c-Type cytochromes are characterized by covalent attachment of haem to protein through thioether bonds between the vinyl groups of the haem and the thiols of a Cys-Xaa-Xaa-Cys-His motif. Proteins of this type play crucial roles in the biochemistry of the nitrogen cycle. Many Gram-negative bacteria use the Ccm (cytochrome c maturation) proteins for the post-translational haem attachment to their c-type cytochromes. The Ccm system can correctly mature c-type cytochromes with CCXXCH, CCXCH, CXCCH and CXXCHC motifs, even though these are not found naturally and the extra cysteine might, in principle, disrupt the biogenesis proteins. The non-occurrence of these motifs probably relates to the destructive chemistry that can occur if a free thiol reacts with haem iron to generate a radical.
The Escherichia coli cytochrome c maturation (Ccm) apparatus can mature cytochromes with an extra cysteine within or adjacent to the CXXCH motif
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J.W.A. Allen, S.J. Ferguson; The Escherichia coli cytochrome c maturation (Ccm) apparatus can mature cytochromes with an extra cysteine within or adjacent to the CXXCH motif. Biochem Soc Trans 1 February 2006; 34 (1): 91–93. doi: https://doi.org/10.1042/BST0340091
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