA Novel.

They succeed with The Troll Twins of Underbridge Academy, but the publisher won’t agree to put it out until they find an “author” with an inspirational story. It’s a deeply satirical episode, spoofing not only the Twilights of the world, but also Ocean’s Eleven (Andy García plays the hack publisher). “The Book Job” shows that even after two-plus decades, The Simpsons still has bite to it.

Even in 1993, it was tough to find a new angle on the Beatles. So, even though “Homer’s Barbershop Quartet,” which substitutes John, Paul, George, and Ringo for the Be Sharps (consisting of Homer, Skinner, Wiggum, I mean, Barney, and Apu de Beaumarchais), leans heavily on the Beatles’ origin story and malware hunter census - Crack Key For U rise and fall, there’s one major difference: It went bigger.

Instead of John Lennon saying the Beatles were “more popular than Jesus,” the Be Sharps named their second album, Bigger Than Jesus. Instead of Homer putting down his dessert so he could be appropriately starstruck by George Harrison, he eats an endless stream of brownies. Instead of Ringo, something something Burt Ward? “Homer’s Barbershop Quartet” could have been a lazy parody of the world’s most famous band, but the writers were self-aware enough to sprinkle some welcome weirdness around the references.

At its best, The Simpsons balances its heartfelt sentiments with uproarious disarray. Case in point: “Lisa on Ice,” an episode that starts with hyperbolic explosions on Kent Brockman’s “Action News” and ends with one of the show’s sweetest moments between Bart and Lisa. It’s a classic tale of sibling rivalry that’s prompted by the threat of Lisa failing gym. (Though let’s not forget Ralph’s “Me, fail English? That’s unpossible!”)

When Apu realizes Lisa’s potential to be a star goalie, she joins his hockey team and begins to outshine Bart. Homer’s hilariously bad parenting agitates the situation further, especially when he’s flickering the lights and yelling “FIGHT! FIGHT! FIGHT!” upon finding out Bart and Lisa will square off in a match. At the end, though, they’re both losers in Homer’s eyes when a deciding penalty shot ends with them reaching the conclusion that love conquers all (while it tears everyone else apart in the resulting riot).

Because it breaks from the preexisting timeline, few episodes of The Simpsons are as controversial among fans as “That ’90s Episode.” But as this list shows, very often the controversial episodes are great, as they are bold enough to mess with our expectations of the series. Formatted like a classic sitcom flashback episode, “That ’90s Episode” succeeds as a perfect send-up of the ’90s and deepens our understanding of Marge and Homer’s relationship, telling the story of how Marge went to college and fell for one of her professors.

The episode is colored by the brilliant irony of the one fixture of the ’90s that isn’t featured: The Simpsons. “That ’90s Episode” portrays what pop culture would’ve looked like without the show, with both cynicism and pretentiousness running rampant. At the time, The Simpsons was an anecdote for both. Some fans criticize “That ’90s Episode” for ignoring the show’s past, but I’d argue there are few episodes of the show more deferential to it.

“Lisa’s First Word” isn’t so much a full-length episode as it is a giant collection of quotes. Nearly every line from the season-four classic, which largely takes place after Lisa is born, is endlessly repeatable, from “can’t sleep, clown’ll eat me” to “everybody mirillis action 3.9.1 crack - Free Activators me, I am so great” to “a baby and a free burger, could this be the best day of my life?” to “iron helps us play.” (Hello, Joe!)

But there is also a plot, one involving the family trying to convince Maggie to say her first word, and it’s more sweet and adorable and loving than any animated series’ flashback episode ought to be. Twenty-two years later, Maggie’s still only uttered a single word — daddy (voiced by Elizabeth Taylor, no less) — but that’s all that needs to be said.

Homer finally gets everything he’s ever wanted, only to realize he can’t find the ANY key. That, in a nutshell, is the plot of “King-Size Homer,” in which the titular giant gains weight to be able to work at home (basically, he turns into a proto-blogger). It’s a rare instance of a TV-show character being proud of the extra pounds he’s put on.

The neat trick this episode pulls is that despite being about a man who inspires his son to wash himself with a rag on a stick, it’s incredibly lively. Homer’s unable to sit still while working in his living room, and during the conclusion, his daring climb up the power-plant tower to prevent a “potential Chernobyl” is paired with Mr. Burns’s surprisingly lively “push out the jive, bring in the love” exercise class.

