Biochemical requirements for the maturation of mitochondrial c-type cytochromes

Cytochromes c are metalloproteins that function in electron transfer reactions and contain a heme moiety covalently attached via thioether linkages between the co-factor and a CXXCH motif in the protein. Covalent attachment of the heme group occurs on the positive side of all energy-transducing membranes (bacterial periplasm, mitochondrial intermembrane space and thylakoid lumen) and requires minimally: 1) synthesis and translocation of the apocytochromes c and heme across at least one biological membrane, 2) reduction of apocytochromes c and heme and maintenance under a reduced form prior to 3) catalysis of the heme attachment reaction. Surprisingly, the conversion of apoforms of cytochromes c to their respective holoforms occurs through at least three different pathways (systems I, II and III). In this review, we detail the assembly process of soluble cytochrome c and membrane-bound cytochrome c1, the only two mitochondrial c-type cytochromes that function in respiration. Mitochondrial c-type cytochromes are matured in the intermembrane space via the system I or system III pathway, an intriguing finding considering that the biochemical requirements for cytochrome c maturation are believed to be common regardless of the energy-transducing membrane under study.