cytochrome P450

Definition: The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids.

Cytochrome P450 (abbreviated CYP, P450, infrequently CYP450) is a very large and diverse superfamily of hemoproteins found in all domains of life.

Cytochromes P450 use a plethora of both exogenous and endogenous compounds as substrates in enzymatic reactions. Usually they form part of multicomponent electron transfer chains, called P450-containing systems.

The most common reaction catalysed by cytochrome P450 is a monooxygenase reaction, e.g. insertion of one atom of oxygen into an organic substrate (RH) while the other oxygen atom is reduced to water:

RH + O2 + 2H+ + 2e– → ROH + H2O

CYPs enzymes have been identified from all lineages of life, including mammals, birds, fish, insects, worms, sea squirts, sea urchins, plants, fungi, slime molds, bacteria and archaea.

More than 8100 distinct CYPs sequences are known (P450 Nomenclature Committee).

The name cytochrome P450 is derived from the fact that these are colored (’chrome’) cellular (’cyto’) proteins, with a "pigment at 450 nm", so named for the characteristic Soret peak formed by absorbance of light at wavelengths near 450 nm when the heme iron is reduced (often with sodium dithionite) and complexed to carbon monoxide.

Types

Detoxication

Some cytochrome P450 heme-thiolate enzymes (CYPs) heme participate in the detoxication and, paradoxically, the formation of reactive intermediates of thousands of chemicals that can damage DNA, as well as lipids and proteins.

CYP expression can also affect the production of molecules derived from arachidonic acid, and alters various downstream signal-transduction pathways. Such changes can be precursors to malignancy.

Nomenclature

Genes encoding CYP enzymes, and the enzymes themselves, are designated with the abbreviation "CYP", followed by an Arabic numeral indicating the gene family, a capital letter indicating the subfamily, and another numeral for the individual gene. The convention is to italicise the name when referring to the gene.

For example, CYP2E1 is the gene that encodes the enzyme CYP2E1 – one of the enzymes involved in paracetamol (acetaminophen) metabolism.

The "CYP" nomenclature is the officially preferred naming convention. However, some gene or enzyme names for CYPs may differ from this nomenclature, denoting the catalytic activity and the name of the compound used as substrate.

Examples include CYP5A1, thromboxane A2 synthase, abbreviated to TBXAS1 (ThromBoXane A2 Synthase 1), and CYP51A1, lanosterol 14-α-demethylase, sometimes unofficially abbreviated to LDM according to its substrate (Lanosterol) and activity (DeMethylation).

The current nomenclature guidelines suggest that members of new CYP families share >40% amino acid identity, while members of subfamiles must share >55% amino acid identity. There is a Nomenclature Committee that keeps track of and assigns new names.

References

 Nebert DW, Dalton TP. The role of cytochrome P450 enzymes in endogenous signalling pathways and environmental carcinogenesis. Nat Rev Cancer. 2006 Dec;6(12):947-60. PMID: 17128211