ATP hydrolysis-driven H⁺ translocation is stimulated by sulfate, a strong inhibitor of mitochondrial ATP synthesis

Abstract

Sulfate is a partial inhibitor at low and a non-essential activator at high [ATP] of the ATPase activity of F₁. Therefore, a catalytically-competent ternary F₁•ATP•sulfate complex can be formed. In addition, the ANS fluorescence enhancement driven by ATP hydrolysis in submitochondrial particles is also stimulated by sulfate, clearly showing that the ATP hydrolysis in its presence is coupled to H⁺ translocation. However, sulfate is a strong linear inhibitor of the mitochondrial ATP synthesis. The inhibition was competitive (K _i=0.46 mM) with respect to Pi and mixed (K _i=0.60 and K′_i=5.6 mM) towards ADP. Since it is likely that sulfate exerts its effects by binding at the Pi binding subdomain of the catalytic site, we suggest that the catalytic site involved in the H⁺ translocation driven by ATP hydrolysis has a more open conformation than the half-closed one (β_HC), which is an intermediate in ATP synthesis. Accordingly, ATP hydrolysis is not necessarily the exact reversal of ATP synthesis.