The presence of phosphate at a catalytic site suppresses the formation of the MgADP-inhibited form of F(1)-ATPase.

F1-ATPase is inactivated by entrapment of MgADP in catalytic sites and reactivated by MgATP or P(i). Here, using a mutant alpha(3)beta(3)gamma complex of thermophilic F(1)-ATPase (alpha W463F/beta Y341W) and monitoring nucleotide binding by fluorescence quenching of an introduced tryptophan, we found that P(i) interfered with the binding of MgATP to F(1)-ATPase, but binding of MgADP was interfered with to a lesser extent. Hydrolysis of MgATP by F(1)-ATPase during the experiments did not obscure the interpretation because another mutant, which was able to bind nucleotide but not hydrolyse ATP (alpha W463F/beta E190Q/beta Y341W), also gave the same results. The half-maximal concentrations of P(i) that suppressed the MgADP-inhibited form and interfered with MgATP binding were both approximately 20 mm. It is likely that the presence of P(i) at a catalytic site shifts the equilibrium from the MgADP-inhibited form to the enzyme-MgADP-P(i) complex, an active intermediate in the catalytic cycle.