Thermochemistry of the biochemical reaction: { pyrophosphate(aq) + H2O(l) = 2 phosphate(aq)}

Abstract

Calorimetric enthalpies of reaction have been measured for the overall biochemical reaction{pyrophosphate(aq) + H2O(l) = 2phosphate (aq)} . The reaction was catalyzed by alkaline phosphatase and, to simplify the thermochemistry, was carried out in the absence of Mg 2+(aq). Measurements were performed with phosphate buffer ( pH = 7.19 and 7.94), PIPES buffer ( pH = 7.13), and HEPES buffer ( pH = 7.86). The results of these measurements were analyzed by using an equilibrium model. These calculations lead to the standard molar enthalpy changeΔrHmo=−(17.3 ± 0.6)kJ·mol−1 (temperature T= 298.15 K and ionic strengthI= 0) for the reference reaction{HP2O73−(aq) + H2O(l) = 2HPO42−(aq) + H+(aq)} . Values of the apparent equilibrium constantK′ for the overall biochemical reaction from the literature were also analyzed by using the equilibrium model in order to obtain what is believed to be a reliable value for the equilibrium constantK= 4.7 · 10−4 for the reference reaction. The values ofK and ΔrHmo for the reference reaction have been used together with values from the CODATA tables to calculate standard molar formation properties for the pyrophosphate species.