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109  structures 2518  species 4  interactions 12139  sequences 229  architectures

Family: Cyt-b5 (PF00173)

Summary: Cytochrome b5-like Heme/Steroid binding domain

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This is the Wikipedia entry entitled "Cytochrome b5". More...

Cytochrome b5 Edit Wikipedia article

Cytochrome b5
1jex cyto b5.png
Rat cytochrome b5 bound to heme
Symbol CYB5A
Alt. symbols CYB5
Entrez 1528
HUGO 2570
OMIM 250790
RefSeq NM_001914
UniProt P00167
Other data
Locus Chr. 18 q23
Cytochrome b5
Symbol Cyt_B5
Pfam PF00173
InterPro IPR001199

Cytochromes b5 are ubiquitous electron transport hemoproteins found in animals, plants, fungi and purple phototrophic bacteria. The microsomal and mitochondrial variants are membrane-bound, while bacterial and those from erythrocytes and other animal tissues are water-soluble. The family of cytochrome b5-like proteins includes (besides cytochrome b5 itself) hemoprotein domains covalently associated with other redox domains in flavocytochrome cytochrome b2 (L-lactate dehydrogenase; EC, sulfite oxidase (EC, plant and fungal nitrate reductases (EC, EC, EC, and plant and fungal cytochrome b5/acyl lipid desaturase fusion proteins.


3-D structures of a number of cytochrome b5 and yeast flavocytochrome b2 are known. The fold belongs to the α+β class, with two hydrophobic cores on each side of a β-sheet. The larger hydrophobic core constitutes the heme-binding pocket, closed off on each side by a pair of helices connected by a turn. The smaller hydrophobic core may have only a structural role and is formed by spatially close N-terminal and C-terminal segments. The two histidine residues provide the fifth and sixth heme ligands, and the propionate edge of the heme group lies at the opening of the heme crevice. Two isomers of cytochrome b5, referred to as the A (major) and B (minor) forms, differ by a 180° rotation of the heme about an axis defined by the α- and γ-meso carbons.

Cytochrome b5 in some biochemical reactions

EC cytochrome-b5 reductase

NADH + H+ + 2 ferricytochrome b5 → NAD+ + 2 ferrocytochrome b5

EC L-ascorbate—cytochrome-b5 reductase

L-ascorbate + ferricytochrome b5 → monodehydroascorbate + ferrocytochrome b5

EC CMP-N-acetylneuraminate monooxygenase

CMP-N-acetylneuraminate + 2 ferrocytochrome b5 + O2 + 2 H+ → CMP-N-glycoloylneuraminate + 2 ferricytochrome b5 + H2O

EC stearoyl-CoA 9-desaturase

stearoyl-CoA + 2 ferrocytochrome b5 + O2 + 2 H+ → oleoyl-CoA + 2 ferricytochrome b5 + H2O

EC linoleoyl-CoA 9-desaturase

linoleoyl-CoA + 2 ferrocytochrome b5 + O2 + 2 H+ → γ-linolenoyl-CoA + 2 ferricytochrome b5 + H2O

See also


  • Lederer, F. (1994). "The cytochrome b5-fold: an adaptable module". Biochimie 76 (7): 674–692. doi:10.1016/0300-9084(94)90144-9. PMID 7893819. 
  • Napier, J.A., Michaelson, L.V. and Sayanova, O. (2003). "The role of cytochrome b5 fusion desaturases in the synthesis of polyunsaturated fatty acids". Prostaglandins, Leukotrienes and Essential Fatty Acids 68 (2): 135–143. doi:10.1016/S0952-3278(02)00263-6. PMID 12538077. 
  • Rivera, M., Barillas-Mury, C., Christensen, K.A., Little, J.W., Wells, M.A. and Walker, F.A. (1992). "Gene synthesis, bacterial expression, and 1H NMR spectroscopic studies of the rat outer mitochondrial membrane cytochrome b5". Biochemistry 31 (48): 12233–12240. doi:10.1021/bi00163a037. PMID 1333795. 
  • Schenkman, J.B. and Jansson, I. (2003). "The many roles of cytochrome b5". Pharmacol. Ther. 97 (2): 139–152. doi:10.1016/S0163-7258(02)00327-3. PMID 12559387. 

External links

  • PDB: 1B5A​ - Solution structure of rat cytochrome b5 (form A)
  • PDB: 1B5B​ - Solution structure of rat cytochrome b5 (form B)
  • PDB: 1CXY​ - X-ray structure of cytochrome b558 from Ectothiorhodospira vacuolata
  • Online 'Mendelian Inheritance in Man' (OMIM) 250790 - Methemoglobinemia due to deficiency of cytochrome b5

This page is based on a Wikipedia article. The text is available under the Creative Commons Attribution/Share-Alike License.

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.

Cytochrome b5-like Heme/Steroid binding domain Provide feedback

This family includes heme binding domains from a diverse range of proteins. This family also includes proteins that bind to steroids. The family includes progesterone receptors such as O00264 [1,2]. Many members of this subfamily are membrane anchored by an N-terminal transmembrane alpha helix. This family also includes a domain in some chitin synthases. There is no known ligand for this domain in the chitin synthases.

Literature references

  1. Gerdes D, Wehling M, Leube B, Falkenstein E; , Biol Chem 1998;379:907-911.: Cloning and tissue expression of two putative steroid membrane receptors. PUBMED:9705155 EPMC:9705155

  2. Meyer C, Schmid R, Scriba PC, Wehling M; , Eur J Biochem 1996;239:726-731.: Purification and partial sequencing of high-affinity progesterone-binding site(s) from porcine liver membranes. PUBMED:8774719 EPMC:8774719

  3. Mifsud W, Bateman A; , Genome Biol 2002;3:0-0.: Membrane-bound progesterone receptors contain a cytochrome b5-like ligand-binding domain. PUBMED:12537557 EPMC:12537557

Internal database links

External database links

This tab holds annotation information from the InterPro database.

InterPro entry IPR001199

Cytochromes b5 are ubiquitous electron transport proteins found in animals, plants and yeasts [PUBMED:2752049]. The microsomal and mitochondrial variants are membrane-bound, while those from erythrocytes and other animal tissues are water-soluble [PUBMED:4030743, PUBMED:8439576].

The 3D structure of bovine cyt b5 is known, the fold belonging to the alpha+beta class, with 5 strands and 5 short helices forming a framework for supporting a central haem group [PUBMED:1167544]. The cytochrome b5 domain is similar to that of a number of oxidoreductases, such as plant and fungal nitrate reductases, sulphite oxidase, yeast flavocytochrome b2 (L-lactate dehydrogenase) and plant cyt b5/acyl lipid desaturase fusion protein.

Gene Ontology

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Domain organisation

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Curation and family details

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Seed source: Bateman A
Previous IDs: heme_1;
Type: Domain
Author: Bateman A
Number in seed: 92
Number in full: 12139
Average length of the domain: 84.00 aa
Average identity of full alignment: 26 %
Average coverage of the sequence by the domain: 17.33 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 80369284 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 20.3 20.3
Trusted cut-off 20.3 20.3
Noise cut-off 20.2 20.2
Model length: 76
Family (HMM) version: 24
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Species distribution

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Archea Archea Eukaryota Eukaryota
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There are 4 interactions for this family. More...

FMN_dh Cyt-b5 FMN_dh Oxidored_molyb


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 Cyt-b5 domain has been found. There are 109 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 seqence.

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