ABSTRACT
Chemical equations are written in terms of specific ionic and elemental species and balance elements and charge, whereas biochemical equations are written in terms of reactants that often consist of species in equilibrium with each other and do not balance elements that are assumed fixed, such as hydrogen at constant pH. Both kinds of reaction equations are needed in biochemistry. When the pH and the free concentrations of certain metal ions are specified, the apparent equilibrium constant K′ for a biochemical reaction is written in terms of sums of species and can be used to calculate a standard transformed Gibbs energy of reaction Δr G′°. Transformed thermodynamic properties can be calculated directly from conventional thermodynamic properties of species. Calorimetry or the dependence of K′ on temperature can be used to obtain the standard transformed enthalpy of reaction Δr H′°. Standard transformed Gibbs energies of formation Δf G′°(i) and standard transformed enthalpies of formation Δf H′°(i) for reactants (sums of species) can be calculated at various T, pH, pMg, and ionic strength (I) if sufficient information about the chemical reactions involved is available. These quantities can also/be calculated from measurement of K′ for a number of reactions under the desired conditions. Tables can be used to calculate Δr G′° and Δr H′° for many more reactions.
