Theory of double pH optima of enzymes

Abstract

Steady state rate equations have been developed for various mechanisms involving enzyme, hydrogen ion, substrate and inhibitor. By differentiating these equations with respect to hydrogen ion concentration and applying Descartes Rule of signs to the resulting equations it has been possible to ascertain which of these enzyme reaction mechanisms can result in double pH optima. The following three general situations can give rise to double pH optima: (i) the presence of two distinct isoenzymes with different pH optima; (ii) formation of active enzymesubstrate complexes by two ionic species of the enzyme which differ by at least two protons; (iii) the presence of an ampholyte inhibitor, only one ionic species of which can combine with the enzyme. Properties of a system of category (iii) are graphically illustrated by numerical substitution in the appropriate rate equations.