Consideration of the contribution of chemical (non-enzymatic) conversion of substrate in the general mechanism of enzyme reaction

Abstract

When enzyme-catalyzed reactions are studied, it is necessary to take into account the contribution of the chemical (non-enzymatic) conversion of the substrate to the product, which is carried out together with the enzyme-catalyzed conversion of the substrate. It is generally believed that the difference of the product concentration that was formed in the presence of the enzyme and in its absence (during the same time interval) is the concentration of the product that was formed directly in the enzyme-catalyzed reaction, i.e. that there is additivity of the product concentrations at each time point. In this paper, we have analyzed when there is additivity and how to correctly take into account the contribution of chemical (non-catalytic) substrate conversion when the enzyme-catalyzed reactions are investigated. We have shown that the additivity of product­ concentrations and initial rates is observed only for a period when the product concentration increases linear­ly with time. The longer the reaction proceeds the more the deviation from the additivity. Under equilibrium condition, there is no additivity of equilibrium product concentrations but under conditions of detailed balance the equilibrium product concentration of the overall reaction, including the enzyme-catalyzed and chemical (non-enzymatic) conversion of the substrate, is also at the same time the equilibrium concentration of the product of the enzyme-catalyzed conversion of the substrate.

Keywords

• catalysis
• cyclic equilibrium
• cyclic reactions
• detailed balance
• dissociation constant
• equilibrium constant
• equilibrium state
• initial rate
• kinetics of the enzyme-catalyzed reactions
• rate constant