Structural Basis of Multifunctional Bovine Mitochondrial Cytochrome bc 1 Complex

The mitochondrial cytochrome bc₁ complex is a multifunctional membrane protein complex. Itcatalyzes 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 centersand inhibitor binding sites, but also suggests a movement of the head domain of the iron–sulfurprotein (ISP) during bc₁ catalysis and inhibition of peptide-processing activity during complexmaturation. The functional importance of the movement of extramembrane (head) domain ofISP in the bc₁ complex is confirmed by analysis of the Rhodobacter sphaeroides bc₁ complexmutants with increased rigidity in the ISP neck and by the determination of rate constants foracid/base-induced intramolecular electron transfer between [2Fe–2S] and heme c₁ in nativeand inhibitor-loaded beef complexes. The peptide-processing activity is activated in bovineheart mitochondrial bc₁ complex by nonionic detergent at concentrations that inactivate electrontransfer activity. This peptide-processing activity is shown to be associated with subunits Iand II by cloning, overexpression and in vitro reconstitution. The superoxide-generation siteof the cytochrome bc₁ complex is located at reduced b_L and Q^–. The reaction is membranepotential-, and cytochrome c-dependent.