Steroid 21-hydroxylase (also known as steroid 21-monooxygenase, cytochrome P450C21, 21α-hydroxylase and less commonly 21β-hydroxylase) is an enzyme that hydroxylates steroids at the C21 position and is involved in biosynthesis of aldosterone and cortisol. The enzyme converts progesterone and 17α-hydroxyprogesterone into 11-deoxycorticosterone and 11-deoxycortisol, respectively, within metabolic pathways that ultimately lead to aldosterone and cortisol. Deficiency in the enzyme may cause congenital adrenal hyperplasia. Steroid 21-hydroxylase is a member of the cytochrome P450 family of monooxygenase enzymes that uses an iron containing heme cofactor to oxidize substrates. The enzyme is localized in endoplasmic reticulum membranes of adrenal cortex, and is encoded by the gene in humans, which is located near the CYP21A1P pseudogene with high degree of sequence similarity. This similarity makes it difficult to analyze the gene at the molecular level, and sometimes leads to loss-of-function mutations of the gene due to intergenic exchange of DNA. Function The steroid 21-hydroxylase enzyme hydroxylates steroids at the C21 position. The enzyme catalyzes the chemical reaction in which the hydroxyl group (-OH) is added at the C21 position of the steroid biomolecule. The enzyme is a member of the cytochrome P450 superfamily of monooxygenase enzymes. The cytochrome P450 enzymes catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids. 21-hydroxylase is localized in microsomes of endoplasmic reticulum membranes within adrenal cortex. It is one of three microsomal steroidogenic P450 enzymes, the others being 17-hydroxylase and aromatase. 21-hydroxylase is essential for the biosynthesis of cortisol and aldosterone. Structure 21-hydroxylase, as a member of the cytochrome P450 family of monooxygenase enzymes, contains a conserved core of a four α-helix bundle, two additional alpha helices, two sets of β-sheets, and a heme cofactor binding loop. Each subunit in the human enzyme consists of a total of 13 α-helices and 9 β-strands that folds into a triangular prism-like tertiary structure. The iron(III) heme group that defines the active site resides in the center of each subunit. The human enzyme binds one substrate at a time. In contrast, the well-characterized bovine enzyme can bind two substrates. The human and bovine enzyme share 80% amino acid sequence identity, but are structurally different, particularly in loop regions, and also evident in secondary structure elements. Mechanism 21-Hydroxylase is a cytochrome P450 enzyme that is notable for its substrate specificity and relatively high catalytic efficiency. Variations of the 21-hydroxylase enzyme can be found in all vertebrates. Like