Throughout Springfield and all over its airwaves, people are talking about the mysterious Gabbo, who’s eventually revealed to be a kid’s-show host: a ventriloquist’s dummy whose aggressively outré antics and addictive catchphrase “I’m a bad widdle boy!” dampen Krusty’s ratings and move the network to cancel him. Bart and Lisa and Krusty’s half-brother Luke Perry (playing himself) come to Krusty’s aide as he spirals into depression; the kids finally defeat Gabbo by letting kids see him trash-talking them on a live feed that was supposed to go dark for a commercial (that same trick that undoes the reactionary talk show host in 1957’s A Face in the Crowd).

Krusty’s never funnier than when he’s wallowing in melodramatic self-pity, and “Krusty Gets Kancelled” helpfully provides him with an Olympic-size pool in which to marinate. He gets a job at a fast-food restaurant, eats his way to obesity, and bemoans his fate every chance he gets. (Krusty: “Ugh, 35 years in show business and already no one remembers me, just like what’s-his-name and whose-it, and you know, that guy, always wore a shirt?” Bart: “Ed Sullivan?”) And like many Simpsons episodes, this one has great fun observing the populace falling for a grandiose hard-sell, this one tinged with subliminal mind-control techniques. (“Gabbo! Gabbo! Gabbo!” the mysterious strobe-flash ads proclaim.)

The Simpsons’ humor operates on several comedic levels at once — situational, character-based, visual, audio, and just the individual jokes themselves — and when all of those are operating in concert, at maximum efficacy, it’s a sight to behold. This episode, in which Prohibition is reinstated in Springfield and Homer becomes a bootlegger known as the Beer Baron, pursued by an Elliot Ness-type crime fighter named Rex Banner, is one such episode. It’s known primarily for its closing line, which is truly one for the ages (“To alcohol: the cause of, and solution to, all of life’s problems.”), but “Homer vs. the 18th Amendment” is full of bons mots.

Voice acting on The Simpsons is often taken for granted, but Dave Thomas’s performance as the humorless Banner is such a perfect homage to The Untouchables and Dragnet that’s it’s hard to imagine the episode succeeding without it. “Now hold on a minute, missy,” he tells Marge. “It’s not up to us to choose which laws we want to obey. If it were, I’d kill everyone who looked at me cockeyed!”

Homer’s physical resemblance to Krusty (bald head, dumpling body) gets turned to the Simpsons’ advantage when he attends Krusty’s clown college and gets work as a Krusty impersonator (or “Krustacean”). “If there had to be a bastardized version of Krusty, I’m glad it’s you,” Lisa tells her dad. Unfortunately, Homer’s so convincing that he gets kidnapped by the Springfield Mafia, who demand repayment of the debt that Krusty opened his clown college to cover. (The first few minutes show Krusty lighting a cigarette with a copy of Action Comics #1.)

This is one of the series’ flat-out funniest episodes, and as delightful as the broad bits are (the finale prompts the Don to tell Homer, “You’ve brought great-a joy to this old Italian stereotype!”), the subtle moments of body language and expression are even funnier. (Krusty’s shifty-eyed expression when Homer exclaims, “You came here to save me!” will never not be funny.) Homer is battered and humiliated during his menial quest to “do kids’ parties, swap meets, and all the other piddling crap I wouldn’t touch with a ten-foot clown pole,” as Krusty puts it; but even at his most desperate, the fake Krusty never becomes as callous as the original — or as desperate as Dick Cavett (a good sport, playing himself), who name-drops as if he’s being paid by the anecdote. Krusty’s debt, by the way, is $48. He pays with a $50; the Don makes change.

A Flowers for Algernon homage that is arguably better than its source material. At the least, it is funnier and more touching. Here, instead of an experimental procedure that results in a higher IQ, Homer becomes smart when a long-embedded crayon is removed from his brain. It becomes increasingly apparent to Homer how frustrating and isolating the world is for intelligent people.

It’s a theme that runs through a series that is written by a bunch of Harvard nerds, but is especially well-articulated here. (The episode was written by Al Jean, one of the original Harvard nerds and showrunner for the past 13 seasons.) The episode’s ending is one of the series’ most poignant moments ever. When he got smarter, Homer and Lisa grew instantly closer, and when he decides to reverse the procedure and become dumb again, Homer first writes his daughter a note that is truly tear-jerking.

