Cytochrome c2 of Rhodospirillum rubrum

The molecular properties of cytochrome c2 isolated from light grown cells of the facultative photoanaerobe, Rhodospirillum rubrum, are described as evaluated after final purification by isoelectric fractionation. The protein was found to consist of a single polypeptide chain comprising 112 amino acid residues, with freely available glutamic acid and lysine marking the amino-terminal and the carboxyl-terminal positions, respectively. Based on the presence of a single protoheme IX, a formula weight of 12,840 was obtained. The distinctively different physiological and physicochemical properties of this c-type cytochrome, as compared to mammalian cytochromes c, are discussed. The peptides resulting from digestion of the protein by trypsin and by thermolysin were isolated and purified; their compositions and amino acid sequences are presented in several tables. Extensive application of the Edman degradation to these peptides, together with positive identification of the resulting phenylthiohydantoins, afforded direct evidence for all sequence assignments within the peptides obtained. The advantages and disadvantages of hydrolysis by trypsin and thermolysin as applied to cytochrome c2 are discussed.