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Cytochrome P450, family 21, subfamily A, polypeptide 2
Available structures
PDB Ortholog search: PDBe, RCSB
Symbols CYP21A2 ; CA21H; CAH1; CPS1; CYP21; CYP21B; P450c21B
External IDs OMIM613815 MGI88591 HomoloGene68063 IUPHAR: 1364 ChEMBL: 2759 GeneCards: CYP21A2 Gene
EC number
RNA expression pattern
PBB GE CYP21A2 214622 at tn.png
More reference expression data
Species Human Mouse
Entrez 1589 13079
Ensembl ENSG00000198457 ENSMUSG00000024365
UniProt P08686 P03940
RefSeq (mRNA) NM_000500 NM_009995
RefSeq (protein) NP_000491 NP_034125
Location (UCSC) Chr 6:
32.01 – 32.01 Mb
Chr 17:
34.8 – 34.8 Mb
PubMed search [1] [2]

Steroid 21-hydroxylase is a cytochrome P450 enzyme that is involved with the biosynthesis of the steroid hormones aldosterone and cortisol.[1]

In humans, 21-Hydroxylase is encoded by the gene CYP21A2.[2]


This gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids. This protein localizes to the endoplasmic reticulum and hydroxylates steroids at the 21 position. Its activity is required for the synthesis of steroid hormones including cortisol and aldosterone.[3]

Clinical significance[edit]

A defect within the CYP21A2 gene causes a disturbance of the development of the enzyme, which leads to congenital adrenal hyperplasia due to 21-hydroxylase deficiency. A related pseudogene is located near this gene; gene conversion events involving the functional gene and the pseudogene are thought account for many cases of steroid 21-hydroxylase deficiency.[3]

Names and classification[edit]

Steroid 21-monooxygenase
EC number
CAS number 9029-68-9
IntEnz IntEnz view
ExPASy NiceZyme view
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO

21-Hydroxylase is also called steroid 21-monooxygenase, 21α-Hydroxylase, and, less commonly 21β-Hydroxylase.


21-Hydroxylase catalyses the hydroxylation of the carbon atom 21 in steroids (adding an "–OH"), which is necessary with the formation of these hormones.

Steroid numbering - #21 is near center top 


Human steroidogenesis, showing both reactions of 21-Hydroxylase at center top.
Corticosteroid biosynthetic pathway in the rat.


  1. ^ Ryan KJ, Engel LL (Mar 1957). "Hydroxylation of steroids at carbon 21" (PDF). The Journal of Biological Chemistry 225 (1): 103–14. PMID 13416221. 
  2. ^ Higashi Y, Yoshioka H, Yamane M, Gotoh O, Fujii-Kuriyama Y (May 1986). "Complete nucleotide sequence of two steroid 21-hydroxylase genes tandemly arranged in human chromosome: a pseudogene and a genuine gene". Proceedings of the National Academy of Sciences of the United States of America 83 (9): 2841–5. Bibcode:1986PNAS...83.2841H. doi:10.1073/pnas.83.9.2841. PMC 323402. PMID 3486422. 
  3. ^ a b "Entrez Gene: CYP21A2 cytochrome P450, family 21, subfamily A, polypeptide 2". 

Further reading[edit]

  • White PC, Tusie-Luna MT, New MI, Speiser PW (1994). "Mutations in steroid 21-hydroxylase (CYP21)". Human Mutation 3 (4): 373–8. doi:10.1002/humu.1380030408. PMID 8081391. 
  • Helmberg A (Aug 1993). "Twin genes and endocrine disease: CYP21 and CYP11B genes". Acta Endocrinologica 129 (2): 97–108. doi:10.1530/acta.0.1290097. PMID 8372604. 
  • de-Araujo M et al. (Jan 1996). "Molecular analysis of CYP21 and C4 genes in Brazilian families with the classical form of steroid 21-hydroxylase deficiency". Brazilian Journal of Medical and Biological Research = Revista Brasileira De Pesquisas Médicas E Biológicas / Sociedade Brasileira De Biofísica ... [Et Al.] 29 (1): 1–13. PMID 8731325. 
  • Yu CY (1999). "Molecular genetics of the human MHC complement gene cluster". Experimental and Clinical Immunogenetics 15 (4): 213–30. doi:10.1159/000019075. PMID 10072631. 
  • Forest MG et al. (Jun 2005). "21-Hydroxylase deficiency: an exemplary model of the contribution of molecular biology in the understanding and management of the disease". Annales D'EnDocrinologiE 66 (3): 225–32. doi:10.1016/s0003-4266(05)81754-8. PMID 15988383. 

External links[edit]

This article incorporates text from the United States National Library of Medicine, which is in the public domain.