cytochrome P450 family 3, subfamily AThe cytochrome P450 3A (CYP3A) subfamily, the most abundant CYP subfamily in the liver, consists of drug-metabolizing enzymes. In humans, there are at least four isoforms: CYP3A4, 3A5, 3A7, and 3A3. CYP3A enzymes are embedded in the endoplasmic reticulum, where they can catalyze a wide variety of biochemical reactions including hydroxylation, N-demethylation, O-dealkylation, S-oxidation, deamination, or epoxidation of substrates. They oxidize a variety of structurally unrelated compounds including steroids, fatty acids, and xenobiotics. The CYP3A subfamily 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.