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>Ambler [<cite idref="PUB00000610"/>] recognised four classes of cytC.</p> <p>Class I includes the low-spinsoluble cytC of mitochondria and bacteria, with the haem-attachment sitetowards the N terminus, and the sixth ligand provided by a methionineresidue about 40 residues further on towards the C terminus. On the basisof sequence similarity, class I cytC were further subdivided into fiveclasses, IA to IE.Class ID (cyt c8) includes such bacterial proteins as Pseudomonas spp. cyt c-551, <taxon tax_id="940">Hydrogenobacter thermophilus</taxon> cyt c-552 and <taxon tax_id="1066">Rhodocyclus tenuis</taxon> (Rhodospirillum tenue) cyt c-553 [<cite idref="PUB00000610"/>]. Sequence characteristics include severalPro residues around the sixth ligand Met, and a conserved Trp residue nearthe C terminus.</p> <p>The 3D structures of cyt C-551 from <taxon tax_id="287">Pseudomonas aeruginosa</taxon> and <taxon tax_id="316">Pseudomonas stutzeri</taxon> have been determined [<cite idref="PUB00004589"/>]. The proteins consist of 5 alpha-helices;three 'core' helices form a 'basket' around the haem group, with one haem edge exposed to the solvent.</p> Cytochrome c, class ID