<biochemistry> Any electron transfer haemoprotein having a mode of action in which the transfer of a single electron is effected by a reversible valence change of the central iron atom of the haem prosthetic group between the +2 and +3 oxidation states.

Classified as cytochromes a in which the haem contains a formyl side chain, cytochromes b, which contain protohaem or a closely similar haem that is not covalently bound to the protein, cytochromes C in which protohaem or other haem is covalently bound to the protein and cytochromes d in which the iron tetrapyrrole has fewer conjugated double bonds than the haems have. Well known cytochromes have been numbered consecutively within groups and are designated by subscripts (beginning with no subscript), for example cytochromes C, c1, C2,. New cytochromes are named according to the wavelength in nanometres of the absorption maximum of the a band of the iron (II) form in pyridine, for example, C 555.

Origin: Gr. Chroma = colour

(11 Mar 2008)