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 cytochrome b 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 cytochrome b primarily result inexercise intolerance in human patients; though more rare, severe multi-system pathologies have also been reported.[6]
Single-point mutations in cytochrome b 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.
