Cytochrome b

Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.

Function
In the mitochondrion of eukaryotes and in aerobic prokaryotes, cytochrome b is a component of respiratory chain complex III - also known as the bc1 complex or ubiquinol-cytochrome c reductase. In plant chloroplasts and cyanobacteria, there is an analogous protein, cytochrome b6, a component of the plastoquinone-plastocyanin reductase, also known as the b6f complex. These complexes are involved in electron transport and the generation of ATP and thus play a vital role in the cell.

Structure
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 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.

Use in phylogenetics
Cytochrome b is commonly used to determine 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 deepen the understanding of evolutionary relationships.

Clinical significance
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 Plasdmodium falciparum and P. berghei are associated with resistance to the anti-malarial drug atovaquone.

Human genes
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