cytochrome P450 family 2, subfamily ACytochrome P450 family 2, subfamily A (CYP2A) includes CYP2A1, 2A2, and 2A3 in rats; CYP2A4, 2A5, 2A12, 2A20p, 2A21p, 2A22, and 2A23p in mice; CYP2A6, 2A7, 2A13, 2A18P in humans; CYP2A8, 2A9, 2A14, 2A15, 2A16, and 2A17 in hamsters; CYP2A10 and 2A11 in rabbits; and CYP2A19 in pigs. CYP2A enzymes metabolize numerous xenobiotic compounds, including coumarin, aflatoxin B1, nicotine, cotinine, 1,3-butadiene, and acetaminophen, among others, as well as endogenous compounds, including testosterone, progesterone, and other steroid hormones. Human CYP2A6 is responsible for the systemic clearance of nicotine, while CYP2A13 activates the nicotine-derived procarcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) into DNA-altering compounds that cause lung cancer. The CYP2A 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.