Cytochrome b

Cytochrome b/b6 is an integral membrane protein of approximately 400 amino acid residues that probably has 8 transmembrane segments. In plants and cyanobacteria, cytochrome b6 consists of two protein subunits encoded by the petB and petD genes. Cytochrome b/b6 non-covalently binds two heme groups, known as b562 and b566. Four conserved histidine residues are postulated to be the ligands of the iron atoms of these two heme groups.

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.

Cytochrome b is commonly used as a region of mitochondrial DNA for determining phylogenetic relationships between organisms, due to its sequence variability. It is considered to be most useful in determining relationships within families and genera. 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.

Mutations in cytochrome b primarily result in exercise intolerance in human patients; though more rare, severe multi-system pathologies have also been reported.

Single-point mutations in cytochrome b of Plasmodium falciparum and P. berghei are associated with resistance to the anti-malarial drug atovaquone.

Human genes encoding cytochrome b proteins include:

• CYB5A, cytochrome b5 type A (microsomal)
• CYB5B, cytochrome b5 type B (outer mitochondrial membrane)
• CYBASC3, cytochrome b, ascorbate dependent 3
• MT-CYB, mitochondrially encoded cytochrome b

Cyt b is targeted by the Q_oI class of fungicides, Fungicide Resistance Action Committee group 11. The cyt b mutations G143A and F129L provide resistance against the main body of group 11, although G143A does not work against metyltetraprole (11A). G143A is significant in Botrytis cinerea in California strawberry production.