Cytochrome P450 | |||||||||
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Cytochrome P450 Oxidase (CYP2C9) | |||||||||
Identifiers | |||||||||
Symbol | p450 | ||||||||
Pfam | PF00067 | ||||||||
InterPro | IPR001128 | ||||||||
PROSITE | PDOC00081 | ||||||||
SCOP | 2cpp | ||||||||
OPM family | 41 | ||||||||
OPM protein | 1w0f | ||||||||
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The cytochrome P450 superfamily (officially abbreviated as CYP) is a large and diverse group of enzymes. The function of most CYP enzymes is to catalyze the oxidation of organic substances. The substrates of CYP enzymes include metabolic intermediates such as lipids and steroidal hormones, as well as xenobiotic substances such as drugs and other toxic chemicals. CYPs are the major enzymes involved in drug metabolism and bioactivation, accounting for about 75% of the total number of different metabolic reactions.[1]
The most common reaction catalyzed by cytochromes 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
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. Often, they form part of multi-component electron transfer chains, called P450-containing systems. Cytochromes P450 have been named on the basis of their cellular (cyto) location and spectrophotometric characteristics (chrome): when the reduced heme iron forms an adduct with CO, P450 enzymes absorb light at wavelengths near 450 nm, identifiable as a characteristic Soret peak.
CYP enzymes have been identified in all domains of life, i.e., in animals, plants, fungi, protists, bacteria, archaea, and even viruses.[2][3] More than 11,500 distinct CYP proteins are known.[4]
Contents |
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 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).[5]
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).
The active site of cytochrome P450 contains a heme iron center. The iron is tethered to the P450 protein via a thiolate ligand derived from a cysteine residue. This cysteine and several flanking residues are highly conserved in known CYPs and have the formal PROSITE signature consensus pattern [FW] - [SGNH] - x - [GD] - {F} - [RKHPT] - {P} - C - [LIVMFAP] - [GAD].[6] Because of the vast variety of reactions catalyzed by CYPs, the activities and properties of the many CYPs differ in many aspects. In general, the P450 catalytic cycle proceeds as follows:
S: An alternative route for mono-oxygenation is via the "peroxide shunt": Interaction with single-oxygen donors such as peroxides and hypochlorites can lead directly to the formation of the iron-oxo intermediate, allowing the catalytic cycle to be completed without going through steps 3, 4, and 5.[9] A hypothetical peroxide "XOOH" is shown in the diagram.
C: If carbon monoxide (CO) binds to reduced P450, the catalytic cycle is interrupted. This reaction yields the classic CO difference spectrum with a maximum at 450 nm.
Because most CYPs require a protein partner to deliver one or more electrons to reduce the iron (and eventually molecular oxygen), CYPs are part of P450-containing systems of proteins. Five general schemes are known:
Human CYPs are primarily membrane-associated proteins[12] located either in the inner membrane of mitochondria or in the endoplasmic reticulum of cells. CYPs metabolize thousands of endogenous and exogenous chemicals. Some CYPs metabolize only one (or a very few) substrates, such as CYP19 (aromatase), while others may metabolize multiple substrates. Both of these characteristics account for their central importance in medicine. Cytochrome P450 enzymes are present in most tissues of the body, and play important roles in hormone synthesis and breakdown (including estrogen and testosterone synthesis and metabolism), cholesterol synthesis, and vitamin D metabolism. Cytochrome P450 enzymes also function to metabolize potentially toxic compounds, including drugs and products of endogenous metabolism such as bilirubin, principally in the liver.
The Human Genome Project has identified 57 human genes coding for the various cytochrome P450 enzymes.[13]
CYPs are the major enzymes involved in drug metabolism, accounting for about 75% of the total metabolism.[1] Most drugs undergo deactivation by CYPs, either directly or by facilitated excretion from the body. Also, many substances are bioactivated by CYPs to form their active compounds.
Many drugs may increase or decrease the activity of various CYP isozymes either by inducing the biosynthesis of an isozyme (enzyme induction) or by directly inhibiting the activity of the CYP (enzyme inhibition). This is a major source of adverse drug interactions, since changes in CYP enzyme activity may affect the metabolism and clearance of various drugs. For example, if one drug inhibits the CYP-mediated metabolism of another drug, the second drug may accumulate within the body to toxic levels. Hence, these drug interactions may necessitate dosage adjustments or choosing drugs that do not interact with the CYP system. Such drug interactions are especially important to take into account when using drugs of vital importance to the patient, drugs with important side-effects and drugs with small therapeutic windows, but any drug may be subject to an altered plasma concentration due to altered drug metabolism.
A classical example includes anti-epileptic drugs. Phenytoin, for example, induces CYP1A2, CYP2C9, CYP2C19, and CYP3A4. Substrates for the latter may be drugs with critical dosage, like amiodarone or carbamazepine, whose blood plasma concentration may either increase because of enzyme inhibition in the former, or decrease because of enzyme induction in the latter.
