Cytochrome c belongs to class I of the c-type cytochrome family and contains a characteristic CXXCH (cysteine-any-any-cysteine-histidine) amino acid motif that binds heme. This motif is located towards the N-terminus of the peptide chain and contains a histidine as the 5th ligand of the heme iron. The 6th ligand is provided by a methionine residue found towards the C-terminus. The protein backbone is folded into five α-helices that are numbered α1-α5 from N-terminus to C-terminus. Helices α3, α4 and α5 are referred to as 50s, 60s and 70s helices, respectively, when referring to mitochondrial cytochrome c.