An enzyme (from Greek énsymo (ένζυμο), formed by én = at or in and simo = leaven or Yeast) is a Protein that catalyzes, or speeds up, a Chemical reaction.
Enzymes are essential to sustain life because most chemical reactions in biological cells would occur too slowly, or would lead to different products, without enzymes. A malfunction (mutation, overproduction, underproduction or deletion) of a single critical enzyme can lead to a severe disease. For example, Phenylketonuria is caused by an enzyme malfunction in the enzyme Phenylalanine hydroxylase, which catalyses the first step in the degradation of phenylalanine. If this enzyme does not function, the resulting build-up of phenylalanine leads to Mental retardation.
Like all catalysts, enzymes work by lowering the Activation energy of a reaction, thus allowing the reaction to proceed much faster. Enzymes may speed up reactions by a factor of many thousands. An enzyme, like any catalyst, remains unaltered by the completed reaction and can therefore continue to function. Because enzymes, like all catalysts, do not affect the relative energy between the products and reagents, they do not affect Equilibrium of a reaction. However, the advantage of enzymes compared to most other catalysts is their sterio-, regio- and chemoselectivity and specificity.
Enzyme activity can be affected by other molecules. Inhibitors are molecules that decrease or abolish enzyme activity; activators are molecules that increase the activity. Suicide inhibitors are inhibitors that incorporate themselves into the enzyme, permanently deactivating it. Inhibitors can be either natural or man-made. Many drugs are enzyme inhibitors. Aspirin, for example, inhibits an enzyme that produces the Inflammation messenger Prostaglandin, thus suppressing pain and inflammation.
Enzymes are also used in everyday products such as washing detergents, where they speed up chemical reactions involved in cleaning the clothes (for example, breaking down starch stains).
More than 5,000 enzymes are known. To name different enzymes, one typically uses the ending -ase with the name of the chemical being transformed (substrate), e.g., Lactase is the enzyme that catalyzes the cleavage of Lactose.
Studying the Fermentation of sugar to alcohol by yeast, Louis Pasteur came to the conclusion that this fermentation was catalyzed by "ferments" in the yeast, which were thought to function only in the presence of living organisms.
In 1897, Hans and Eduard Buchner inadvertently used yeast extracts to ferment sugar, despite the absence of living yeast cells. They were interested in making extracts of yeast cells for medical purposes, and, as one possible way of preserving them, they added large amounts of sucrose to the extract. To their surprise, they found that the sugar was fermented, even though there were no living yeast cells in the mixture. The term "enzyme" was used to describe the substance(s) in yeast extract that brought about the fermentation of sucrose. An example of an enzyme would be amylase
As with any protein, each monomer is actually produced as a long, linear chain of amino acids, which folds in a particular fashion to produce a three-dimensional product. Individual monomers may then combine via non-covalent interactions to form a multimeric protein.
[image] Most enzymes are far larger molecules than the substrates they act on and that only a very small portion of the enzyme, around 10 amino acids, come into direct contact with the substrate(s). This region, where binding of the substrate(s) and then the reaction occurs, is known as the Active site of the enzyme. Sometimes enzymes contain additionally other binding sites. Some enzymes have a binding site for a cofactor, which is needed for catalysis. Some enzymes have a binding site that serve regulatory functions, which increase or decrease the enzyme's activity. These typically bind small molecules, often direct or products or substrates of the reaction catalyzed. This provides a means for Feedback regulation.
The amino acid sidechains of an enzyme are either involved in forming the active site or a binding site, or are needed to form the 3D-structure of the protein. Some amino acid sidechains are not needed for function or structure of the enzyme.
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A suitable analogy would be that of a hand changing the shape of a glove as the glove is put on.
Another kind of posttranslational modification is the cleavage and splicing of the polypeptide chain. E.g. 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 prevents the enzyme from harmful digestion of the pancreas or other tissue. This type of inactive precursor to an enzyme is known as a Zymogen.
