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8  structures 1320  species 0  interactions 7404  sequences 206  architectures

Family: Cytochrom_B561 (PF03188)

Summary: Eukaryotic cytochrome b561

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This is the Wikipedia entry entitled "Cytochrome b561". More...

Cytochrome b561 Edit Wikipedia article

Eukaryotic cytochrome b561
Pfam clanCL0328
OPM superfamily92
OPM protein4o6y
Prokaryotic cytochrome b561
Pfam clanCL0328
OPM superfamily92
OPM protein5oc0

Cytochrome b561 is an integral membrane protein responsible for electron transport, binding two heme groups non-covalently.[1] It is a family of ascorbate-dependent oxidoreductase enzymes.[2]

Human proteins containing this domain


  1. ^ Cytochrome b561, Uniprot
  2. ^ Lu P, Ma D, Yan C, Gong X, Du M, Shi Y (February 2014). "Structure and mechanism of a eukaryotic transmembrane ascorbate-dependent oxidoreductase". Proceedings of the National Academy of Sciences of the United States of America. 111 (5): 1813–8. doi:10.1073/pnas.1323931111. PMC 3918761. PMID 24449903.

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This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.

Eukaryotic cytochrome b561 Provide feedback

Cytochrome b561 is a secretory vesicle-specific electron transport protein. It is an integral membrane protein, that binds two heme groups non-covalently. This is a eukaryotic family. Members of the 'prokaryotic cytochrome b561' family can be found in Pfam: PF01292.

Literature references

  1. Okuyama E, Yamamoto R, Ichikawa Y, Tsubaki M; , Biochim Biophys Acta. 1998;1383:269-278.: Structural basis for the electron transfer across the chromaffin vesicle membranes catalyzed by cytochrome b561: analyses of cDNA nucleotide sequences and visible absorption spectra. PUBMED:9602148 EPMC:9602148

Internal database links

External database links

This tab holds annotation information from the InterPro database.

InterPro entry 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.

Domain organisation

Below is a listing of the unique domain organisations or architectures in which this domain is found. More...

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Pfam Clan

This family is a member of clan 2heme_cytochrom (CL0328), which has the following description:

This superfamily includes a variety of different heme binding cytochromes.

The clan contains the following 10 members:

COX15-CtaA Cytochrom_B561 Cytochrom_B_N_2 Cytochrom_C_asm Cytochrome_B DUF2427 DUF4405 DUF6529 Ferric_reduct Ni_hydr_CYTB


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This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.

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Curation and family details

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Curation View help on the curation process

Seed source: Pfam-B_2927 (release 6.5) & Pfam-B_7165 (Release 8.0)
Previous IDs: Cytochrome_B561;
Type: Family
Sequence Ontology: SO:0100021
Author: Mifsud W
Number in seed: 36
Number in full: 7404
Average length of the domain: 130.40 aa
Average identity of full alignment: 23 %
Average coverage of the sequence by the domain: 36.99 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 57096847 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 23.0 23.0
Trusted cut-off 23.0 23.0
Noise cut-off 22.9 22.9
Model length: 137
Family (HMM) version: 18
Download: download the raw HMM for this family

Species distribution

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Colour assignments

Archea Archea Eukaryota Eukaryota
Bacteria Bacteria Other sequences Other sequences
Viruses Viruses Unclassified Unclassified
Viroids Viroids Unclassified sequence Unclassified sequence


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For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Cytochrom_B561 domain has been found. There are 8 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein sequence.

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