Cytochrome b-561 / ferric reductase transmembrane domain.
SMART accession number:SM00665
Description: Cytochrome b-561 recycles ascorbate for the generation of norepinephrine by dopamine-beta-hydroxylase in the chromaffin vesicles of the adrenal gland. It is a transmembrane heme protein with the two heme groups being bound to conserved histidine residues. A cytochrome b-561 homologue, termed Dcytb, is an iron-regulated ferric reductase in the duodenal mucosa. Other homologues of these are also likely to be ferric reductases. SDR2 is proposed to be important in regulating the metabolism of iron in the onset of neurodegenerative disorders.
Interpro abstract (IPR006593):

Cytochromes b561 constitute a class of intrinsic membrane proteins containing two haem molecules that are involved in ascorbate (vitamin C) regeneration. They have been suggested to function as electron transporters, shuttling electrons across membranes from ascorbate to an acceptor molecule. The one-electron oxidation product of ascorbate, monodehydro-ascorbate (MDHA) has been shown to function as an electon acceptor for mammalian and plant cytochromes b561. The cytochrome b561-catalysed reduction of MDHA results in the regeneration of the fully reduced ascorbate molecule. Cytochromes b561 have been identified in a large number of phylogenetically distant species, but are absent in prokaryotes. Most species contain three or four cytochrome b561 paralogous proteins [ (PUBMED:12801412) ].

Members of the cytochrome b561 protein family are characterised by a number of structural features, likely to play an essential part in their function. They are highly hydrophobic proteins with six transmembrane helices (named TMH1 through TMH6), four conserved histidine residues, probably coordinating the two haem molecules, and predicted substrate-binding sites for ascorbate and MDHA [ (PUBMED:12801412) ]. The functionally relevant and structurally most conserved region in the cytochrome b561 family is the TMH2 to -5 4-helix core with an amino acid composition that is very well conserved in the inner surface and somewhat less conserved in the outer surface of the core. The two terminal helices (TMH1 and TMH6) are less conserved [ (PUBMED:11532994) (PUBMED:12768339) ].

The entry represents a conserved region containing six transmembrane helices, found in cytochrome b651 and homologous proteins including some ferric reductases.

Family alignment:
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There are 4061 B561 domains in 4037 proteins in SMART's nrdb database.

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