Those inactive protein parts of enzymes are called apoenzymes. An apoenzyme works effectively only in the presence of non-protein cofactors. An apoenzyme together with its cofactor constitutes a Holoenzyme, i.e., an active enzyme. Most of the cofactors are either regenerated or chemically unchanged at the end of the reactions.
The efficiency of an enzyme can be expressed in terms of kcat/Km. The quantity kcat, also called the turnover number, incorporates the rate constants for all steps in the reaction, and is the quotient of Vmax and the total enzyme concentration. kcat/Km is a useful quantity for comparing different enzymes against each other, or the same enzyme with different substrates, because it takes both affinity and catalytic ability into consideration. The theoretical maximum for kcat/Km, called diffusion limit, is about 108 to 109 (l mol-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 that reach this kcat/Km value are called catalytically perfect or kinetically perfect. Example of such enzymes are triose-phosphate isomerase, Carbonic anhydrase, acetylcholinesterase, Catalase, fumarase, ß-lactamase, and superoxide dismutase.
As with all catalysts, all reactions catalyzed by enzymes must be "spontaneous" (containing a net negative Gibbs free energy). With the enzyme, they run in the same direction as they would without the enzyme, just more quickly. However, the uncatalyzed, "spontaneous" reaction might lead to different products than the catalyzed reaction. Furthermore, enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive" a thermodynamically unfavorable one. For example, the cleavage of the high-energy compound ATP is often used to drive other, energetically unfavorable chemical reactions.
Many reactions catalyzed by an enzyme are reversible.
| CO 2 + H 2 O n { Carbonic anhydrase } n!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! n | → | nH 2 CO 3 | } |
| H 2 CO 3 n { Carbonic anhydrase } n!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! n | → | nCO 2 + H 2 O | } |
By changing the conformation (the three-dimensional structure) of the enzyme, the inhibitors either disable the ability of the enzyme to bind or turn over its substrate. The EI and EIS-complex have no catalytic activity.
Another example of irreversible inhibition is provided by the nerve gas diisopropylfluorophosphate (DFP) designed for use in warfare. It combines with the Amino acid Serine (contains the —SH group) at the active site of the enzyme acetylcholinesterase. The enzyme deactivates the Neurotransmitter Acetylcholine. Neurotransmitters are needed to continue the passage of nerve impulses from one Neuron to another across the Synapse. Once the impulse has been transmitted, acetylcholinesterase functions to deactivate the acetycholine almost immediately by breaking it down. If the enzyme is inhibited, acetylcholine accumulates and nerve impulses cannot be stopped, causing prolonged muscle contraction. Paralysis occurs and Death may result since the respiratory muscles are affected. Some insecticides currently in use, including those known as organophosphates (e.g. Parathion), have a similar effect on insects, and can also cause harm to nervous and Muscular system of humans who are overexposed to them.
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Enzymes that are regulated by end-production inhibition are usually allosteric enzymes. An allosteric enzyme molecule has an active site and also an allosteric site. The allosteric site can bind with allosteric effectors that affect the activity of the enzyme molecule. Allosteric effectors include allosteric activators and allosteric inhibitors. The binding with an allosteric activator activates an enzyme molecule because the active site is in the right conformation to bind with substrate molecules. The binding with an allosteric inhibitor inactivates the enzyme molecule because the conformation of the active site is altered. The activation and inhibition of an allosteric enzyme are reversible.
