Catalytic acceleration of reactions by enzymes. Effect of screening of a polar medium by a protein globule

Abstract

When a reaction is transferred from an aqueous medium to an enzyme globule, part of water volume in the vicinity of the reacting particles is substituted by protein—a medium with a much lower static dielectric permittivity. Because of this the degree of reorientation of dipoles necessary for a charge transfer reaction to take place decreases, i.e. the reorganization energy of the medium and hence the activation energy of the reaction decreases. The existence of this effect necessarily follows from the theory of the elementary act of chemical charge transfer reactions. The effect has been quantitatively calculated for a number of reaction models, including the hydrolysis of acylchymotrypsin. Calculations show that the expected decrease of the activation energies amounts to several (from 2 to 7) kcal, which corresponds to an acceleration of the reaction by 1·5-5 orders of magnitude. At first the catalytic effect increases rapidly with increasing globule size, but when globule radii of about 15–20 Å are reached, it becomes practically constant. These optimum radii are close to real dimensions of many enzyme molecules. It is shown that immersion of the charge transfer centers into the globule to the depth of the order of 7–8 Å strongly decreases the charge transfer activation energy. The effect is of importance for charge transfer reactions not involving substrate sorption-desorption, such as e.g. in electron-transport chains.