DEPENDENCE OF KINETIC-PARAMETERS OF CHLOROPLAST ATP SYNTHASE ON EXTERNAL PH, INTERNAL PH, AND DELTA-PH

ATP synthesis by the membrane-bound chloroplast ATPase in the oxidized state of its gamma-disulfide bridge was studied as a function of the ADP concentration, DELTA-pH, and external pH values, under conditions where DELTA-pH was clamped and delocalized. At a given pH, the rate of phosphorylation at saturating ADP concentration (V(max)) and the Michaelis constant K(m) (ADP) depend strictly on DELTA-pH, irrespective of the way the DELTA-pH is generated: there evidently is no specific interaction between the redox carriers and the ATPase. It was also shown that both K(m) (ADP) and V(max) depend on DELTA-pH, not on the external or internal pH. This suggests that internal proton binding and external proton release are concerted, so that net proton translocation is an elementary step of the phosphorylation process. These results appear to be consistent with a modified "proton substrate' model, provided the DELTA-G0 of the condensation reaction within the catalytic site is low. At least one additional assumption, such as a shift in the pK of bound phosphate or the existence of an additional group transferring protons from or to reactants, is nevertheless required to account for the strict DELTA-pH dependence of the rate of ATP synthesis. A purely "conformational" model, chemically less explicit, only requires constraints on the pK's of the groups involved in proton translocation.