Cytochrome cc3 <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>This family, found in sulphate-reducers, represents a cytochrome containing sixteen haem groups which forms part of a transmembrane protein complex that allows electron flow from the periplasmic hydrogenase to the cytoplasmic enzymes that catalyse reduction of sulphates [<cite idref="PUB00028007"/>]. HmcA (<db_xref db="SWISSPROT" dbkey="P24092"/>) from <taxon tax_id="882">Desulfovibrio vulgaris</taxon> strain Hildenborough is composed of three distinct regions; an N-terminal three-haem domain homologous to cytochrome c7, a four-haem cyctochrome c3-like domain, and a C-terminal nine-haem cytochrome Hcc-like domain [<cite idref="PUB00025401"/>, <cite idref="PUB00021717"/>]. This last domain is composed of two cytochrome c3-like domains with an isolated haem group inserted between them. HcmA interacts specifically with cytochrome c3 [<cite idref="PUB00028008"/>], where the last haem group of HcmA and haem four of cytochrome c3 interact to provide the likely site of electron transfer between molecules [<cite idref="PUB00025401"/>].</p>