CHARACTERIZATION OF CYTOCHROME C-556 FROM THE PURPLE PHOTOTROPHIC BACTERIUM RHODOBACTER-CAPSULATUS AS A CYTOCHROME-C PEROXIDASE

A cytochrome c-556 was purified from Rhodobacter capsulatus and the co mplete amino acid sequence was determined. It contains 328 amino acid residues and two typical heme-binding sites at cysteine residues 54 an d 57 and at residues 200 and 203. It is homologous to the family of ba cterial cytochrome c peroxidases (BCCP) with 69% identity to Paracoccu s denitrificans BCCP and 60% identity to Pseudomonas aeruginosa BCCP f or which there is a three-dimensional structure. There is lesser simil arity to the mauG gene products from methylotrophic bacteria which are thought to be involved in biosynthesis of the quinone cofactor of met hylamine dehydrogenase. Translated genes from Escherichia coli and Hel icobacter pylori are also related to the bacterial cytochrome c peroxi dases. The divergence of this family of proteins is reflected in the f act that the reported sixth heme ligands are not conserved, except in Pseudomonas, Rhodobacter and Paracoccus. This suggests that homologs o f BCCP may fold differently and/or may not have the same enzymatic act ivity as the prototypic protein from Ps. aeruginosa. We found that the Rb. capsulatus BCCP is active with both Rb. capsulatus cytochrome c, and with horse cytochrome c as substrates (K-m values 60 mu m and 6 mu m, respectively). The turnover number was 40 s(-1) and the K-m for pe roxide was 33 mu m. We have thus confirmed that the Rb. capsulatus pro tein is a cytochrome c peroxidase.