The Control of the Adenosine Triphosphatase of Rhodospirillum rubrum Chromatophores by Divalent Cations and the Membrane High Energy State

Abstract

The rate of ATP hydrolysis catalyzed by R. rubrum chromatophores is accelerated by low concentrations and inhibited by high concentrations of uncoupling agent. The inhibition at high concentrations of uncoupling agent is potentiated by the presence of free Mg2+. At low uncoupler concentrations Mg has no effect on the rate of ATP hydrolysis. Inhibition of ATP hydrolysis by high concentrations of uncoupling agent and free Mg2+ is reversed by illumination. Illumination has less effect at low Mg concentrations. Free Ca2+ inhibit ATP hydrolysis independently of the coupled state of the membrane. Under coupled conditions, Mg2+ can overcome the inhibition induced by Ca. The 2 ions compete for the same site on the enzyme. Inhibition by free Mg in highly uncoupled chromatophores and inhibition by free Ca are both non-competitive with respect to the divalent cation-ATP substrate. These data are consistent with a model in which divalent cations can bind to a site on the enzyme which is distinct from the substrate site. The regulation of the enzyme activity by the high energy state of the membrane is dependent on the occupant of this site.