Cytochrome c maturation system on the negative side of bioenergetic membranes: CCB or System IV.

Cytochromes of the c-type contain hemes covalently attached via one or, more generally, two thioether bonds between the vinyls of heme b and the thiols of cysteine residues of apocytochromes. This post-translational modification relies on membrane-associated specific biogenesis proteins, referred to as cytochrome c maturation systems. At least three different versions (i.e. Systems I-III) are found on the positive side of bioenergetic membranes in different organisms and compartments. The present minireview is concerned with systems on the negative side of the membranes. It describes System IV, also referred to as cofactor assembly on complex C subunit B, for heme binding on cytochrome b(6) through one thioether bond; this covalent heme is usually called c(i) . This system is found in all organisms with oxygenic photosynthesis but not in Firmicutes, although they also have a cytochrome b protein with an additional heme c(i) covalently attached via a single thioether bond.