Apparent Equilibrium Constants and Standard Transformed Gibbs Energies of Biochemical Reactions Involving Carbon Dioxide

Abstract

When carbon dioxide is produced in a biochemical reaction, the expression for the apparent equilibrium constantK′ can be written in terms of the partial pressure of carbon dioxide in the gas phase or the total concentration of species containing CO2in the aqueous phase, referred to here as [TotCO2]. The values of these two apparent equilibrium constants are different because they correspond to different ways of writing the biochemical equations. Their dependencies on pH and ionic strength are also different. The ratio of these two apparent equilibrium constants is equal to the apparent Henry's law constantKH′. This article provides derivations of equations for the calculation of the standard transformed Gibbs energies of formation of TotCO2and values of the apparent Henry's law constant at various pH levels and ionic strengths. These equations involve the four equilibrium constants interconnecting the five species [CO2(g), CO2(aq), H2CO3, HCO3−, and CO32−] of carbon dioxide. In the literature there are many errors in the treatment of equilibrium data on biochemical reactions involving carbon dioxide, and so several examples are discussed here, including calculation of standard transformed Gibbs energies of formation of reactants. This approach also applies to net reactions, and the net reaction for the oxidation of glucose to carbon dioxide and water is discussed.