Cytochrome c biogenesis in mitochondria – Systems III and V

In c‐type cytochromes, heme becomes covalently attached to the polypeptide chain by a reaction between the vinyl groups of the heme and cysteine thiols from the protein. There are two such cytochromes in mitochondria: cytochrome c and cytochrome c1. The heme attachment is a post‐translational modification that is catalysed by different biogenesis proteins in different organisms. Three types of biogenesis system are found or predicted in mitochondria: System I (the cytochrome c maturation system); System III (termed holocytochrome c synthase (HCCS) or heme lyase); and System V. This review focuses primarily on cytochrome c maturation in mitochondria containing HCCS (System III). It describes what is known about the enzymology and substrate specificity of HCCS; the role of HCCS in human disease; import of HCCS into mitochondria; import of apocytochromes c and c1 into mitochondria and the close relationships with HCCS‐dependent heme attachment; and the role of the fungal cytochrome c biogenesis accessory protein Cyc2. System V is also discussed; this is the postulated mitochondrial cytochrome c biogenesis system of trypanosomes and related organisms. No cytochrome c biogenesis proteins have been identified in the genomes of these organisms whose c‐type cytochromes also have a unique mode of heme attachment. In c‐type cytochromes, heme becomes covalently attached to the polypeptide chain in a posttranslational modification that is catalysed by distinct proteins in different organisms. This review describes cytochrome c biogenesis in mitochondria that use the enzyme holocytochrome c synthase (also called heme lyase and System III) and in the mitochondria of trypanosomes and related organisms which use biogenesis System V