Cytochrome P450, E-class, CYP3A (IPR008072)

Short name: Cyt_P450_E_CYP3A

Overlapping homologous superfamilies


Family relationships


Cytochrome P450 enzymes are a superfamily of haem-containing mono-oxygenases that are found in all kingdoms of life, and which show extraordinary diversity in their reaction chemistry. In mammals, these proteins are found primarily in microsomes of hepatocytes and other cell types, where they oxidise steroids, fatty acids and xenobiotics, and are important for the detoxification and clearance of various compounds, as well as for hormone synthesis and breakdown, cholesterol synthesis and vitamin D metabolism. In plants, these proteins are important for the biosynthesis of several compounds such as hormones, defensive compounds and fatty acids. In bacteria, they are important for several metabolic processes, such as the biosynthesis of antibiotic erythromycin in Saccharopolyspora erythraea (Streptomyces erythraeus).

Cytochrome P450 enzymes use haem to oxidise their substrates, using protons derived from NADH or NADPH to split the oxygen so a single atom can be added to a substrate. They also require electrons, which they receive from a variety of redox partners. In certain cases, cytochrome P450 can be fused to its redox partner to produce a bi-functional protein, such as with P450BM-3 from Bacillus megaterium [PMID: 17023115], which has haem and flavin domains.

Organisms produce many different cytochrome P450 enzymes (at least 58 in humans), which together with alternative splicing can provide a wide array of enzymes with different substrate and tissue specificities. Individual cytochrome P450 proteins follow the nomenclature: CYP, followed by a number (family), then a letter (subfamily), and another number (protein); e.g. CYP3A4 is the fourth protein in family 3, subfamily A. In general, family members should share >40% identity, while subfamily members should share >55% identity.

Cytochrome P450 proteins can also be grouped by two different schemes. One scheme was based on a taxonomic split: class I (prokaryotic/mitochondrial) and class II (eukaryotic microsomes). The other scheme was based on the number of components in the system: class B (3-components) and class E (2-components). These classes merge to a certain degree. Most prokaryotes and mitochondria (and fungal CYP55) have 3-component systems (class I/class B) - a FAD-containing flavoprotein (NAD(P)H-dependent reductase), an iron-sulphur protein and P450. Most eukaryotic microsomes have 2-component systems (class II/class E) - NADPH:P450 reductase (FAD and FMN-containing flavoprotein) and P450. There are exceptions to this scheme, such as 1-component systems that resemble class E enzymes [PMID: 16042601, PMID: 15128046, PMID: 8637843]. The class E enzymes can be further subdivided into five sequence clusters, groups I-V, each of which may contain more than one cytochrome P450 family (eg, CYP1 and CYP2 are both found in group I). The divergence of the cytochrome P450 superfamily into B- and E-classes, and further divergence into stable clusters within the E-class, appears to be very ancient, occurring before the appearance of eukaryotes.

This entry represents the CYP2J family from group I, class E, cytochrome P450 proteins, as well as other CYP2 family proteins. The CYP2 family comprises 15 subfamilies (A-H, J-N, P and Q). The first five (A-E) are present in mammalian liver, but in differing amounts and with different inducibilities. These five subfamilies show varied substrate specificities, with some degree of overlap. CYP3A family enzymes are of major importance in the mammalian (especially human) detoxification of xenobiotics. CYP3A4, CYP3A5 and CYP3A7 catalyses the metabolism of a wide variety of substrates, including over 50% of therapeutic drugs. CYP3A enzymes are predominantly (not exclusively) expressed in the liver and intestine. Both genetic and environmental factors such as diet (especially grapefruit and St John's wort) can affect CYP3A activity, which can alter the efficacy and clearance of drugs [PMID: 12406645]. In addition, CYP3A may play a role in breast and prostate carcinogenesis through its role in controlling the level of se hormones drugs [PMID: 15496535].

GO terms

Biological Process

GO:0055114 oxidation-reduction process

Molecular Function

GO:0020037 heme binding
GO:0005506 iron ion binding
GO:0016712 oxidoreductase activity, acting on paired donors, with incorporation or reduction of molecular oxygen, reduced flavin or flavoprotein as one donor, and incorporation of one atom of oxygen

Cellular Component

No terms assigned in this category.

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.