In the mitochondrion of eukaryotes and in aerobic prokaryotes, cytochrome b is a component of respiratory chain complex III (EC184.108.40.206) -- 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 (EC220.127.116.11), also known as the b6f complex. These complexes are involved in electron transport, the pumping of protons to create a proton-motive force (PMF). This proton gradient is used for the generation of ATP. These complexes play a vital role in cells.
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:
^Siregar JE, Syafruddin D, Matsuoka H, Kita K, Marzuki S (June 2008). "Mutation underlying resistance of Plasmodium 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.