a Faculty of Pharmaceutical Sciences, University of Tokushima, Shomachi-1, Tokushima 770, Japan
b College of General Education, University of Tokushima, Minami-Jyosanjima, Tokushima 770, Japan
Abstract:
The exchange rate constant between free Mg2+ and Mg2+ bound to adenosine 5′-triphosphate (ATP) was determined at various temperatures from the 31P-NMR spectra of ATP in the absence and presence of Mg2+. The activation free energy of this exchange reaction showed that Mg2+ binds asymmetrically to the β- and γ-phosphoryl groups and that it coordinates with the β-phosphoryl group more tightly than with the γ-phosphoryl group of ATP. On binding, Mg2+ becomes located closer to the β-phosphoryl group. This asymmetric location of Mg2+ weakens the chemical bond of the terminal bridged phosphoryl group, thus causing specific cleavage of this group. This mechanism was confirmed by an ab initio molecular orbital calculation, and by experiments on the stability of ATP in aqueous solution.