The fundamental equation of thermodynamics for biochemical reaction systems

Abstract

In biochemistry it is useful to consider pH and pMg as independent variables along with T and P. When T, P, pH, and pMg are specified, the Gibbs energy G is not the criterion of equilibrium, but a transformed Gibbs energy G can be defined by use of a Legendre transform involving the specified chemical potentials of H+and Mg+2. It can be shown that (dG')T,P,pH,pMg ≤ 0 is the criterion of equilibrium. The fundamental equation for G' is the basis for deriving the expression for the apparent equilibrium constant K for a biochemical reaction, the standard transformed Gibbs energy of formation ΔfG'o, and the standard transformed enthalpy of formation ΔfH'oof a biochemical reactant at specified T, P, pH, pMg, and I. These properties can be calculated if ΔfGoand ΔfHoare known for the various species of the reactant. If these properties are not known for the species of a reactant, values of ΔfG'oand ΔfH'ocan be calculated on the assumption that these properties are equal to zero for one of the species. © 1993, Walter de Gruyter GmbH.