Lisa and Abe are widely cast as wet blankets on The Simpsons. But this episode is a prime example as to why these two are just as entertaining as Homer or Bart. By focusing their defining characteristics — Lisa’s stern activism and Abe’s blundering old age — this episode builds story lines that force them to tackle these traits head-on.

Lisa’s battle with the sexist Malibu Stacy doll company may be permanently stalled when her rival doll is dropped in favor of a last-minute hat upgrade, but it still results in Lisa’s strongest feminist statement on the show yet. While Abe tries to be hip and young again through a small-time fast-food gig, he soon realizes that he doesn’t want to be on that side of the counter anymore. Here, Abe’s disorienting rambles drive people away, but it’s an attribute that earns him the spotlight in many other memorable episodes. Lisa and Abe will always be grouches, but there is value in being the antithesis.

The road-trip episode is a time-honored sitcom tradition, right up there with having two dates at the same time with two different women. “Bart on the Road” tweaks the trope, though, by ditching the coming-of-age nonsense and replacing it with something completely absurd — a story about Nelson threatening to beat up Bart, Milhouse, and Martin if they don’t go to an Andy Williams concert (“Bam, second encore!”). It’s also about how being an adult isn’t all that it’s cracked up to be. Sure, you can go see Naked Lunch whenever you want, but you’re probably going to think there are at least two things wrong with that title.

Even Homer needs the imagination of Lisa, who’s sent to the power planet for Go to Work With Your Parents Day, to reveal the magic in the mundane — it’s amazing how easily radiation suits can make you feel like you’re an astronaut. Cherish your youth, “Bart on the Road” brilliantly suggests, because before too long, you’re working as a courier, hauling a cooler labeled “HUMAN EYES” through an airport in Hong Kong.

You can literally pinpoint the second when audiences fall in love with “I Love Lisa”: It’s that fateful scene in which Ralph clutches his chest after Lisa, who is fed up with his courting, explodes and tells the poor guy, “Now, listen to me, I don’t like you! I’ve never liked you!” What started off as a well-intentioned deed spirals into an episode that chronicles all the stages of very young love, from crushes to heartbreak and the eventual heartwarming return to friendship.

Not only is that scene one of The Simpsons’ most notable moments, but the rest of “I Love Lisa”’s charming story line has also found its way into the hearts of fans. The iconic “I Choo-Choo-Choose You” is one of the series’ most oft-used quotes.

“You? Went into outer space? You?” “Sure. You’ve never been?” There are a lot of words one could use to describe Homer: fat, drunk, bald, fat (again). But high up on the list is lucky. Homer’s a lucky guy. He’s got a loving wife, two wonderful kids, and Bart; and no matter how many times he quits or just forgets to come in, he’s always got his job at the nuclear power-plant waiting for him.

He’s the opposite of self-made man Frank Grimes, whom we’re introduced to as the newest employee of the plant in “Homer’s Enemy.” It’s also Grimes’s final episode. He can live in a single room above a bowling alley and below another bowling alley without going insane, but the minute he meets Homer, he crumbles like an abandoned factory, partially out of jealousy, but mostly because he can’t understand why one man can have such a charmed life without even trying. That’s what makes this a somewhat controversial episode — it has a different tone than any one before it, and none have come close to touching its super-dark humor since. You should hate Homer for basically making Grimey commit suicide, yet there’s something so innocent, so sweet, so dumb about him, you just can’t.

Homer has yet another job, this time, as an adult-education teacher of a class that shares its name with the episode title. The problem is, those “secrets” end up being Homer and Marge’s actual marital problems and sexual particularities. Things escalate, Homer gets kicked out, he makes a new wife out of a plant, and then tries very desperately to win his non-green wife back.

The episode is a testament to the show’s ability to pack a ton into 22 minutes — not just classic bits, which the episode has plenty of, like Homer’s Jack Nicholson quote onslaught, but the story itself. It goes from being about Homer’s slowness to a tour of the town’s adult-education classes (including Moe teaching “Funk Dancing for Self-Defense”), to Homer becoming a teacher, to Marge kicking Homer out, leaving him insane, homeless, and “as dirty as a Frenchman.” Any of these stories would’ve been enough for a normal show, but The Simpsons weaves it all together seamlessly.

It all started here, with Bart needing to get a tattoo removed and draining the family’s savings, and Homer not getting his Christmas bonus and having to save the holiday. Besides being the first episode that aired, it is also the first of a very popular type of Simpsons episode: one in which Homer gets a different job. In this case, he becomes a mall Santa.

