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Cytochromes P450 (CYPs) belong to the superfamily of proteins containing a heme cofactor and, therefore, are hemoproteins. CYPs use a variety of small and large molecules as substrates in enzymatic reactions. They are, in general, the terminal oxidase enzymes in electron transfer chains, broadly categorized as P450-containing systems. The term ''P450'' is derived from the spectrophotometric peak at the wavelength of the absorption maximum of the enzyme (450 nm) when it is in the reduced state and complexed with CO. CYP enzymes have been identified in all domains of life - animals, plants, fungi, protists, bacteria, archaea, and even in viruses. However, the enzymes have not been found in ''E. coli''. More than 21,000 distinct CYP proteins are known. Most CYPs require a protein partner to deliver one or more electrons to reduce the iron (and eventually molecular oxygen). Based on the nature of the electron transfer proteins, CYPs can be classified into several groups: *Microsomal P450 systems in which electrons are transferred from NADPH via cytochrome P450 reductase (variously CPR, POR, or CYPOR). Cytochrome b5 (cyb5) can also contribute reducing power to this system after being reduced by cytochrome b5 reductase (CYB5R). *Mitochondrial P450 systems, that employ adrenodoxin reductase and adrenodoxin to transfer electrons from NADPH to P450. *Bacterial P450 systems, that employ a ferredoxin reductase and a ferredoxin to transfer electrons to P450. *CYB5R/cyb5/P450 systems in which both electrons required by the CYP come from cytochrome b5. *FMN/Fd/P450 systems originally found in ''Rhodococcus'' sp. in which a FMN-domain-containing reductase is fused to the CYP. *P450 only systems, which do not require external reducing power. Notable ones include thromboxane synthase (CYP5), prostacyclin synthase (CYP8), and CYP74A (allene oxide synthase). The most common reaction catalyzed by cytochromes P450 is a monooxygenase reaction, e.g., insertion of one atom of oxygen into the aliphatic position of an organic substrate (RH) while the other oxygen atom is reduced to water: RH + O2 + NADPH + H+ → ROH + H2O + NADP+ ==Nomenclature== Genes encoding CYP enzymes, and the enzymes themselves, are designated with the abbreviation CYP, followed by a number 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 official naming convention, although occasionally (and incorrectly) CYP450 or CYP450 is used. 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 subfamilies must share >55% amino acid identity. There are nomenclature committees that assign and track both base gene names ((Cytochrome P450 Homepage )) and allele names ((CYP Allele Nomenclature Committee )). (詳細はウィキペディア（Wikipedia）』 ■ウィキペディアで「Cytochrome P450」の詳細全文を読む スポンサード リンク
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