Nucleotide/H(+)-dependent change in Mg2+ affinity at the ATPase inhibitory site of the mitochondrial F1-F0 ATP synthase.

The interactions between ADP and Mg2+ that result in the slowly reversible inhibition of the mitochondrial F1-F0 ATPase were studied. The Ki for the inhibitory Mg2+ is shown to be strongly dependent on the occupation of the nucleotide-binding sites. The inhibitory binding site for Mg2+ is not seen unless a stoichiometric amount of ADP is added [Biochem. J. 276 (1991) 149-156]; it appears (Ki = 2.10(-6) M) in the presence of stoichiometric ADP and the affinity for inhibitory Mg2+ decreases to a Ki value of 7.10(-5) M when the second nucleotide binding site with Kd = 5.10(-6) M is loaded with ADP. The binding of the inhibitory Mg2+ is competitively inhibited by H+ ions within the pH interval 6.8-8.2. The nucleotide-dependent affinity transition of the Mg(2+)-specific site suggests that H+/Mg2+ exchange may play an important role in the catalytic mechanism of ATP synthesis/hydrolysis at the active site(s) of F1-F0 ATP synthase.