Kinetic Equivalence of Transmembrane pH and Electrical Potential Differences in ATP Synthesis

ATP synthase is the key player of Mitchell's chemiosmotic theory, converting the energy of transmembrane proton flow into the high energy bond between ADP and phosphate. The proton motive force that drives this reaction consists of two components, the pH difference (Delta pH) across the membrane and transmembrane electrical potential (Delta psi). The two are considered thermodynamically equivalent, but kinetic equivalence in the actual ATP synthesis is not warranted, and previous experimental results vary. Here, we show that with the thermophilic Bacillus PS3 ATP synthase that lacks an inhibitory domain of the epsilon subunit, Delta pH imposed by acid-base transition and Delta psi produced by valinomycin-mediated K+ diffusion potential contribute equally to the rate of ATP synthesis within the experimental range examined (Delta pH -0.3 to 2.2, Delta psi -30 to 140 mV, pH around the catalytic domain 8.0). Either Delta pH or Delta psi alone can drive synthesis, even when the other slightly opposes. Delta psi was estimated from the Nernst equation, which appeared valid down to 1 mM K+ inside the proteoliposomes, due to careful removal of K+ from the lipid.