21-Hydroxylase

Steroid 21-hydroxylase is a 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.

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

Function
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.

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

Clinical significance
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.