Cytochrome c biogenesis System I

Cytochromes c are widespread respiratory proteins characterized by the covalent attachment of heme. The formation of c-type cytochromes requires, in all but a few exceptional cases, the formation of two thioether bonds between the two cysteine sulfurs in a -CXXCH- motif in the protein and the vinyl groups of heme. The vinyl groups of the heme are not particularly activated and therefore the addition reaction does not physiologically occur spontaneously in cells. There are several diverse post-translational modification systems for forming these bonds. Here, we describe the complex multiprotein cytochrome c maturation (Ccm) system (in Escherichia coli comprising the proteins CcmABCDEFGH), also called System I, that performs the heme attachment. System I is found in plant mitochondria, archaea and many Gram-negative bacteria; the systems found in other organisms and organelles are described elsewhere in this minireview series. System 1 for the assembly of c-type cytochromes is a multi-component system, found in many bacterial species and in mitochondria of some eukaryotes, for example plants. Amongst its constituents are an ABC-type protein, a novel chaperone for haem that binds this cofactor via a novel histidine to haem bond, and proteins for reduction of unwanted disulfides in the CXXCH motif