InterPro domain: IPR023655

Cytochrome c (CytC) proteins can be defined as electron-transfer proteins having one or several haem 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. CytC possess a wide range of properties and function in a large number of different redox processes.

Ambler [ 1 ] recognised 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 IC, 'split-alpha-band' Cyt C, possess a widened or split alpha-band of lowered absorptivity. This class includes dihaem Cyt C4 and monohaem Cyt C6 (Cyt C-553) and Cyt C-554.

The 3D structures of Chlamydomonas reinhardtii Cyt C6 [ 2 ] and Desulfovibrio vulgaris Cyt C-553 [ 3 ] have been determined. The proteins consist of 4 alpha-helices; three 'core' helices form a 'basket' around the haem group, with one haem edge exposed to the solvent.

This entry also includes Cytochrome c6 from Arabidopsis, which functions as an electron carrier between membrane-bound cytochrome b6-f and photosystem I in oxygenic photosynthesis [ 4 ].

1. Sequence variability in bacterial cytochromes c. Biochim. Biophys. Acta 1058, 42-7
2. The structure of chloroplast cytochrome c6 at 1.9 A resolution: evidence for functional oligomerization. J. Mol. Biol. 250, 627-47
3. S-class cytochromes c have a variety of folding patterns: structure of cytochrome c-553 from Desulfovibrio vulgaris determined by the multi-wavelength anomalous dispersion method. J. Biochem. 108, 701-3
4. Structure of cytochrome c6A, a novel dithio-cytochrome of Arabidopsis thaliana, and its reactivity with plastocyanin: implications for function. J. Mol. Biol. 360, 968-77