Thermodynamic Data for the Hydrolysis of Adenosine Triphosphate as a Function of pH, Mg”+ Ion Concentration, and Ionic Strength*

Equations are set up to calculate the observed standard free energy change for the hydrolysis of ATP (namely, AGobs, the quantity evaluated on the basis of total ATP, ADP, and orthophosphate concentrations at equilibrium, irrespective of the state of ionization or complex formation, or both) as a function of pH and Mg2+ ion concentration. The variation of AGobs is analyzed in terms of AG, the standard free energy change with respect to the 1 molal standard states for the H+ and Mg Z+ ions, and the driving force due to the prevailing Hf and Mg2+ ion concentrations with data extrapolated to zero ionic strength so that the standard free energy, enthalpy, and entropy changes are free from minor contributions arising from unspecific ionic interactions. The variation of AG is then discussed in terms of the component enthalpy and entropy changes for the individual hydrolysis equilibria which predominate over the pH range 6 to 9 as first the ATP, then the ADP, and finally the orthophosphate are converted into Mg2+ complexes. The calculations, based on data for the glutamine synthetase and hydrolysis reactions, lead to a revised value for AG,,,, at pH 7.5, 25”, and p = 0.2 of -10.7 kcal per mole. Series of values are listed in an appendix for various ionic strengths at 25” and 37” over the pH range 6 to 9 with Mg2+ concentrations up to 50 mu, from which AG,b, for other reaction conditions can be obtained by interpolation.