From Big Medical Encyclopedia

CATALYSTS (grech, katalysis destruction) — substances which small amounts increase the speed of chemical changes as a result of interaction with reactants which amount can be very considerable. A specific place in the nature is held by powerful biocatalysts of the proteinaceous nature — enzymes (see) with the participation of which in organisms all exchange reactions of substances are carried out.

To. are classified by type of the catalyzed reactions (kislotnoosnovny, oxidation-reduction), by the nature To. (metal, sulphidic, etc.), to destination specific catalytic processes (synthesis of ammonia, cracking of oil products).

Along with ability to accelerate chemical reactions by the major property K. their selectivity {specificity) is. Most brightly specificity is expressed at enzymes; it provides orderliness of a metabolism. Many enzymes are absolutely specific, i.e. affect only one substance (substrate) or, in case of a reversible test, the corresponding couple of substances — substrate and a product. E.g., enzyme fumarase (fumaratehydratase; KF catalyzes reversible transformation apple to - you in fumaric and affects only this couple of substances; at the same time fumarase shows stereospecificity, catalyzing transformation only of one of stereoisomers apple to - you (see. Isomerism ).

In the presence of certain substances — inhibitors (see) activity of enzymes considerably decreases or in general disappears therefore, entering specific inhibitors into an organism, it is possible to regulate activity of certain enzymes.

See also Catalysis .

Bibliography: See bibliogr, to St. Catalysis .

L. M. Ginodman.