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 K1 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 (K(i) = 2.10(-6) M) in the presence of stoichiometric ADP and the affinity for inhibitory Mg2+ decreases to a K(i) value of 7-10(-5) M when the second nucleotide binding site with K(d) = 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 Mg2+-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.