The Principle for Proton Motive Force Generated by Quinones Oxidoreduction

The proton motive force across the inner mitochondrial membrane drives adenosine triphosphate synthesis. Q-cycle is a mechanism of the proton motive force generation. Its essential feature is a bifurcation and forming semiquinone, which remains contentious. Three cathodic waves of benzoquinone appeared in dilute phosphate buffered solution (pH 3.35), which was attributed to dramatic pH change at electrode surface. These waves indicated that at different potential of the electrode, benzoquinone accepted the protons from H 3 O + , H 2 PO 4 − , and H 2 O, and produced H 2 O, HPO 4 2− , and OH − , respectively. It revealed that the pK a of hydroquinone was dependent on the potential of the electrode and external oxidant, which was attributed to the nature of proton-coupled electron transfer of quinone oxidoreduction. We proposed a mechanism that the proton motive force was directly produced by the reductant at Q i site and the oxidant at Q o site through quinone oxidoreduction.