29 The Comparison of the Steps of Some Enzyme-Catalyzed and Base-Catalyzed Hydrolysis Reactions

Abstract

It has been shown that enzyme-catalyzed hydrolysis reactions, especially those involving trypsin, chymotrypsin, and some plant peptidases, proceed through a number of well-defined steps. The first of these steps is a rapid second-order reaction, which is thought to be an adsorption of the substrate on to the “specificity site” of the enzyme. Subsequent first-order reactions involve two or more basic groups of the “catalytic site” of the enzyme molecule and the carbonyl carbon of the substrate. Two pre-steady-state methods for the study of consecutive steps in enzyme-catalyzed reactions are described. The first involves the initial acceleration of the rate of formation of the final products, and the second the observation of reaction intermediates. Some results of the application of these methods to the characterization of intermediate steps in several hydrolysis reactions, as well as a model for the path of such enzyme reactions, are given. This model is based on the identification of the basic groups on the catalytic sites and can be extended to explain the nature of transfer reactions. The kinetic consequences of such a sequence of reaction steps and their contribution to the efficiency of enzyme reactions as compared with homogeneous base-catalyzed reactions are discussed.