Primary structure of cytochrome c' of Methylococcus capsulatus Bath: evidence of a phylogenetic link between P460 and c'-type cytochromes.

Cytochrome c' of Methylococcus capsulatus Bath is involved in electron flow from the enzyme responsible for hydroxylamine oxidation, cytochrome P460, to cytochrome C555. This cytochrome is spectrally similar to other cytochromes c' but is larger (16,000 Da) and has a lower midpoint potential (-205 mV). By a combination of Edman degradation, mass spectroscopy, and gene sequencing, we have obtained the primary structure of cytochrome c' from M. capsulatus Bath. The cytochrome shows low sequence similarity to other cytochromes c', only residues R12, Y53, G56, and the C-terminal heme-binding region (GXXCXXCHXXXK) being conserved. In contrast, cytochrome c' from M. capsulatus Bath shows considerable sequence similarity to cytochromes P460 from M. capsulatus Bath (31% identity) and from Nitrosomonas europaea (18% identity). This suggests that P460-type cytochromes may have originated from a c'-type cytochrome which developed a covalent cross-link between a lysine residue and the c'-heme.