Naturally occurring compounds may also induce or inhibit CYP activity. For example, bioactive compounds found in grapefruit juice and some other fruit juices, including bergamottin, dihydroxybergamottin, and paradicin-A, have been found to inhibit CYP3A4-mediated metabolism of certain medications, leading to increased bioavailability and, thus, the strong possibility of overdosing.[15] Because of this risk, avoiding grapefruit juice and fresh grapefruits entirely while on drugs is usually advised.[16]
Other examples:
A subset of cytochrome P450 enzymes play important roles in the synthesis of steroid hormones (steroidogenesis) by the adrenals, gonads, and peripheral tissue:
Humans have 57 genes and more than 59 pseudogenes divided among 18 families of cytochrome P450 genes and 43 subfamilies.[22] This is a summary of the genes and of the proteins they encode. See the homepage of the Cytochrome P450 Nomenclature Committee for detailed information.[13]
Family | Function | Members | Names |
CYP1 | drug and steroid (especially estrogen) metabolism | 3 subfamilies, 3 genes, 1 pseudogene | CYP1A1, CYP1A2, CYP1B1 |
CYP2 | drug and steroid metabolism | 13 subfamilies, 16 genes, 16 pseudogenes | CYP2A6, CYP2A7, CYP2A13, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1, CYP2F1, CYP2J2, CYP2R1, CYP2S1, CYP2U1, CYP2W1 |
CYP3 | drug and steroid (including testosterone) metabolism | 1 subfamily, 4 genes, 2 pseudogenes | CYP3A4, CYP3A5, CYP3A7, CYP3A43 |
CYP4 | arachidonic acid or fatty acid metabolism | 6 subfamilies, 12 genes, 10 pseudogenes | CYP4A11, CYP4A22, CYP4B1, CYP4F2, CYP4F3, CYP4F8, CYP4F11, CYP4F12, CYP4F22, CYP4V2, CYP4X1, CYP4Z1 |
CYP5 | thromboxane A2 synthase | 1 subfamily, 1 gene | CYP5A1 |
CYP7 | bile acid biosynthesis 7-alpha hydroxylase of steroid nucleus | 2 subfamilies, 2 genes | CYP7A1, CYP7B1 |
CYP8 | varied | 2 subfamilies, 2 genes | CYP8A1 (prostacyclin synthase), CYP8B1 (bile acid biosynthesis) |
CYP11 | steroid biosynthesis | 2 subfamilies, 3 genes | CYP11A1, CYP11B1, CYP11B2 |
CYP17 | steroid biosynthesis, 17-alpha hydroxylase | 1 subfamily, 1 gene | CYP17A1 |
CYP19 | steroid biosynthesis: aromatase synthesizes estrogen | 1 subfamily, 1 gene | CYP19A1 |
CYP20 | unknown function | 1 subfamily, 1 gene | CYP20A1 |
CYP21 | steroid biosynthesis | 2 subfamilies, 1 gene, 1 pseudogene | CYP21A2 |
CYP24 | vitamin D degradation | 1 subfamily, 1 gene | CYP24A1 |
CYP26 | retinoic acid hydroxylase | 3 subfamilies, 3 genes | CYP26A1, CYP26B1, CYP26C1 |
CYP27 | varied | 3 subfamilies, 3 genes | CYP27A1 (bile acid biosynthesis), CYP27B1 (vitamin D3 1-alpha hydroxylase, activates vitamin D3), CYP27C1 (unknown function) |
CYP39 | 7-alpha hydroxylation of 24-hydroxycholesterol | 1 subfamily, 1 gene | CYP39A1 |
CYP46 | cholesterol 24-hydroxylase | 1 subfamily, 1 gene | CYP46A1 |
CYP51 | cholesterol biosynthesis | 1 subfamily, 1 gene, 3 pseudogenes | CYP51A1 (lanosterol 14-alpha demethylase) |
Many animals have as many or more CYP genes than humans do. For example, mice have genes for 101 CYPs, and sea urchins have even more (perhaps as many as 120 genes).[23] Most CYP enzymes are presumed to have monooxygenase activity, as is the case for most mammalian CYPs that have been investigated (except for, e.g., CYP19 and CYP5). However, gene and genome sequencing is far outpacing biochemical characterization of enzymatic function, although many genes with close homology to CYPs with known function have been found.
The classes of CYPs most often investigated in non-human animals are those either involved in development (e.g., retinoic acid or hormone metabolism) or involved in the metabolism of toxic compounds (such as heterocyclic amines or polyaromatic hydrocarbons). Often there are differences in gene regulation or enzyme function of CYPs in related animals that explain observed differences in susceptibility to toxic compounds.
CYPs have been extensively examined in mice, rats, dogs, and less so in zebrafish, in order to facilitate use of these model organisms in drug discovery and toxicology.
CYPs have also been heavily studied in insects, often to understand pesticide resistance. For example, CYP6G1 is linked to insecticide resistance in DDT-resistant Drosophila melanogaster[24] and CYP6Z1 in the mosquito malaria vector Anopheles gambiae is capable of directly metabolizing DDT.[25]
Microbial cytochromes P450 are often soluble enzymes and are involved in critical metabolic processes. Three examples that have contributed significantly to structural and mechanistic studies are listed here, but many different families exist.
The commonly used azole class antifungal drugs work by inhibition of the fungal cytochrome P450 14α-demethylase. This interrupts the conversion of lanosterol to ergosterol, a component of the fungal cell membrane. (This is useful only because humans' P450 have a different sensitivity; this is how this class of antifungals work.)[29]
Significant research is ongoing into fungal P450s, as a number of fungi are pathogenic to humans (such as Candida yeast and Aspergillus) and to plants.
Plant cytochromes P450 are involved in a wide range of biosynthetic reactions, leading to various fatty acid conjugates, plant hormones, defensive compounds, or medically important drugs. Terpenoids, which represent the largest class of characterized natural plant compounds, are often substrates for plant CYPs.
InterPro subfamilies:
The remarkable reactivity and substrate promiscuity of P450s have long attracted the attention of chemists.[30] Recent progress towards realizing the potential of using P450s towards difficult oxidations have included; i) eliminating the need for natural co-factors by replacing them with inexpensive peroxide containing molecules,[31] ii) exploring the compatibility of p450s with organic solvents,[32] and iii) the use of small, non-chiral auxiliaries to predictably direct P450 oxidation.
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