Interactions of quinone with the iron-sulfur protein of the bc₁ complex: Is the mechanism spring-loaded?

Since available structures of native bc₁ complexes show a vacant Q_o-site, occupancy by substrate and product must be investigated by kinetic and spectroscopic approaches. In this brief review, we discuss recent advances using these approaches that throw new light on the mechanism. The rate-limiting reaction is the first electron transfer after formation of the enzyme-substrate complex at the Q_o-site. This is formed by binding of both ubiquinol (QH₂) and the dissociated oxidized iron-sulfur protein (ISP_ox). A binding constant of ∼14 can be estimated from the displacement of E_m or pK for quinone or ISP_ox, respectively. The binding likely involves a hydrogen bond, through which a proton-coupled electron transfer occurs. An enzyme-product complex is also formed at the Q_o-site, in which ubiquinone (Q) hydrogen bonds with the reduced ISP (ISPH). The complex has been characterized in ESEEM experiments, which detect a histidine ligand, likely His-161 of ISP (in mitochondrial numbering), with a configuration similar to that in the complex of ISPH with stigmatellin. This special configuration is lost on binding of myxothiazol. Formation of the H-bond has been explored through the redox dependence of cytochrome c oxidation. We confirm previous reports of a decrease in E_m of ISP on addition of myxothiazol, and show that this change can be detected kinetically. We suggest that the myxothiazol-induced change reflects loss of the interaction of ISPH with Q, and that the change in E_m reflects a binding constant of ∼4. We discuss previous data in the light of this new hypothesis, and suggest that the native structure might involve a less than optimal configuration that lowers the binding energy of complexes formed at the Q_o-site so as to favor dissociation. We also discuss recent results from studies of the bypass reactions at the site, which lead to superoxide (SO) production under aerobic conditions, and provide additional information about intermediate states.