Cytochrome P450, E-class, CYP3A (Cyt_P450_E_CYP3A)
Type: Family

CYP3A family of enzymes is predominantly (but not exclusively) expressed in the liver and intestine, being the most abundant CYPP450 subfamily in the liver. This family is of major importance in mammals (especially human) for detoxification of xenobiotics (drug-metabolizing enzymes). They are embedded in the endoplasmic reticulum, where they can catalyse 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 (including over 50% of therapeutic drugs). In human there are several isoforms, for instance, CYP3A4, 3A5, 3A7, and 3A3 . 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 . In addition, CYP3A may play a role in breast and prostate carcinogenesis through its role in controlling the level of se hormones drugs .

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 , 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 . 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.