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enzyme
Значение термина enzyme в knolik
enzyme - enzyme
enzyme - A protein which is a catalyst (i.e. a substance which in minute amounts promotes chemical change without itself being used up in the reaction), by virtue of its power of increasing the reactivity of a specific substance or specific substances (called the Substrate). There are many different kinds, each kind directly promoting only a very limited range of chemical reactions. Within all living things a large number of chemical reactions are always in progress. See also: Metabolism. Most of the reactions of metabolism would not occur perceptibly in the absence of enzymes, at the temperature and in the other conditions in which living things exist; and so metabolism is entirely dependent on enzymes. An enzyme produces its effect on the substrate by combining with it and activating it. so that the substrate undergoes further chemical change, at the same time losing its combination with the enzyme. (The substrate molecule being usually small in relation to the enzyme molecule, only a specific small part of the latter, the active site, takes part in the combination.) As die result of such a process, therefore, the enzyme is not consumed, but at the end of the process is free to deal with more substrate. Since the activation of the substrate is rapid, a very small amount of enzyme can produce a very great effect. One molecule of catalase can decompose 40,000 molecules of hydrogen peroxide per second at freezing point. Most enzymes are in fact present in relatively minute amounts. Most enzymes activate only one kind of substrate each. Other enzymes are less highly specific, but even so each reacts only with chemically related substrates. There is correspondingly a large number of different enzymes, for all the different reactions of metabolism.
Each enzyme requires certain definite conditions for optimum performance, particularly as regards pH, the presence of specific accessory substances (co-enzymes, activators) and the absence of specific inhibiting substances.
Enzymes are unstable substances, and are easily destroyed or inactivated, e.g. by high temperature, or by a great range of chemical substances. Although they are not consumed by the reaction, enzymes slowly break down and have to be synthesized. Many of the micro-nutrients required by organisms probably go to regenerating certain enzymes and co-enzymes. Enzymes are all produced within living cells and most of them do their work there, though they can often be extracted and can be studied outside the cell. Some enzymes (e.g. digestive enzymes of vertebrates) are normally secreted to the outside of cells. In metabolic processes of cells, and also in digestion, enzymes work very much as systems, the products of one reaction becoming the substrate of another.
The effective amount of any particular enzyme within a cell seems to be controlled in two main ways, (1) The synthesis of new enzyme molecules may be repressed or permitted through control of the activity of the gene ultimately responsible for their formation. Enzyme induction is the production of active enzyme, by removal of the repression of the relevant gene, when a metabolite, often a substrate for the enzyme, is administered to the cells (See also: Adaptive Enzyme). (2) The activity of already existing enzyme molecules may be inhibited, often by a product, not of the enzyme's own activity, but of the activity of another enzyme farther along the chain of enzymes which constitutes the enzyme system; thus providing an automatic self-limitation of the activity of the whole system. (See also: End-product inhibition).
Closely similar enzymes, and enzyme systems, have been found in a wide variety of organisms, including plants, animals, and bacteria, and they account for the fundamental similarities of many aspects of their metabolism.
Several enzymes have been obtained pure and crystalline. Some of these are entirely protein (e.g. proteolytic enzymes); some are protein plus a prosthetic group (e.g. many concerned with oxidation and reduction). Enzymes are usually named by attaching the suffix -ase to a root indicating the nature of the substrate or of the reaction. See also: Amylase, Dehydrogenase, Proteolytic Enzymes Adaptive Enzyme, Isoenzyme.
Рядом со словом enzyme в knolik
| environment | В начало буква "" буквосочетание "" | eocene |
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