Studies on the CoQH2-cytochrome c reductase segment of the respiratory chain of yeast mitochondria, using mutants of the cytochrome b split gene

Our work relating to the role of cytochrome b in the CoQH2-cytochrome c reductase segment of the respiratory chain of S. cerevisiae mitochondria is reviewed here and new results are reported. The results concerning the structure-function relationship of cytochrome b in this complex, analyzed within the framework of the eight transmembrane alpha helice cytochrome b folding model, agree with the following features of the proton motive Q cycle (or SQ cycle): i) the antimycin A and myxothiazol binding domains are located on opposite sides of the inner mitochondrial membrane; and ii) the antimycin A binding domain is associated with the b562 domain, the myxothiazol domain with the b565 domain. These results were obtained from structural data derived from amino-acid sequence studies on mit- mutants and from biochemical studies of these mutants. However, functional studies are reported here that are not in agreement with the following features of the above models: i) the serial arrangement of the two hemes of cytochrome b and ii) the isolation of cytochrome b from redox changes with the couple fumarate/succinate in the presence of antimycin A and myxothiazol.