The role of the `Rieske' iron sulfur protein in the hydroquinone oxidation (QP) site of the cytochrome bc 1 complex

摘要

>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 <sub>L</sub> 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.