<p>Cytochromes c (cytC) 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.</p> <p>Ambler [<cite idref="PUB00000610"/>] recognised four classes of cytC.</p> <p>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 <taxon tax_id="1071">Rhodopila globiformis</taxon> cyt c2; class IA includes 'long' cyt c2, such as <taxon tax_id="1085">Rhodospirillum rubrum</taxon> cyt c2 and <taxon tax_id="189">Aquaspirillum itersonii</taxon> cyt c-550, which have several extra loops by comparison with class IB cytC.</p> Cytochrome c, class I