Cytochrome b/b6 <p>This entry represents the mitochondrial cytochrome b subunit [<cite idref="PUB00037404"/>] and the cytochrome b6 subunit of the cytochrome b6f complex, which provides the electronic connection between the photosystem I and II reaction centres of oxygenic photosynthesis, and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis [<cite idref="PUB00032072"/>]. </p><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 an 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. 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. 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> The cyt b6/f complex is central to the functions of oxygenic photosynthetic electron transport in cyanobacteria and its equivalents in algae and higher plants, mediating electron transfer between photosystem II (PSII) and photosystem I (PSI) reaction centre complexes. The cyt b6f complex consists of a cytochrome b core, in addition to cytochrome f, Rieske 2Fe-2S, and subunits IV, V, VI, VII. The complex has a central quinone exchange cavity, which is defined by two monomers that resemble the respiratory cytochrome bc1 complex [<cite idref="PUB00034763"/>]. Cyt b6f has three prosthetic groups not found in cytbc1: chlorophyll-a, beta-carotene and a structurally unique covalently bound haem. The haem is thought to function in cyclic electron transport via anionic ferredoxin. Each monomer of the complex contains a molecule of chlorophyll-a and beta-carotene. Subunit IV is one of the principal subunits for the binding of the redox prosthetic groups. </p>