The role of the 'Rieske' iron sulfur protein in the hydroquinone oxidation (Qp) site of the cytochrome bc(1) complex - The 'proton-gated affinity change' mechanism

The essential reaction in the widely accepted protonmotive Q-cycle mechanism of the bc(1) complex is the bifurcation of the electron flow during hydroquinone oxidation at the hydroquinone oxidation (Q(P)) site formed by the 'Rieske' iron sulfur protein and by the heme b(L) domain of cytochrome b, The 'Rieske' [2Fe-2S] cluster has a unique structure containing two exposed histidine ligands, which are the binding site for quinones, The affinity of the 'Rieske'' cluster for quinones increases several orders of magnitude upon reduction; this will stabilize semiquinone at the Qp site, Based on this affinity change, a reaction scheme is presented,which can explain the bifurcation of the electron flow without invoking highly unstable semiquinone species. (C) 1997 Federation of European Biochemical Societies.