Proton translocation in the respiratory chain involving ubiquinone - a hypothetical semiquinone switch mechanism for complex I

The protonmotive Q-cycle is the generally accepted reaction scheme of the cytochrome bc(1) complex of the respiratory chain. It employs the redox-dependent protonation and deprotonation of ubiquinone (coenzyme Q(10)) to translocate protons accross the inner mitochondrial or bacterial plasma membrane. The requirements for the operation of a similar mechanism in proton translocating NADH:ubiquinone oxidoreductase (complex I) and the specific roles of the 'Rieske' iron-sulfur center in the cytochrome bet complex and iron-sulfur center N-2 in complex I are discussed. Recent results suggest that there is only one ubiquinone-reactive site in complex I which seems Co exclude a classical Q-cycle type mechanism. Therefore, a "semiquinone switch" mechanism is proposed involving one lightly bound and one substrate quinone. It is based on the same principles as a Q-cycle, but is a localized rather than a ligand conduction type mechanism.