Two different ways that hydrogen ions are involved in the thermodynamics and rapid-equilibrium kinetics of the enzymatic catalysis of S=P and S+H2O=P.

Hydrogen ions are involved in two different ways in the thermodynamics and rapid-equilibrium kinetics of enzyme-catalyzed reactions. The two ways are through pKs and through the production or consumption of hydrogen ions in the mechanism. These ways are examined for the catalyzed reactions S=P and S+H2O=P. Since the apparent equilibrium constant K' can be calculated from the kinetic parameters by use of the Haldane equation, the treatment of the effects of pH must be consistent in thermodynamics and kinetics. This leads to a new kind of Haldane equation that involves 10(pH) or 10(-pH) in addition to the kinetic parameters when hydrogen ions are produced or consumed. These concepts are applicable to more complicated reactions and rate equations. Derivations of equations for calculating these two types of pH effects are discussed in thermodynamics and rapid-equilibrium kinetics. A computer program is used to make four plots of apparent equilibrium constants and changes in the binding of hydrogen ions in the catalyzed reaction.