InterPro domain: IPR016174

This entry represents a haem-binding domain with a 4-helical bundle structure that is found in transmembrane di-haem cytochromes. The domain contains four transmembrane helices in an up-and-down bundle, and binds two haem groups in between the helices; three of the four haem-binding residues is conserved between family members. Proteins containing this domain include:

• N-terminal domain of mitochondrial cytochrome b subunit, in which the domain contains an extra transmembrane linker helix that is absent in plant and cyanobacteria subunits [ 1 ].
• Cytochrome b6 subunit of the cytochrome b6f complex, which provides the electronic connection between the photosystems I and II reaction centres of oxygenic photosynthesis, and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis [ 2 ].
• Cytochrome gamma subunit of formate dehydrogenase-N (Fdn-N), which acts as a major component of Escherichia coli nitrate respiration [ 3 ].

Please note, this entry also identifies a number of proteins that are cleaved into two chains - a truncated non-functional cytochrome oxidase 1 and an intron-encoded endonuclease.

1. Binding of the respiratory chain inhibitor antimycin to the mitochondrial bc1 complex: a new crystal structure reveals an altered intramolecular hydrogen-bonding pattern. J. Mol. Biol. 351, 573-97
2. Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity. Science 302, 1009-14
3. Molecular basis of proton motive force generation: structure of formate dehydrogenase-N. Science 295, 1863-8