BINDING OF CYTOCHROME-C TO THE CYTOCHROME-BC1 COMPLEX (COMPLEX-III) AND ITS SUBUNITS CYTOCHROME-C1 AND CYTOCHROME-B

Cytochrome c1, the electron donor for cytochrome c, is a subunit of the mitochondrial cytochrome bc1 complex (complex III, cytochrome c reductase). To test if cytochrome c1 is the cytochrome c-binding subunit of the bc1 complex, binding of cytochrome c to the complex and to isolated cytochrome c1 was compared by a gel-filtration method under non-equilibrium conditions (a bc1 complex lacking the Rieske ironsulfur protein was used; von Jagow et al. (1977) Biochim. Biophys. Acta 462, 549-558). The approximate stoichiometries and binding affinities were found to be very similar. Binding of cytochrome c to isolated cytochrome b which is another subunit of the reductase was not detectable by the gel-filtration method. Further, the same lysine residues of cytochrome c were shielded towards chemical acetylation in the complexes c:c1 and c:bc1. From this we conclude that the same surface area of cytochrome c is in direct contact with cytochrome bc1 and with cytochrome c1 in the respective complexes and that therefore cytochrome c is most probably the structural ligand for cytochrome c in mitochondrial cytochrome c reductase. © 1979.