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 bc1 complex of the respiratory chain. It employs the redox-dependent protonation and deprotonation of ubiquinone (coenzyme Q10) to translocate protons across 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.
Universitätsklinikum Frankfurt, Institut für Biochemie I, Frankfurt am Main, Germany. brandt@zbc.klinik.uni-frankfurt.de
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D000818:Animals; D014450:Electron Transport Complex III / Q000737:chemistry / Q000378:metabolism; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism; D008956:Models, Chemical; D016660:NAD(P)H Dehydrogenase (Quinone) / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D010101:Oxygen Consumption; D011522:Protons; D014451:Ubiquinone / Q000737:chemistry / Q000378:metabolism*
D007506:Iron-Sulfur Proteins; D011522:Protons; C037989:Rieske iron-sulfur protein; D014451:Ubiquinone; D016660:NAD(P)H Dehydrogenase (Quinone); D014450:Electron Transport Complex III