Cytochrome P450 (P450, CYP) is a large 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 with > 40% sequence identity are members of the same family. There are approximately 2250 CYP families: mammals, insects, plants, fungi, bacteria, and archaea have around 18, 208, 277, 805, 591, and 14 families, respectively. 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. Their monooxygenase activity relies on the reductive scission of molecular oxygen bound to the P450 heme iron, and the delivery of two electrons to the heme iron during the catalytic cycle. CYPs use a variety of redox partners, such as the eukaryotic diflavin enzyme NADPH-cytochrome P450 oxidoreductase and the bacterial/mitochondrial NAD(P)H-ferredoxin reductase and ferredoxin partners. Some CYPs are naturally linked to their redox partners and others have evolved to bypass requirements for redox partners, and instead react directly with hydrogen peroxide or NAD(P)H to facilitate oxidative or reductive catalysis.
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