Kinetic properties and equilibrium constant of the adenosine triphosphate-creatine transphosphorylase-catalyzed reaction.

catalyzed by adenosine triphosphate-creatine transphosphorylase can be described by a reasonable kinetic scheme. It was shown that MgATPzis the “true substrate,” that the Michaelis constant is the dissociation constant of the enzyme-substrate complex, and that the pH-activity curve resembles a single ionization curve with pK N 6.5. It was of interest to study the kinetics of the reverse process, the formation of ATP and creatine. Using ADP and creatine phosphate as the substrates, we found the effects of Mg++ and pH on the enzymic activity to be analogous to their effects on the forward process, and the equilibrium constant of Reaction 1, which has been previously determined at various magnesium ion concentrations (2), satisfied the Haldane equation (3), when magnesium nucleotide was considered as the “true substrate.” Furthermore, experiments with inosine and cytidine nucleotides as substrate and the effect of various anions as competitor with creatine phosphate gave some information about the active site of the enzyme.