Etymology and history
Structures and mechanisms
"Lock and key" model
- Lowering the activation energy by creating an environment in which the transition state is stabilized (e.g. straining the shape of a substrate—by binding the transition-state conformation of the substrate/product molecules, the enzyme distorts the bound substrate(s) into their transition state form, thereby reducing the amount of energy required to complete the transition).
- Lowering the energy of the transition state, but without distorting the substrate, by creating an environment with the opposite charge distribution to that of the transition state.
- Providing an alternative pathway. For example, temporarily reacting with the substrate to form an intermediate ES complex, which would be impossible in the absence of the enzyme.
- Reducing the reaction entropy change by bringing substrates together in the correct orientation to react. Considering ΔH‡alone overlooks this effect.
- Increases in temperatures speed up reactions. Thus, temperature increases help the enzyme function and develop the end product even faster. However, if heated too much, the enzyme's shape deteriorates and the enzyme becomes denatured. Some enzymes like thermolabile enzymes work best at low temperatures.
Transition state stabilization
Dynamics and function
Cofactors and coenzymes
- Competitive inhibition
- Uncompetitive inhibition
- Non-competitive inhibition
- Mixed inhibition
- Uses of inhibitors
Control of activity
- 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 and inhibition. For example, bacteria may become resistant to antibiotics such as penicillin because enzymes called beta-lactamases are induced that hydrolyze the crucial beta-lactam ring within the penicillin molecule. Another example are enzymes in the liver called cytochrome P450 oxidases, which are important in drug metabolism. Induction or inhibition of these enzymes can cause drug interactions.
- 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 the Golgi apparatus and used by a different set of enzymes as a source of energy in the mitochondrion, through β-oxidation.
- Enzymes can be regulated by inhibitors and activators. 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. Negative feedback mechanism can effectively adjust the rate of synthesis of intermediate metabolites according to the demands of the cells. This helps allocate materials and energy economically, and prevents the manufacture of excess end products. The control of enzymatic action helps to maintain a stable internal environment in living organisms.
- Enzymes can be regulated through post-translational modification. This can include phosphorylation, myristoylationand glycosylation. For example, in the response to insulin, the phosphorylation of multiple enzymes, includingglycogen synthase, helps control the synthesis or degradation of glycogen and allows the cell to respond to changes inblood sugar. 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.
- Some enzymes may become activated when localized to a different environment (e.g., from a reducing (cytoplasm) to an oxidizing (periplasm) environment, high pH to low pH, etc.). For example, hemagglutinin in theinfluenza virus is activated by a conformational change caused by the acidic conditions, these occur when it is taken up inside its host cell and enters the lysosome.
Involvement in disease
- EC 1 Oxidoreductases: catalyze oxidation/reduction reactions
- EC 2 Transferases: transfer a functional group (e.g. a methyl or phosphate group)
- EC 3 Hydrolases: catalyze the hydrolysis of various bonds
- EC 4 Lyases: cleave various bonds by means other than hydrolysis and oxidation
- EC 5 Isomerases: catalyze isomerization changes within a single molecule
- EC 6 Ligases: join two molecules with covalent bonds.
|Food processing||Amylases from fungi and plants||Production of sugars from starch, such as in making high-fructose corn syrup. In baking, catalyze breakdown of starch in the flour to sugar. Yeast fermentation of sugar produces the carbon dioxide that raises the dough.|
|Proteases||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 for fermentation.|
|Industrially produced barley enzymes||Widely used in the brewing process to substitute for the natural enzymes found in barley.|
|Amylase, glucanases, proteases||Split polysaccharides and proteins in themalt.|
|Betaglucanases and arabinoxylanases||Improve the wort and beer filtration characteristics.|
|Amyloglucosidase and pullulanases||Low-calorie beer and adjustment of fermentability.|
|Proteases||Remove cloudiness produced during storage of beers.|
|Acetolactatedecarboxylase (ALDC)||Increases fermentation efficiency by reducing diacetyl formation.|
|Fruit juices||Cellulases, pectinases||Clarify fruit juices.|
|Dairy industry||Rennin, derived from the stomachs of young ruminant animals (like calves and lambs)||Manufacture of cheese, used tohydrolyze protein|
|Microbially produced enzyme||Now finding increasing use in the dairy industry|
|Lipases||Is implemented during the production ofRoquefort cheese to enhance the ripening of the blue-mold cheese.|
|Lactases||Break down lactose to glucose and galactose.|
|Meat tenderizers||Papain||To soften meat for cooking|
|Starch industry||Amylases, amyloglucosideases and glucoamylases||Converts starch into glucose and varioussyrups.|
|Glucose isomerase||Converts glucose into fructose in production of high-fructose syrups from starchy materials. These syrups have enhanced sweetening properties and lower calorific values than sucrose for the same level of sweetness.|
|Paper industry||Amylases, Xylanases, Cellulases andligninases||Degrade starch to lower viscosity, aidingsizing and coating paper. Xylanases reduce bleach required for decolorizing; cellulases smooth fibers, enhance water drainage, and promote ink removal; lipases reduce pitch and lignin-degrading enzymes remove lignin to soften paper.|
|Biofuel industry||Cellulases||Used to break down cellulose into sugars that can be fermented (seecellulosic ethanol)|
|Ligninases||Use of lignin waste|
|Biological detergent||Primarily proteases, produced in anextracellular form from bacteria||Used for presoak conditions and direct liquid applications helping with removal of protein stains from clothes|
|Amylases||Detergents for machine dish washing to remove resistant starch residues|
|Lipases||Used to assist in the removal of fatty and oily stains|
|Cellulases||Used in biological fabric conditioners|
|Contact lens cleaners||Proteases||To remove proteins on contact lens to prevent infections|
|Rubber industry||Catalase||To generate oxygen from peroxide to convert latex into foam rubber|
|Photographic industry||Protease (ficin)||Dissolve gelatin off scrap film, allowing recovery of its silver content.|
|Molecular biology||Restriction enzymes, DNA ligase andpolymerases||Used to manipulate DNA in genetic engineering, important in pharmacology,agriculture and medicine. Essential forrestriction digestion and the polymerase chain reaction. Molecular biology is also important in forensic science.|