Cytochrome c superfamily

After cytochrome c is synthesized in the cytoplasm as apocytochrome c, it is transported through the outer mitochondrial membrane to the intermembrane space, where haem is covalently attached by thioester bonds to two cysteine residues located in the cytochrome c centre. Cytochrome c is required during oxidative phosphorylation as an electron shuttle between Complex III (cytochrome c reductase) and IV (cytochrome c oxidase). In addition, cytochrome c is involved in apoptosis in more complex organisms such as Xenopus, rats and humans. Cellular stress can induce cytochrome c release from the mitochondrial membrane. In mammals, cytochrome c triggers the assembly of the apoptosome, consisting of cytochrome c, Apaf-1 and dATP, which activates caspase-9, leading to cell death [12729583, 10707095]. There are several different members of the cytochrome c family with different functional roles, for instance cytochrome c549 is associated with photosystem II [11315568].

The known structures of c-type cytochromes have six different classes of fold. Of these, four are unique to c-type cytochromes [12594933, 2166169]. The consensus sequence for the cytochrome c centre is Cys-X-X-Cys-His, where the histidine residue is one of the two axial ligands of the haem iron [10647174]. This arrangement is shared by all proteins known to belong to the cytochrome c family, which presently includes both mono-haem proteins and multi-haem proteins. This entry represents mono-haem cytochrome c proteins (excluding class II and f-type cytochromes), such as cytochromes c, c1, c2, c5, c555, c550 to c553, c556, and c6.

Cytochrome c-type centres are also found in the active sites of many enzymes, including cytochrome cd1-nitrite reductase as the N-terminal haem c domain, in quinoprotein alcohol dehydrogenase as the C-terminal domain, in Quinohemoprotein amine dehydrogenase A chain as domains 1 and 2, and in the cytochrome bc1 complex as the cytochrome bc1 domain.