Structural basis of multifunctional bovine mitochondrial cytochrome bc₁ complex

The mitochondrial cytochrome bc₁ complex is a multifunctional membrane protein complex. It catalyzes electron transfer, proton translocation, peptide processing, and superoxide generation. Crystal structure data at 2.9 Å resolution not only establishes the location of the redox centers and inhibitor binding sites, but also suggests a movement of the head domain of the iron-sulfur protein (ISP) during bc₁ catalysis and inhibition of peptide-processing activity during complex maturation. The functional importance of the movement of extramembrane (head) domain of ISP in the bc₁ complex is confirmed by analysis of the Rhodobacter sphaeroides bc₁ complex mutants with increased rigidity in the ISP neck and by the determination of rate constants for acid/base-induced intramolecular electron transfer between [2Fe-2S] and heme c₁ in native and inhibitor-loaded beef complexes. The peptide-processing activity is activated in bovine heart mitochondrial bc₁ complex by nonionic detergent at concentrations that inactivate electron transfer activity. This peptide-processing activity is shown to be associated with subunits I and II by cloning, overexpression and in vitro reconstitution. The superoxide-generation site of the cytochrome bc₁ complex is located at reduced b(L) and Q(·-). The reaction is membrane potential-, and cytochrome c-dependent.