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

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Abstract
The protonmotive Q‐cycle is the generally accepted reaction scheme of the cytochrome bc1 complex of the respiratory chain. It employs the redox‐dependent protonation and deprotonation of ubiquinone (coenzyme Q10) 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 bc1 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 to exclude a classical Q‐cycle type mechanism. Therefore, a “semiquinone switch” mechanism is proposed involving one tightly 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.