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Cytochrome P450
Cytochrome P450 (abbreviated CYP, P450, infrequently CYP450) is a very large and diverse superfamily of hemoproteins found in all domains of life.[1] 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 CYP 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 7700 distinct CYP sequences are known (as of September 2007; see the web site of the P450 Nomenclature Committee for current counts).[2] 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.
NomenclatureGenes 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 CYP51, lanosterol 14-?-demethylase, sometimes unofficially abbreviated to LDM according to its substrate (Lanosterol) and activity (DeMethylation). [3] 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. Mechanism
The P450 catalytic cycle 1: The substrate binds to the active site of the enzyme, in close proximity to the heme group, on the side opposite to the peptide chain. The bound substrate induces a change in the conformation of the active site, often displacing a water molecule from the distal axial coordination position of the heme iron[5], and sometimes changing the state of the heme iron from low-spin to high-spin[6]. This gives rise to a change in the spectral properties of the enzyme, with an increase in absorbance at 390 nm and a decrease at 420 nm. This can be measured by difference spectrometry and is referred to as the "type I" difference spectrum (see inset graph in figure). Some substrates cause an opposite change in spectral properties, a "reverse type I" spectrum, by processes that are as yet unclear. Inhibitors and certain substrates that bind directly to the heme iron give rise to the type II difference spectrum, with a maximum at 430 nm and a minimum at 390 nm (see inset graph in figure). If no reducing equivalents are available, this complex may remain stable, allowing the degree of binding to be determined from absorbance measurements in vitro[7] 2: The change in the electronic state of the active site favors the transfer of an electron from NAD(P)H via cytochrome P450 reductase or another associated reductase[8]. This takes place by way of the electron transfer chain, as described above, reducing the ferric heme iron to the ferrous state. 3: Molecular oxygen binds covalently to the distal axial coordination position of the heme iron. The cysteine ligand is a better electron donor than histidine, with the oxygen consequently being activated to a greater extent than in other heme proteins. However, this sometimes allows the bond to dissociate, the so-called "decoupling reaction", releasing a reactive superoxide radical, interrupting the catalytic cycle[5]. 4: A second electron is transferred via the electron-transport system, either from cytochrome P450 reductase, ferredoxins, or cytochrome b5, reducing the dioxygen adduct to a negatively charged peroxo group. This is a short-lived intermediate state. 5: The peroxo group formed in step 4 is rapidly protonated twice by local transfer from water or from surrounding amino-acid side chains, releasing one water molecule, and forming a highly reactive iron(V)-oxo species[5]. 6: Depending on the substrate and enzyme involved, P450 enzymes can catalyse any of a wide variety of reactions. A hypothetical hydroxylation is shown in this illustration. After the product has been released from the active site, the enzyme returns to its original state, with a water molecule returning to occupy the distal coordination position of the iron nucleus. 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[7]. 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 properly speaking part of P450-containing systems of proteins. Five general schemes are known:
P450s in humansHuman CYPs are primarily membrane-associated proteins, located either in the inner membrane of mitochondria or in the endoplasmic reticulum of cells. CYPs metabolize thousands of endogenous and exogenous compounds. Most CYPs can metabolize multiple substrates, and many can catalyze multiple reactions, which accounts for their central importance in metabolizing the extremely large number of endogenous and exogenous molecules. In the liver, these substrates include drugs and toxic compounds as well as metabolic products such as bilirubin (a breakdown product of hemoglobin). Cytochrome P450 enzymes are present in most other 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. Hepatic cytochromes P450 are the most widely studied. The Human Genome Project has identified 57 human genes coding for the various cytochrome P450 enzymes.[9] Drug metabolismCYPs are the major enzymes involved in drug metabolism, accounting for ?75% of the total metabolism.[10]. Cytochrome P450 is the most important element of oxidative metabolism (also known as phase I metabolism). (Metabolism in this context is the chemical modification or degradation of drugs.) Drug interactionMany drugs may increase or decrease the activity of various CYP isozymes in a phenomenon known as enzyme induction and 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, possibly causing an overdose. Hence, these drug interactions may necessitate dosage adjustments or choosing drugs which do not interact with the CYP system. Such drug interactions are extra 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 decrease because of enzyme induction. Interaction of other substancesIn addition, naturally occurring compounds may cause a similar effect. For example, bioactive compounds found in grapefruit juice and some other fruit juices, including bergamottin, dihydroxybergamottin, and paradisin-A, have been found to inhibit CYP3A4-mediated metabolism of certain medications, leading to increased bioavailability and thus the strong possibility of overdosing.[11] Because of this risk, avoiding grapefruit juice and fresh grapefruits entirely while on drugs is usually advised. Other examples:
Other specific CYP functions
Steroidogenesis, showing many of the enzyme activities that are performed by cytochrome P450 enzymes. HSD: Hydroxysteroid dehydrogenase
CYP Families in HumansHumans have 57 genes and more than 59 pseudogenes divided among 18 families of cytochrome P450 genes and 43 subfamilies.[12] 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.[9]
P450s in animalsMany 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).[13] 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 involved in either 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, and 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. P450s in bacteriaBacterial 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.
P450s in fungiThe commonly used azole antifungal agents work by inhibition of the fungal cytochrome P450 14?-demethylase. This interrupts the conversion of lanosterol to ergosterol, a component of the fungal cell membrane. P450s in plantsPlant cytochrome P450s 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 subfamiliesInterPro subfamilies:
ReferencesExternal links
de:Cytochrom P450 es:Citocromo P450 fr:Cytochrome p450 it:Citocromo P450 nl:Cytochroom P450 ja:?????P450 no:Cytokrom P450 pl:Cytochrom P450 pt:Citocromo P450 sv:Cytokrom P450 Source: Wikipedia | The above article is available under the GNU FDL. | Edit this article
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