cytochrome P450 21, also called steroid 21-hydroxylase
Cytochrome P450 21 (CYP21 or Cyp21), also called steroid 21-hydroxylase (EC 184.108.40.206) or cytochrome P-450c21 or CYP21A2 (in humans), catalyzes the 21-hydroxylation of steroids such as progesterone and 17-alpha-hydroxyprogesterone (17-alpha-OH-progesterone) to form 11-deoxycorticosterone and 11-deoxycortisol, respectively. It is required for the adrenal synthesis of mineralocorticoids and glucocorticoids. Deficiency of this CYP is involved in ~95% of cases of human congenital adrenal hyperplasia, a disorder of adrenal steroidogenesis. There are two CYP21 genes in the human genome, CYP21A1 (a pseudogene) and CYP21A2 (the functional gene). Deficiencies in steroid 21-hydroxylase activity lead to a type of congenital adrenal hyperplasia, which has three clinical forms: a severe form with concurrent defects in both cortisol and aldosterone biosynthesis; a form with adequate aldosterone biosynthesis; and a mild, non-classic form that can be asymptomatic or associated with signs of postpubertal androgen excess without cortisol deficiency. CYP21A2 is also the major autoantigen in autoimmune Addison disease. Cyp21 belongs to the large cytochrome P450 (P450, CYP) superfamily of heme-containing proteins that catalyze a variety of oxidative reactions of a large number of structurally different endogenous and exogenous compounds in organisms from all major domains of life. CYPs bind their diverse ligands in a buried, hydrophobic active site, which is accessed through a substrate access channel formed by two flexible helices and their connecting loop.