Cytochrome b6, PetB <p>In the mitochondrion of eukaryotes and in aerobic prokaryotes, cytochrome b is a component of respiratory chain complex III (<db_xref db="EC" dbkey="1.10.2.2"/>) - also known as the bc1 complex or ubiquinol-cytochrome c reductase. In plant chloroplasts and cyanobacteria, there is a analogous protein, cytochrome b6, a component of the plastoquinone-plastocyanin reductase (<db_xref db="EC" dbkey="1.10.99.1"/>), also known as the b6f complex. Both of these complexes are involved in electron transport and the generation of ATP and are therefore vitally important to the cell.</p><p>Cytochrome b/b6 [<cite idref="PUB00003404"/>, <cite idref="PUB00000640"/>] 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. The sequence of petB is colinear with the N-terminal part of mitochondrial cytochrome b, while petD corresponds to the C-terminal part. Cytochrome b/b6 non-covalently binds two haem groups, known as b562 and b566. Four conserved histidine residues are postulated to be the ligands of the iron atoms of these two haem groups.</p><p>Apart from regions around some of the histidine haem ligands, there are a few conserved regions in the sequence of b/b6. The best conserved of these regions includes an invariant P-E-W triplet which lies in the loop that separates the fifth and sixth transmembrane segments. It seems to be important for electron transfer at the ubiquinone redox site - called Qz or Qo (where o stands for outside) - located on the outer side of the membrane. This entry is the N terminus of these proteins.</p><p>Proteins in this entry belong to the PetB family of cytochrome b6.</p>