Standard Gibbs Free Energy, Enthalpy, and Entropy Changes as a Function of pH and pMg for Several Reactions Involving Adenosine Phosphates

R A Alberty

doi:10.1016/s0021-9258(18)93127-3

Abstract

Abstract The standard Gibbs free energy change ΔG0obs, number (nh) of moles of H+ produced, number (nmg) of moles of Mg2+ produced, standard enthalpy change ΔH0obs, and standard entropy change ΔS0obs have been calculated as functions of pH and pMg for the following phosphohydrolyase and phosphotransferase reactions at 25° and 0.2 ionic strength: ATP + H2O = ADP + Pi (1) ATP + AMP = 2ADP (2) ATP + H2O = AMP + PPi (3) ADP + H2O = AMP + Pi (4) ATP + 2H2O = AMP + 2Pi (5) PPi + H2O = 2Pi (6) The values of these thermodynamic quantities are presented by means of contour diagrams for the range pH 4 to 10 and pMg 1 to 7. These diagrams make the general features of the pH and pMg dependences readily discernible and summarize the results of some 2500 calculations per diagram. There are significant changes in the heat evolved by these reactions over this range of the independent variables. Equations are derived which make it possible to calculate the standard entropy of reaction ΔS0obs from the entropy change of the reaction written in terms of particular ionic species, entropy changes of the various acid and metal ion dissociation reactions, the entropies of mixing of the various forms of each reactant and product, and the entropies of dilution of H+ and Mg2+. The relative contributions of enthalpy and entropy to the equilibrium constants of these reactions may be accessed from the diagrams as a function of pH and pMg.

Highlights

The values of these thermodynamic quantities are presented by means of contour diagrams for the range pH 4 to 10 and pMg 1 to 7
Equations are derived which make it possible to calculate the standard entropy of reaction AS& from the entropy change of the reaction written in terms of particular ionic species, entropy changes of the various acid and metal ion dissociation reactions, the entropies of mixing of the various forms of each reactant and product, and the entropies of dilution of H+ and Mg2+
The relative contributions of enthalpy and entropy to the equilibrium constants of these reactionsmay be accessedfrom the diagramsas a function of pH and pMg

Summary

SUMMARY

If the equilibrium constant for a biochemical reaction is known at one temperature, ionic strength, pH, and known free concentrations of metal ions that form complexes with the reactants, the equilibrium constant may be calculated for. Writing the equilibrium expressions in terms of concentrations means that the chemical potentials p of the ions are given by the ideal solution equation p = /.LO+ RT ln c, The number, nn, of molesof H+ producedby the reaction where c is concentration, in media of constant ionic strength, pH, and the number,nMs, of molesof Mg2+producedare related by and pMg.l The standard Gibbs free energy change, AG!&, for a reaction is made up of two contributions, the standard enthalpy change,. AG$,. in kilocalories per mole; middle, AHibs in kilocalories per mole; right, TA&% in kilocalories per mole

PM t-4 I

DISCUSSION