21-Hydroxylase

Steroid 21-hydroxylase, also called steroid 21-monooxygenase, 21α-hydroxylase, P45021A2, and, less commonly 21β-hydroxylase, is a cytochrome P450 enzyme that is involved with the biosynthesis of the steroid hormones aldosterone and cortisol. These syntheses take place in the adrenal cortex. Specifically, 21-hydroxylase converts progesterone and 17α-hydroxyprogesterone into 11-deoxycorticosterone and 11-deoxycortisol, respectively, by hydroxylating at the C21 position. The products of the conversions then continue through their appropriate pathways towards creation of aldosterone and cortisol. Like other cytochrome P450 enzymes, 21-hydroxylase participates in the cytochrome P450 catalytic cycle, and engages in one-electron transfer with NADPH-P450 reductase. Its structure includes an essential iron heme group centered within the protein, also common to all P450 enzymes. Variations of the 21-hydroxylase enzyme can be found in all vertebrates. However, understanding of human 21-hydroxylase structure and function is of particular clinical value, as a failure of the enzyme to act appropriately results in congenital adrenal hyperplasia. The x-ray crystal structure for human 21-hydroxylase, with bound progesterone, was realized and published in 2015, providing opportunity for further study. The enzyme is notable for its substrate specificity and relatively high catalytic efficiency.4Y8W,%%s2GEG158913079ENSG00000235134ENSMUSG00000024365P08686Q08AG9P03940NM_000500NM_001128590NM_009995NP_001122062.3NP_034125 Steroid 21-hydroxylase, also called steroid 21-monooxygenase, 21α-hydroxylase, P45021A2, and, less commonly 21β-hydroxylase, is a cytochrome P450 enzyme that is involved with the biosynthesis of the steroid hormones aldosterone and cortisol. These syntheses take place in the adrenal cortex. Specifically, 21-hydroxylase converts progesterone and 17α-hydroxyprogesterone into 11-deoxycorticosterone and 11-deoxycortisol, respectively, by hydroxylating at the C21 position. The products of the conversions then continue through their appropriate pathways towards creation of aldosterone and cortisol. Like other cytochrome P450 enzymes, 21-hydroxylase participates in the cytochrome P450 catalytic cycle, and engages in one-electron transfer with NADPH-P450 reductase. Its structure includes an essential iron heme group centered within the protein, also common to all P450 enzymes. Variations of the 21-hydroxylase enzyme can be found in all vertebrates. However, understanding of human 21-hydroxylase structure and function is of particular clinical value, as a failure of the enzyme to act appropriately results in congenital adrenal hyperplasia. The x-ray crystal structure for human 21-hydroxylase, with bound progesterone, was realized and published in 2015, providing opportunity for further study. The enzyme is notable for its substrate specificity and relatively high catalytic efficiency. 21-hydroxylase is a complex of three independent and identical enzyme subunits. Each subunit in the human enzyme consists of 13 ??α-helices and 9 ß-strands, formed 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. 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. The 21-hydroxylase enzyme is one of three microsomal steroidogenic P450 enzymes, the others being 17-hydroxylase and aromatase. 21-hydroxylase is an essential enzyme in the biosynthetic pathways that produce cortisol and aldosterone. 21-Hydroxylase catalyzes the addition of hydroxyl (-OH) to the C21 position of two steroids: progesterone and 17α-hydroxyprogesterone. 21-Hydroxylase is a cytochrome P450 enzyme and follows the P450 catalytic cycle. 21-Hydroxylase is highly specific for hydroxylation of progesterone and 17-hydroxyprogesterone. No studies have reported sufficient binding of alternate substrates. In this way, it differs from the evolutionarily and functionally related P450 enzyme 17-hydroxylase, which has a large range of substrates. Earlier studies of the human enzyme expressed in yeast classified 17-hydroxyprogesterone as the best substrate for 21-hydroxylase. However, recent analysis of the purified human enzyme found a lower KM and greater catalytic efficiency for progesterone over 17-hydroxyprogesterone. The 2015 analysis found the catalytic efficiency of 21-hydroxylase for conversion of progesterone in humans to be approximately 1.3 x 10^7 M-1s-1 at 37 °C. This makes it the most catalytically efficient P450 enzyme of those reported, as of 2015, and more catalytically efficient than the closely related bovine 21-hydroxylase enzyme. C-H bond breaking to create a primary carbon radical is thought to be the rate-limiting step in the hydroxylation.