Mitochondrial cytochrome c biogenesis: no longer an enigma

•Mitochondrial HCCS attaches heme to cytochrome c by a four-step mechanism.•Heme controls biogenesis by binding HCCS, thus allowing cytochrome to bind.•Stereochemical attachment leads to the release of cytochrome c from HCCS.•Mechanisms are compared to cytochrome biogenesis in bacteria with unifying themes.

Cytochromes c (cyt c) and c1 are heme proteins that are essential for aerobic respiration. Release of cyt c from mitochondria is an important signal in apoptosis initiation. Biogenesis of c-type cytochromes involves covalent attachment of heme to two cysteines (at a conserved CXXCH sequence) in the apocytochrome. Heme attachment is catalyzed in most mitochondria by holocytochrome c synthase (HCCS), which is also necessary for the import of apocytochrome c (apocyt c). Thus, HCCS affects cellular levels of cyt c, impacting mitochondrial physiology and cell death. Here, we review the mechanisms of HCCS function and the roles of heme and residues in the CXXCH motif. Additionally, we consider concepts emerging within the two prokaryotic cytochrome c biogenesis pathways.