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Cytochromes ''c'' (cytC) are electron-transfer proteins having one or several heme c groups, bound to the protein by one or, more generally, two thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cytochromes ''c'' possess a wide range of properties and function in a large number of different redox processes. The founding member of this family is mitochondrial cytochrome c. Ambler recognized four classes of cytC. *Class I includes the low-spin soluble cytC of mitochondria and bacteria, with the haem-attachment site towards the N-terminus, and the sixth ligand provided by a methionine residue about 40 residues further on towards the C-terminus. On the basis of sequence similarity, class I cytC were further subdivided into five classes, IA to IE. Class IB includes the eukaryotic mitochondrial cytC and prokaryotic 'short' cyt c2 exemplified by ''Rhodopila globiformis'' cyt c2; class IA includes 'long' cyt c2, such as ''Rhodospirillum rubrum'' cyt c2 and ''Aquaspirillum itersonii'' cytc-550, which have several extra loops by comparison with class IB cytC. *Class II includes the high-spin cytC' and a number of low-spin cytochromes, e.g. cyt c-556. The haem-attachment site is close to the C terminus. The cytC' are capable of binding such ligands as CO, NO or CN(-), albeit with rate and equilibrium constants 100 to 1,000,000-fold smaller than other high-spin haemoproteins. This, coupled with its relatively low redox potential, makes it unlikely that cytC' is a terminal oxidase. Thus cytC' probably functions as an electron transfer protein. The 3D structures of a number of cytC' have been determined. The molecule usually exists as a dimer, each monomer folding as a four-alpha-helix bundle incorporating a covalently-bound haem group at the core.〔 The ''Chromatium vinosum'' cytC' exhibits dimer dissociation upon ligand binding. *Class III comprises the low redox potential multiple haem cytochromes: cyt C7 (trihaem), C3 (tetrahaem), and high-molecular-weight cytC, HMC (hexadecahaem), with only 30-40 residues per haem group. The haem c groups, all bis-histidinyl coordinated, are structurally and functionally nonequivalent and present different redox potentials in the range 0 to -400 mV. The 3D structures of a number of cyt C3 proteins have been determined. The proteins consist of 4-5 alpha-helices and 2 beta-strands wrapped around a compact core of four non-parallel haems, which present a relatively high degree of exposure to the solvent. The overall protein architecture, haem plane orientations and iron-iron distances are highly conserved.〔 *Class IV includes complex proteins containing other prosthetic groups besides haem c, such as flavocytochromes c and cytochromes cd.〔 ==Subfamilies== *Bacterial cytochrome c, class IC 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cytochrome c family」の詳細全文を読む スポンサード リンク
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