Abstract
Cytochrome c 1, the electron donor for cytochrome c , is a subunit of the mitochondrial cytochrome bc 1 complex (complex III, cytochrome c reductase). To test if cytochrome c 1 is the cytochrome c -binding subunit of the bc 1 complex, binding of cytochrome c to the complex and to isolated cytochrome c 1 was compared by a gel-filtration method under non-equilibrium conditions (a bc 1 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: c 1 and c: bc 1. From this we conclude that the same surface area of cytochrome c is in direct contact with cytochrome bc 1 and with cytochrome c 1 in the respective complexes and that therefore cytochrome c is most probably the structural ligand for cytochrome c in mitochondrial cytochrome c reductase.
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