The physiological ligands adp and Pi modulate the degree of intrinsic coupling in the ATP synthase of the photosynthetic bacterium rhodobacter capsulatus

In ATP synthases from several sources an uncoupling between ATP hydrolysis and proton pumping has been occasionally reported as a result of chemical modifications or mutagenesis.In the work to be presented, an intrinsic uncoupling has been found in the native, wild-type enzyme. The proton pumping and the ATP hydrolysis activities of the ATP synthase of Rb. capsulatus have been compared under the same experimental conditions, as a function of ADP and Pi concentration. The ADP concentrations were clamped in the steady state by coupling the ATP hydrolysis to the pyruvate kinase reaction. The proton pumping was measured either with the pH difference probe ACMA or with the electric potential difference probe oxonol VI, obtaining consistent results. The comparison indicates that an intrinsic uncoupling of ATP synthase is induced when the concentration of either ligand is decreased. The half-maximal effect was found in the submicromolar range for ADP and at about 70 AM for Pi (at pH 8). These data indicate that a switching from a partially uncoupled state of ATP synthase to the coupled state is induced by the simultaneous binding of ADP and Pi. It is proposed that this switching could correspond to the position switching of the C-terminal alpha-helix hairpin of the epsilon subunit from FO(‘‘down-state’’) to F1 to (‘‘up-state’’).