The role of the 'Rieske' iron sulfur protein in the hydroquinone oxidation (Q(P)) site of the cytochrome bc1 complex. The 'proton-gated affinity change' mechanism.

The essential reaction in the widely accepted proton-motive Q-cycle mechanism of the bc1 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 bL 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 Q(P) 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.