Standard Gibbs Energy of Metabolic Reactions: I. Hexokinase Reaction

The standard Gibbs energy of reaction enables calculation of the driving force of a (bio)chemical reaction. Gibbs energies of reaction are required in thermodynamic approaches to determine fluxes as well as single reaction conversions of metabolic bioreactions. The hexokinase reaction (phosphorylation of glucose) is the entrance step of glycolysis, and thus its standard Gibbs energy of reaction (Δ^Rg°) is of great impact. Δ^Rg° is accessible from equilibrium measurements, and the very small concentrations of the reacting agents cause usually high error bars in data reduction steps. Even worse, works from literature do not account for the nonideal behavior of the reacting agents (activity coefficients were assumed to be unity); thus published Δ^Rg° values are not standard data. Consistent treatment of activity coefficients of reacting agents is crucial for the accurate determination of standard Gibbs energy from equilibrium measurements. In this work, equilibrium molalities of hexokinase reaction were measured with an enzyme kit. These results were combined with reacting agents' activity coefficients obtained with the thermodynamic model ePC-SAFT. Pure-component parameters for adenosine triphosphate (ATP) and adenosine diphosphate (ADP) were fitted to experimental osmotic coefficients (water + Na₂ATP, water + NaADP). Δ^Rg° of the hexokinase reaction at 298.15 K and pH 7 was found to be -17.83 ± 0.52 kJ·mol^-1. This value was compared with experimental literature data; very good agreement between the different Δ^Rg° values was obtained by accounting for pH, pMg, and the activity coefficients of the reacting agents.