Standard molar entropies, standard entropies of formation, and standard transformed entropies of formation in the thermodynamics of enzyme-catalyzed reactions

Abstract

The standard transformed Gibbs energy of reaction provides the criterion for spontaneous change and equilibrium for an enzyme-catalyzed reaction at specified temperature, pressure, and pH. The standard transformed enthalpy of reaction yields the heat of reaction. The standard transformed entropy of reaction has received less attention, but it provides almost all of the pH dependence of the apparent equilibrium constant. When the standard Gibbs energies of formation Δ f G j ∘ ( 298 . 15 K , I = 0 ) and standard enthalpies of formation Δ f H j ∘ ( 298 . 15 K , I = 0 ) of the species of a reactant are known, the standard entropies of formation Δ f S j ∘ ( 298 . 15 K , I = 0 ) can be calculated. With the database BasicBiochemData2 and recent extensions, this can be done for about fifty reactants. However, standard molar entropies S m ∘ ( 298 . 15 K , I = 0 ) of species are of more interest because it may be possible to interpret their values and use them to estimate S m ∘ values for species for which Δ f H j ∘ are unknown. These estimates of S m ∘ of species for which Δ f G j ∘ values are known make it possible to obtain Δ f H j ∘ of species that can be used to calculate effects of temperature on standard transformed Gibbs energies of reactants and apparent equilibrium constants of enzyme-catalyzed reactions.