Though this episode aired first, it wasn’t the first Simpsons episode, as it aired before the first season as a Christmas special, and it wasn’t the first episode produced. It was actually the eighth. That isn’t a knock on the episode. Anything but. “Simpsons Roasting on an Open Fire” is a much, much better episode of television than the next seven that aired, which were still grappling with the show’s tone and pacing. Also, the characters didn’t necessarily feel settled yet. With the show’s signature mix of high concept and heart, “Simpsons Roasting on an Open Fire” surprisingly feels fully formed. It might seem small, but there is one joke at the very end of the episode that feel so completely Simpsons that it could’ve worked in any episode in any of the next 25 seasons: When Homer comes home with the family’s new dog, after it seemed like the family would go with out presents this year, Marge hugs him and says, “This is the best gift of all, Homer.” Adding, “Yes, something to share our love and frighten prowlers.”

When Homer learns that his car has been illegally parked in New York City, the Simpsons decide to take a trip there as a family. There’s only one problem: Homer hates New York City. While Homer’s worst expectations are confirmed as he tries to remove the boot from his car, the rest of the family seems to have a pleasant-enough trip. It was always a well-liked episode. Then 9/11 happened, and it was taken out of syndication.

It seemed reasonable at the time, as a large portion of the episode takes place at the World Trade Center. However, what made the decision controversial among fans, even those from New York — especially those from New York — is that in many ways, the episode is a celebration of the city, capturing through the show’s extremes both what’s terrific and terrifically terrible about New York. Few scenes throughout the history of television and movies capture NYC better than the one in which Homer tries to remove the boot on his car by chewing off one of the bolts. A man in one of the towers shouts, “Hey, when you’re done with that, I got something up here you can bite on.” Then a man in the other tower shouts at the first guy, “Hey, why don’t you be polite, you stinking puss-bag?” and proceeds to give Homer helpful information, all while maintaining that combative New York accent.

The long-awaited live-action-movie version of Radioactive Man is scheduled to shoot in Springfield, starring Rainier Wolfcastle in the title role. Bart auditions for the role of the hero’s sidekick, Fallout Boy, but loses it to Milhouse, who’s an inch taller. Milhouse is unhappy in the role because he only accepted it to appease his money-grubbing parents.

One of the most quotable Simpsons ever — a bold statement, admittedly — “Radioactive Man” stirs together an astounding number of The Simpsons’ ongoing obsessions, including jealousy between friends, comic-book mania, shifting pop-culture styles (apparently, the old Radioactive Man TV series is modeled on the 1960s Adam West Batman), and the way Hollywood warps innocence (Moe recalls his agonizing stint as a Little Rascal named “Smelly”). It’s also a star-making episode for regular cameo player Rainier Wolfcastle, who pronounces the hero’s catchphrase “Up and atom!” as “Up and at dem,” and responds to an onrushing tidal wave of acid with a monotone, “My eyes … de goggles, dey do nothing!”

Longtime Simpsons showrunner Al Jean has said that “Holidays of Future Passed” was originally written to be a finale, just in case the cast members’ contract negotiations didn’t pan out. And you can sense it. By setting it in the future, it allows for the episode to have a backwards-looking, deeply nostalgic tone, focusing on Bart and Lisa’s difficulty with parenthood as a way for them to look at their relationships with their own parents. Also, by setting it in the future, it allows for a lot of incredibly absurdist bits, like Bart’s treehouse tree grumpily coming to life and revealing his resentment.

Speaking of the treehouse, it’s there that a now grown-up (and drunk) Bart and Lisa have a very real, adult conversation about their respect for each other and the specific bond they share by growing up in the same house. It’s one of the (many — we’re talking about 25 seasons here, people) most moving scenes of the entire series (partly thanks to Bart’s court-mandated sincerity chip). “Holidays of Future Passed” just feels like the show operating at its very best. The heart, the satire, the absurdity, the one-liners: It all feels special in the way that the best episodes of The Simpsons feel like something beyond just a great episode of television. It would’ve been a perfect finale.

An extended parody of James Bond in general and You Only Live Twice in particular, this is an almost entirely absurd episode that gins up world-changing events so that they can be forgotten the following week. Homer gets hired at the Globex Corporation, a mysterious company headed by a secret villain named Hank Scorpio; it’s a plum job that lets Homer work for a man who seems to genuinely like him, but Marge and the kids are unhappy with their relocation to Cypress Creek and want to return to Springfield. Homer’s attempt to discuss this conflict with his boss is interrupted by government troops attacking Scorpio’s headquarters. Things only get sillier from there.

