21-Hydroxylase

Steroid 21-hydroxylase is a protein that in humans is encoded by the CYP21A2 gene. The protein is an enzyme that hydroxylates steroids at the C21 position on the molecule. Naming conventions for enzymes are based on the substrate acted upon and the chemical process performed. Biochemically, this enzyme is involved in the biosynthesis of the adrenal gland hormones aldosterone and cortisol, which are important in blood pressure regulation, sodium homeostasis and blood sugar control. The enzyme converts progesterone and 17α-hydroxyprogesterone into 11-deoxycorticosterone and 11-deoxycortisol, respectively, within metabolic pathways which in humans ultimately lead to aldosterone and cortisol creation—deficiency in the enzyme may cause congenital adrenal hyperplasia.

Steroid 21-hydroxylase is a member of the cytochrome P450 family of monooxygenase enzymes that use an iron-containing heme cofactor to oxidize substrates.

In humans, the enzyme is localized in endoplasmic reticulum membranes of cells in adrenal cortex, and is encoded by the CYP21A2 gene which is located near the CYP21A1P pseudogene that has 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.

Steroid 21-hydroxylase in humans is encoded by the CYP21A2 gene that may be accompanied by one or several copies of the nonfunctional pseudogene CYP21A1P, this pseudogene shares 98% of the exonic informational identity with the actual functional gene.

Pseudogenes are common in genomes, and they originate as artifacts during the duplication process. Though often thought of as "junk DNA", research has shown that retaining these faulty copies can have a beneficial role, often providing regulation of their parent genes.

In the mouse genome, the Cyp21a2 is a pseudogene and the Cyp21a1 is a functional gene. In the chicken and quail, there is only a single Cyp21 gene, which locus is located between complement component C4 and TNX gene, along with Cenpa.

CYP21A2 in humans is located in chromosome 6, in the major histocompatibility complex III (MHC class III) close to the Complement component 4 genes C4A and C4B, the Tenascin X gene TNXB and STK19. MHC class III is the most gene-dense region of the human genome, containing many genes that have, as of 2023 - unknown functions or structures.

Inside the MHC class III, CYP21A2 is located within the RCCX cluster (an abbreviation composed of the names of the genes RP (a former name for STK19 serine/threonine kinase 19), C4, CYP21 and TNX), which is the most complex gene cluster in the human genome. The number of RCCX segments varies between one and four in a chromosome, with the prevalence of approximately 15% for monomodular, 75% for bimodular (STK19-C4A-CYP21A1P-TNXA-STK19B-C4B-CYP21A2-TNXB), and 10% for trimodular in Europeans. The quadrimodular structure of the RCCX unit is very rare. In a monomodular structure, all of the genes are functional i.e. protein-coding, but if a module count is two or more, there is only one copy of each functional gene rest being non-coding pseudogenes with the exception of the C4 gene which always has active copies.