31P NMR quantitation of the displacement of equilibria of arginine, creatine, pyruvate, and 3-P-glycerate kinase reactions by substitution of sulfur for oxygen in the beta phosphate of ATP.

31P NMR measurements have been found to be a convenient means for simultaneously measuring the concentrations of several species in the equilibrium mixtures of the reactions catalyzed by arginine kinase and creatine kinase. MgATP + X in equilibrium MgADP + XP where X = arginine or creatine and XP = P-arginine or P-creatine. The free energy of phosphorylaton of various metabolites by adenosine 5'-O-(2-thiotriphosphate) at pH 8.0 and 30 degrees C is more exergonic than the corresponding phosphorylations by ATP by about 2.5 kcal/mol, resulting in a displacement of the equilibrium toward the nucleoside diphosphates by a factor of approximately 60. Since this factor does not depend on the nature of the metabolite, the equilibrium constants of thionucleotide reactions may be used to determined the equilibrium constants of corresponding oxynucleotide reactions which lie too far toward ATP. The equilibrium constants of the oxynucleotide reactions catalyzed by pyruvate kinase and 3-P-glycerate kinase calculated by this method from the experimentally determined equilibrium constants of the corresponding thionucleotide reactions are 3.1 x 10(-4) and 2.9 x 10(-4), respectively, under the experimental conditions used. The equilibrium constants and degree of stereoselectivity of the arginine kinase reaction are altered when Ca2+ replaces Mg2+ as the activating metal ion.