A one-thing-after-another gagfest in mid-period Mel Brooks style, “You Only Move Twice” is one-stop shopping for Bond riffs. (The agent himself even makes an appearance and gets strapped to a table and menaced with a laser, à la Goldfinger, only he’s renamed “James Bont.” Occasional Simpsons guest-star Albert Brooks voices Scorpio, who, of course, sounds like an Albert Brooks character, hyperverbal and always hard-selling himself and his vision. Marge: “Mr. Scorpio, this house is almost too good for us. I keep expecting to get the bum’s rush.” Scorpio: “We don’t have bums in our town, Marge, and if we did, they wouldn’t rush. They’d be allowed to go at their own pace.”) The omnipotent malevolence of Bond villains is played for laughs here: At one point, Scorpio asks Homer if he likes France or Italy better, then launches a missile at Homer’s second choice, France. “Nobody ever says Italy,” Scorpio says.

Equally inspired by 32 Short Films About Glenn Gould, HMA Pro VPN 5.1Crack _ Unblock Websites With HMA Free Fiction (which it extensively references), and meandering, “whatever happens, happens” movies by the likes of Robert Altman and Richard Linklater, this is one of The Simpsons’ most structurally daring episodes: essentially a grab-bag of short stories or vignettes or TV-show parodies connected not by plot but by theme, and sometimes by filmmaking devices (the transitions are unusually clever).

This also contains the immortal scene in which Principal Skinner invites Superintendent Chalmers over to his house for a meal, ruins it, and ends up serving him Krusty Burgers, which he passes off as “steamed hams.” The Marx Bros. would approve.

After Bart chucks her saxophone out the window, Lisa is heartbroken. She can’t remember her life before it, she says, which prompts Homer and Marge to tell her the saga of how the instrument came into their lives. “It all happened in 1990,” Homer says. A 5-year-old Bart was miserable at kindergarten, a secretly gifted Lisa had nothing to stimulate her intellect, and Marge and Homer were scraping together $200 to buy an air conditioner. Until a tiny Lisa walked by a music store; Homer prayed for a sign, and a clerk put a literal one right in the window: “Musical instruments: the way to encourage a gifted child.” In they go, and out they walk with one saxamaphone.

It’s the perfect encapsulation of each character’s true self: Homer is dopey, but ultimately very devoted; Marge is the voice of reason, but not a stick in the mud; Bart’s mischievous and naughty, but only because he’s looking for validation and companionship; Lisa feels a little underappreciated, but comes to see how much her parents really love her. Bonus points for the baby versions of Lisa, Bart, and especially Milhouse.

Marge tries to get out of one of her periodic ruts by throwing a dinner party at the Simpsons’s home and inviting the Flanderses, the Hibberts, the Lovejoys, and the Van Houtens, but Milhouse’s parents have a fight that cracks open the fissures already weakening their marriage, leading them to divorce. Kirk loses his job and tries rather pathetically to reinvent himself as a single guy, even dating a radio-station employee named Starla and recording a demo; Luann has better luck, hooking up with an American Gladiator who goes by the name of Pyro.

“A Milhouse Divided” pulls off that Simpsons trick of digging into incredibly painful real-life situations with enough humor that the experience doesn’t become unpleasant. Its core is the way that people take their significant others for granted or fail to read and respond to their distress until it’s too late. The episode ends with one of Homer’s patented brilliant-stupid Hail Mary passes that somehow manage to connect: He secretly divorces Marge so that he can propose to her and redo their substandard wedding, including the “reception,” revealed in flashback as the two of them eating a Carvel cake at a truck stop.

Although The Simpsons has always been aware of how chaotic the kids’ home lives are, they never really dealt with it on somewhat real terms until this episode, which finds Homer and Marge going away for a spa vacation. They leave Bart, Lisa, and Maggie under the care of Grandpa Abe, who botches the job so badly that Child Protective Services takes them away, names the Flanders as their foster parents, and informs Marge and Homer that they can’t get their kids back until they take a parenting class. Bart chafes at Ned and Maude’s God obsession and strictness, but Lisa finds that she appreciates the routine.

Источник: https://www.vulture.com/article/best-the-simpsons-episodes.html

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