AbstractEquilibria of reactions catalyzed by acetate kinase and adenylate kinase were studied experimentally and theoretically. Nucleotide conversions in excess of 90% were obtained with reactants (acetyl phosphate and one nucleotide) in stoichiometric proportion for ATP regeneration from either ADP via acetate kinase or AMP via the coupled enzyme system. Observed equilibrium constants, measured as a function of [Mg2+] atpH 7.4, ranged from about 1 to 9 for the reaction catalyzed by adenylate kinase and from about 50 to 400 for the reaction catalyzed by acetate kinase. These results agreed well with prediction of a theoretical model for the multiple equilibria between all species present in solution. Theoretical calculations showed that magnesium ion complexes and totally dissociated anions are the predominant species in solution at\documentclass{article}\pagestyle{empty}\begin{document}$p{\rm H}\tilde>7{\rm and}[{\rm Mg}^{2 + } ]\tilde>10^{ - 3} {\rm M}$\end{document}.