In proteins belonging to the c-type cytochrome family [1], the heme group is covalently attached by thioether bonds to two conserved cysteine residues located in the cytochrome c center. Cytochromes c typically function in electron transfer, but c-type cytochrome centers are also found in the active sites of many enzymes, and in eukaryotic cells, cytochrome c has also a role in apoptosis [2].

The known structures of c-type cytochromes have six different classes of fold. Of these, four are unique to c-type cytochromes [3]. The different folds are detailed in the example list below.

The consensus sequence for the cytochrome c center is Cys-x-x-Cys-His, where the histidine residue is one of the two axial ligands of the heme iron [4]. This arrangement is shared by all proteins known to belong to the cytochrome c family, which presently includes:

Monoheme proteins:

• Cytochrome c, an electron carrier protein located in the mitochondrial matrix. Cytochrome c is a globular protein with an all α-helice fold (see <PDB:1HRC>).
• Cytochrome c1. This is the heme-containing component of the cytochrome b-c1 complex, which accepts electrons from Rieske protein and transfers electrons to cytochrome c in the mitochondrial respiratory chain.
• Bacterial class II cytochromes c (c' and c556). Cytochrome c' is a high-spin protein and is the most widely occurring bacterial c-type cytochrome. Cytochrome c556 is a low-spin cytochrome . Both have a C-terminal c-type cytochrome center. Class II cytochromes are composed of four α helices (see <PDB:1CPR>).
• Cytochromes c2, c5 and c6.
• Bacterial cytochromes c550 to c553 and c555.
• Chloroplast and cyanobacteria cytochrome f. It translocates protons across the thylakoid membrane and transfers electrons from photosystem II to photosystem I. Structurally, cytochrome f is unique in the cytochrome c family as it is an all β-strand fold (see <PDB:1CTM>).
• Bacteria cytochrome c oxidase, mono-heme subunit, FixO.

Multiheme proteins (prokaryotes). They are frequently associated with electron-transport processes within the nitrogen and sulphur cycles:

• Cytochrome c nitrite reductase, each monomere contains five heme groups clustered in a pseudo-two fold structure (see <PDB:1QDB>).
• Cytochrome c3. It participates in sulfate respiration coupled with phosphorylation by transferring electrons from the enzyme dehydrogenase to ferredoxin. It binds 4 heme groups per subunit.
• Cytochrome c4. It binds 2 heme groups per subunit.
• Cytochromes cc3/Hmc (High-molecular-weight cytochrome c), binds 16 heme groups per subunit.
• Purple bacteria photosynthetic reaction center. It binds four heme groups.
• HAO (hydroxylamine oxidoreductase). It catalyzes the oxidation of hydroxylamine to nitrite. The electrons released in the reaction are partitioned to ammonium monooxygenase and to the respiratory chain. It binds eight heme groups per subunit.
• Cytochrome c554. It is the immediate acceptor of electrons from HAO. It binds four heme groups per subunit.
• Flavocytochrome fumarate. It catalyzes unidirectional fumarate reduction using artificial electron donors such as methyl viologen. It binds four heme groups per subunit.

To recognize c-type cytochrome family proteins we have developed 4 profiles. The first one recognizes all mono-heme cytochrome c proteins (except class II and f-type cytochromes). The second one recognizes cytochrome c that binds more than one heme group. The third one recognizes class II cytochrome c and the fourth one is directed against cytochrome f family.