Cytochrome b/b6 is an integralmembrane protein of approximately 400amino acid residues that probably has 8transmembrane segments. In plants and cyanobacteria, cytochrome b6 consists of twoprotein subunits encoded by the petB and petD genes. Cytochrome b/b6 non-covalently binds twoheme groups, known as b562 and b566. Four conserved histidine residues are postulated to be theligands of the iron atoms of these two heme groups.[2][3]
The heme groups are key parts of the internal electron transfer pathway and indispensable to the functioning of the two quinol oxidizing complexes. Two units of b/b6 also form a quinol entry pathway.[4]
Cytochrome b is commonly used as a region ofmitochondrial DNA for determiningphylogenetic relationships between organisms, due to its sequence variability. It is considered to be most useful in determining relationships withinfamilies andgenera. Comparative studies involving cytochromeb have resulted in new classification schemes and have been used to assign newly described species to a genus as well as to deepen the understanding of evolutionary relationships.[5]
Mutations in cytochromeb primarily result inexercise intolerance in human patients; though more rare, severe multi-system pathologies have also been reported.[6]
Single-point mutations in cytochromeb ofPlasmodium falciparum andP. berghei are associated with resistance to the anti-malarial drugatovaquone.[7]
^Siregar JE, Syafruddin D, Matsuoka H, Kita K, Marzuki S (June 2008). "Mutation underlying resistance ofPlasmodium berghei to atovaquone in the quinone binding domain 2 (Qo(2)) of the cytochrome b gene".Parasitology International.57 (2):229–32.doi:10.1016/j.parint.2007.12.002.PMID18248769.
