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 [<cite idref="PUB00053356"/>]. <p>Ambler [<cite idref="PUB00000610"/>] recognised four classes of cytC.</p> <p>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 redoxpotentials in the range 0 to -400 mV [<cite idref="PUB00003572"/>]. The 3D structures of a number of cyt C3 proteins have been determined. The proteinsconsist of 4-5 alpha-helices and 2 beta-strands wrapped around a compactcore 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 [<cite idref="PUB00003572"/>].</p> Cytochrome c, class III domain