The mitochondrial cytochrome bc 1 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 bc 1 catalysis and inhibition of peptide-processing activity during complexmaturation. The functional importance of the movement of extramembrane (head) domain ofISP in the bc 1 complex is confirmed by analysis of the Rhodobacter sphaeroides bc 1 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 c 1 in nativeand inhibitor-loaded beef complexes. The peptide-processing activity is activated in bovineheart mitochondrial bc 1 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 bc 1 complex is located at reduced b L and Q •− . The reaction is membranepotential-, and cytochrome c -dependent.
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