Haem type	Formal iron oxidation states
Haem c	FeII; FeIII

Cytochromes c (cyt c) can be defined as electron­transfer proteins having one or several haem c groups, bound to the protein by one or, more commonly two, thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cyt c possess a wide range of properties and function in a large number of different redox processes [1, 2]. Ambler [3] recognised four classes of cyt c:

• Class I includes the low­spin soluble cyt c of mitochondria and bacteria, with the haem­attachment site towards the N­terminus, and the sixth ligand provided by a methionine residue about 40 residues further on towards the C­terminus. The proteins contain three conserved "core" helices which form a "basket" around the haem group with one haem edge exposed to the solvent.
• Class II includes the high­spin cyt c' and a number of low­spin cytochromes, e.g. cyt c­556. The haem­attachment site is close to the C­terminus. The protein fold comprises a four­­helix bundle [4].
• Class III comprises the low redox potential multiple­haem cytochromes: cyt c₇ (trihaem), c₃ (tetrahaem), and high­molecular­weight cyt c (HMC; hexadecahaem), with only 30-40 residues per haem group. The haem c groups, all bis­His coordinated, are structurally and functionally nonequivalent and present different redox potentials in the range 0 to -400 mV [5].
• Class IV was originally created to hold the complex proteins that have other prosthetic groups as well as haem c, e.g. flavocytochrome c and cytochromes cd. Alternatively, Moore and Pettigrew [2] have suggested that Class IV cyt c are tetrahaem proteins containing both bis­His and His­Met coordinated haems, with a 3­D structure exemplified by that of the photosynthetic reaction centre (PRC) cyt c, and form a structurally homogeneous family.

A number of cyt c are known which do not fit readily into either Class I, II or III [3]. For example, in the dihaem cyt c peroxidase from Pseudomonas aeruginosa, which consists of two Class I cyt c­like domains, the high­potential (+320 mV) haem is ligated by His and Met, whereas the low­potential (-330 mV) haem has two His axial ligands [9]. The hexahaem protein nitrite reductase contains five low­spin (bis­His coordinated) haem groups and one high­spin haem, which was found to bind NO [10]. N­Terminal domain of cyt cd₁ nitrite reductase was tentatively assigned to the Class I [2]; however, the threading and connectivity of the helices and haem c iron coordination (bis­His) are different from those of Class I cyt c [11].

The features of cyt c from the Classes I-IV as well as cyt c₁, cyt f, cyt cd₁, cyt c₅₅₄ and HAO are summarised in the table [1, 3].

Class	Haem iron coordination	Axial iron ligands	Number of haems	Redox potential (mV)	Proteins
I	Hexacoordinate	NHis; SMet	1 to 2	0 to +500	IA: `Long' cyt c2 IB: Mitochondrial `Short' cyt c2 IC: Split­­band cyt c6 cyt c­554 cyt c4 (dihaem) ID: Cyt c8 IE: Cyt c5
IIa	Pentacoordinate / Hexacoordinate	NHis; (CO, NO, CN¯ or other ligand)	1	0 to +150	Cyt c'
IIb	Hexacoordinate	NHis; SMet	1	0 to +500	Cyt c­556
III	Hexacoordinate	NHis; NHis	3 to 16	-400 to 0	Cyt c3 (4­haem) Cyt c7 (3­haem) HMC (16­haem)
IV	Hexacoordinate	NHis; SMet	4	-80 to +400	PRC cyt c
	Hexacoordinate	NHis; NHis
c1	Hexacoordinate	NHis; SMet	1	+220 to +290	Cyt c1
c554	Hexacoordinate	NHis; NHis	1	-276 to +47	Cyt c554
	Pentacoordinate	NHis	1	+47
	Hexacoordinate	NHis; NHis	2	-276 to +47
cd1 c­domain	Hexacoordinate	NHis; NHis	1	+190 to +290	Cyt cd1
f	Hexacoordinate	NHis; NTyr	1	+300 to +370	Cyt f
HAO	Pentacoordinate / Hexacoordinate	NHis; (substrate)	1	-322 to -203	Hydroxylamine oxidoreductase
	Hexacoordinate	NHis; NHis	7	-412 to +300

Cytochromes c in PROMISE

PROMISE ID	Description
CYTC1	Cytochromes c1
CYTC554	Tetrahaem cytochrome c554 from Nitrosomonas europaea
CYTCD1	Cytochrome cd1 nitrite reductase (cytochrome oxidase)
CYTCI	Class I cytochromes c (mitochondrial cyt c, cyt c2, cyt c4, cyt c5, cyt c­554, cyt c6, cyt c8)
CYTCII	Class II cytochromes c (cyt c', cyt c­556)
CYTCIII	Class III cytochromes c (bacterial multihaem cytochromes: cyt c3, cyt c7, HMC)
CYTCIV	Class IV cytochromes c (photosynthetic reaction centre cyt c subunit)
CYTF	Cytochromes f
HAO	Hydroxylamine oxidoreductase

Cytochromes c in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
CYTOCHROMEC1	PR00603	CYTOCHROME_C	PS00190	BL00190
CYTCHRMECIAB	PR00604
CYTCHROMECIC	PR00605
CYTCHROMECID	PR00606
CYTCHROMECIE	PR00607
CYTCHROMECII	PR00608
CYTOCHROMEC3	PR00609
CYTOCHROMEF	PR00610

References

1. Pettigrew, G.W. and Moore, G.R. (1987) Cytochromes c. Biological Aspects. Springer­Verlag, Berlin - Heidelberg - New York.
2. Moore, G.R. and Pettigrew, G.W. (1990) Cytochromes c. Evolutionary, Structural, and Physicochemical Aspects. Springer­Verlag, Berlin - Heidelberg - New York.
3. Ambler, R.P. (1991) Sequence variability in bacterial cytochromes c. Biochim. Biophys. Acta 1058, 42-47.
4. Moore, G.R. (1991) Bacterial 4­­helical bundle cytochromes. Biochim. Biophys. Acta 1058, 38-41.
5. Coutinho, I.B. and Xavier, A.V. (1994) Tetraheme cytochromes. Methods Enzymol. 243, 119-140.
6. Prince, R.C. and George, G.N. (1995) Cytochrome f revealed. Trends Biochem. Sci. 20, 217-218.
7. Prince, R.C. and George, G.N. (1997) The remarkable complexity of hydroxylamine oxidoreductase. Nature Struct. Biol. 4, 247-250.
8. Iverson, T.M., Arciero, D.M., Hsu, B.T., Logan, M.S.P., Hooper, A.B. and Rees, D.C. (1998) Heme packing motifs revealed by the crystal structure of the tetra­heme cytochrome c554 from Nitrosomonas europaea. Nature Struct. Biol. 5, 1005-1012.
9. Fülöp, V., Ridout, C.J., Greenwood, C. and Hajdu, J. (1995) Crystal structure of the di­haem cytochrome c peroxidase from Pseudomonas aeruginosa. Structure 3, 1225-1233.
10. Moura, J.J.G., Costa, C., Liu, M.­Y., Moura, I. and LeGall, J. (1991) Structural and functional approach toward a classification of the complex cytochrome c system found in sulfate­reducing bacteria. Biochim. Biophys. Acta 1058, 61-66.
11. Fülöp, V., Moir, J.W.B., Ferguson, S.J. and Hajdu, J. (1995) The anatomy of a bifunctional enzyme: structural basis for reduction of oxygen to water and synthesis of nitric oxide by cytochrome cd₁. Cell 81, 369-377.

Reviews on cytochromes c