Steroid 21- is a hydroxylase cytochrome P450 enzyme that is involved with the biosynthesis of the steroid hormones aldosterone and cortisol. 
In humans, 21-Hydroxylase is encoded by the
gene CYP21A2. 
Function [ edit ]
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. 
Clinical significance [ edit ]
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 to account for many cases of steroid 21-hydroxylase deficiency. 
Names and classification [ edit ]
21-Hydroxylase is also called
steroid 21-monooxygenase, 21α-Hydroxylase, and, less commonly 21β-Hydroxylase.
Reaction [ edit ]
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
Pathway [ edit ]
, showing both reactions of 21-Hydroxylase at center top.
Corticosteroid biosynthetic pathway in the rat.
References [ edit ]
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. 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.