The redox properties and heme environment of cytochrome c-551.5 from Desulfuromonas acetoxidans.

The environment of the three heme groups in cytochrome c-551.5 from Desulfuromonas acetoxidans was investigated by the technique of solvent perturbation difference spectroscopy. The hemeoctapeptide from cytochrome c plus added imidazole was used as a model compound for the fully exposed chromophore. The average heme exposure in both the ferric and ferrous cytochromes c-551.5 was found to be considerably greater than that previously observed for the monoheme mitochondrial cytochrome c and Prosthecochloris cytochrome c-555. Differences in the average heme exposure for ferric and ferrous cytochromes c-551.5 suggested that a change in oxidation state is accompanied by a change in conformation. A spectrophotometric redox titration of the protein yielded a sigmoidal plot of the potential versus the logarithm of the ratio of oxidized to reduced heme. The resolved plot indicated that two hemes were characterized by a E'o of -177 mV and the third E'o of -102 mV. Each of the resolved steps had an n value of 1 indicating that cytochrome c-551.5 transfers electrons singly.