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Definition
An enzyme is a biochemical catalyst that helps to cleave or otherwise modify a substrate. The enzyme facilitates the necessary reaction by reducing the activation energy, which always has to be overcome for metabolism to take place at all. The enzyme participates in the biochemical reaction; even forms a temporary compound (the enzyme-substrate complex) with the substances to be converted, but remains unaltered by the reaction.Enzymes are proteins by their chemical nature. Responsible for the catalytic efficiency is the so-called active site, which consists of specially folded parts of the polypeptide chain or reactive non-protein elements of the enzyme molecule. A special hollow structure in the enzyme enables the active site to make contact with a suitable substrate.
Classes of enzymes
Essential protein enzymes
- Their active site is formed by certain amino acid residues. The hydrolases, which belong to this group of enzymes, hydrolytically cleave a substrate.·
- Besides these essential protein enzymes, there exist enzymes with a reactive non-protein part (cofactor). The cofactor of an enzyme may either be an inorganic ion (e.g. an iron or manganese ion) or a more complex organic molecule, which is called a coenzyme. Some enzymes need a coenzyme and one or several metal ions to become active. The cofactor can be permanently or only temporarily bound to the protein part of the enzyme. One therefore distinguishes between:
Enzymes with prosthetic group
- A prosthetic group is firmly and permanently bound in many enzymes. ·
- In the so-called prosthetic group, the active center is made of a non-protein molecule. This might be, for instance, a vitamin derivative.
Holoenzymes
- Holoenzymes consist of one apoenzyme and one coenzyme(cosubstrate).
- The apoenzyme is the protein portion, whereas the coenzyme or cosubstrate is a separable cofactor. The holoenzyme is the complex consisting of apoenzyme and coenzyme, which only forms temporarily.
The protein portion is responsible for the substrate specificity and for the action specificity (reaction specificity) of an enzyme, i.e. it determines which substances are actually catalyzed and which of the numerous possible reactions the substrate molecule is to take.
The enzyme activity, one of the parameters of enzyme kinetics, depends on exterior factors. A rise in temperature may increase the speed of an enzymatic reaction, but only if the enzyme proteins are not denatured by the higher temperature. Changes in pH value can also affect enzyme activity.
Hundreds of different enzymes are active in our body. If one enzyme is missing, or for instance inactive through vitamin deficiency, severe metabolic disorders can occur. Enzymes are also required by industrial production. Lipase, a fat-cleaving enzyme, is added to detergents to enhance their cleaning performance. Enzymes are also used in the production of some medicines and insecticides. The laboratory enzyme plays a part in cheese production, an enzyme which is extracted from the stomach of calves. Today, many enzymes can be produced with genetically modified microorganisms. The name of an enzyme mostly ends with the syllable -ase, e.g. lipase, amylase, and proteinase. Thus, enzymes are often named after their substrates. But they can also be named after the process which they catalyze. Some exceptions from the rule are the protein-cleaving enzymes trypsin and chymotrypsin from the pancreas and pepsin, which is active in the stomach; these enzymes have kept their traditional trivial names.
Systematic
Systematics By international convention, enzymes can be classified into six groups according to the reactions that are catalyzed by them; each of them is ascribed an EC number ""(Enzymes Commission)"": ·- EC I: Oxidoreductases
- EC II: Transferases
- EC III: Hydrolases
- EC IV: Lyases (synthases)
- EC V: Isomerases
- EC VI: Ligases (synthetases)
Enzyme inhibition
distinguishes four forms of enzyme inhibition:
Competitive inhibition
The substrate competes with the inhibitor for the active site. The inhibitor cannot be converted and thereby stops the work of the enzyme. The enzyme remains inhibited only with a sufficiently high inhibitor concentration. If the inhibitor concentration decreases and the substrate concentration rises, the enzyme can once again cleave the substrate.
Non-competitive inhibition
The inhibitor (mostly a heavy metal ion) binds irreversibly to the active site, and the enzyme is thereby permanently destroyed.
Allosteric inhibition
In this form of inhibition, the inhibitor becomes active at another site of the enzyme, the allosteric site. It binds there reversibly and changes the form of the active site, so that the substrate does not fit the enzyme anymore. When the inhibitor is displaced from the enzyme by an increase in substrate concentration, the reaction can continue.Feedback inhibition A variant of allosteric inhibition, but here the end product acts as an inhibitor, thereby automatically creating a feedback control.
