Cytochrome b6, PetB (IPR023530)

Short name: Cyt_B6_PetB

Family relationships



In the mitochondrion of eukaryotes and in aerobic prokaryotes, cytochrome b is a component of respiratory chain complex III (EC: - 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 (EC:, 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.

Cytochrome b/b6 [PMID: 2509716, PMID: 8329437] 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.

Proteins in this entry belong to the PetB family of cytochrome b6.

GO terms

Biological Process

GO:0022900 electron transport chain

Molecular Function

GO:0045158 electron transporter, transferring electrons within cytochrome b6/f complex of photosystem II activity
GO:0020037 heme binding
GO:0005506 iron ion binding

Cellular Component

GO:0016021 integral component of membrane

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.