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The International Union of Biochemistry and Molecular Biology has developed a Nomenclature for enzymes, the EC numbers; each enzyme is described by a sequence of four numbers, preceded by "EC". The first number broadly classifies the enzyme based on its mechanism:
| Group | Reaction catalyzed | Typical reaction | Enzyme example(s) with trivial name |
| EC 1 Oxidoreductases | To catalyze oxidation/reduction reactions; transfer of H and O atoms or electrons from one substance to another | AH + B → A + BH (reduced) A + O → AO (oxidized) | Dehydrogenase, Oxidase |
| EC 2 Transferases | Transfer of a Functional group from one substance to another. The group may be methyl-, acyl-, amino- or phospate group | AB + C → A + BC | Transaminase, Kinase |
| EC 3 Hydrolases | Formation of two products from a substrate by Hydrolysis | AB + H2O → AOH + BH | Lipase, Amylase, peptidase |
| EC 4 Lyases | Non-hydrolytic addition or removal of groups from substrates. C-C, C-N, C-O or C-S bonds may be cleaved | RCOCOOH → RCOH + CO2 | |
| EC 5 Isomerases | Intramolecule rearrangement, i.e. isomerization changes within a single molecule | AB → BA | Isomerase, mutase |
| EC 6 Ligases | Join together two molecules by synthesis of new C-O, C-S, C-N or C-C bonds with simultaneous breakdown of ATP | X + Y+ ATP → XY + ADP + Pi | Synthetase |
The complete nomenclature can be browsed at /iubmb/enzyme/
Application | Enzymes used | Uses | Notes and examples | ||||
| Biological detergent | Primarily proteases, produced in an extracellular form from Bacteria | Used for presoak conditions and direct liquid applications helping with removal of protein stains from clothes. | [image] Note: The amylases and proteases used in detergents are allergenic for the process workers, although, encapsulation techniques have reduced this problem. | ||||
| Amylase enzymes | Detergents for machine dishwashing to remove resistant starch residues | ||||||
| Baking industry | Fungal alpha-amylase enzymes: normally inactivates about 50 degrees Celsius, destroyed during baking process | Catalyze breakdown of starch in the Flour to sugar. Yeast action on sugar produces carbon dioxide. Used in production of white bread, buns, and rolls | [image] | ||||
| Protease enzymes | Biscuit manufacturers use them to lower the protein level of flour. | ||||||
| Baby foods | Trypsin | To predigest baby foods | |||||
| Brewing industry | Enzymes from barley are released during the mashing stage of beer production. | They degrade starch and proteins to produce simple sugar, amino acids and peptides that are used by yeast to enhance fermentation. | [image] | ||||
| Industrially produced enzymes now widely used in the brewing process to substitute for the natural enzymes found in barley: | |||||||
| Amylase, glucanases, proteases | Split polysaccharides and proteins in the Malt | ||||||
| Betaglucosidase | Improve the filtration characteristics. | ||||||
| Amyloglucosidase | Low-calorie Beer | ||||||
| Proteases | Remove cloudiness during storage of beers. | ||||||
| Fruit juices | Cellulases, pectinases | Clarify fruit juices | |||||
| Dairy industry | Rennin, derived from the stomachs of young ruminant animals (calves, lambs, kids) | Manufacture of cheese, used to split protein | Note: As animals age rennin production decreases and is replaced by another protease, pepsin, which is not suitable for cheese production. In recent years the increase in cheese consumption, as well as increased beef production, has resulted in a shortage of rennin and escalating prices. | ||||
| Microbially produced enzyme | Now finding increasing use in the dairy industry | [image] | |||||
| Lipases | Is implemented during the production of Roquefort cheese to enhance the ripening of the blue-mould cheese. | ||||||
| Lactases | Break down lactose to glucose and galactose | ||||||
| Starch industry | Amylases, amyloglucosideases and glucoamylases | Converts starch into glucose and various syrups |
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| Glucose isomerase | Converts Glucose in fructose (high fructose syrups derived from starchy materials have enhanced sweetening properties and lower calorific values) | ||||||
| Immobilised enzymes | Production of high fructose syrups | Note: Although this process is widely used in the USA and Japan, legislation in the EEC restricts its use to protect Sugar beet farmers. | |||||
| Rubber industry | Catalase | To generate Oxygen from Peroxide to convert LaTeX to foam rubber | |||||
| Paper industry | Amylases | Degrade starch to lower Viscosity product needed for sizing and coating paper | [image] | ||||
| Photographic industry | Protease (ficin) | Dissolve Gelatin off the scrap film allowing recovery of Silver present | |||||