Biochemistry

biochemical transformations, but also often determine the general direction of metabolic processes. Depending on whether the enzyme can catalyze any one reaction (i.e., act on a given substrate) or several reactions (i.e., act on a specific group of substrates), absolute and relative specificity are distinguished. Absolute specificity is characteristic of enzymes that act on the same substrate with a well-defined structure. Any changes in the structure of the substrate make it inaccessible to the action of the enzyme. Such enzymes include urease, which catalyzes the decomposition of urea and does not act on any other compounds, even urea derivatives, such as methylurea. This group of enzymes also includes arginase, which breaks down arginine in vivo. Enzymes that act on a particular type of bond have relative specificity. For example, peptidases hydrolyze all compounds containing the -CO-NH peptide bond (Fig. 3.2). R — ,CH — COOH I ЫН» Amino acid Decarboxyl ase R — C-COOH4-NH, О Ketoacid R — СҚ - NH 2 +CO 2 Amine R' — CH — COOH+ I NH, Amino acid +R-C — COOH+ II О Ketoacid Figure 3.2 - Relative specificity of enzymes Enzymes with relative specificity have a wide spectrum of action. Relative group specificity is characteristic of enzymes that act on substrates having the same type of bond and a certain functional grouping. They not only have a specific effect on a particular type of chemical bond, but also allow changes in the chemical structure of the substrate. For example, lipases and esterases cleave various triacylglycerols (fats) as well as diacylglycerols, monoacylglycerols and esters. However, the cleavage of these substrates occurs at different rates. 84