Cytochrome P450
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 ∼75% of the total metabolism.
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 kingdoms of life, i.e., in animals, plants, fungi, bacteria, and archaea.[1][2] More than 11500 distinct CYP proteins are known.[3]
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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 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).[4]
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 are nomenclature committees that assign and track both base gene names (Cytochrome P450 Homepage) and allele names (CYP Allele Nomenclature Committee).
Mechanism
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].[5] 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:
- 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,[6] and sometimes changing the state of the heme iron from low-spin to high-spin.[7] 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[8]
- 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[9] This takes place by way of the electron transfer chain, as described above, reducing the ferric heme iron to the ferrous state.
- 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, as a consequence, 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.[6]
- A second electron is transferred via the electron-transport system, from either cytochrome P450 reductase, ferredoxins, or cytochrome b5, reducing the dioxygen adduct to a negatively charged peroxo group. This is a short-lived intermediate state.
- 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.[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.[8] 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:
- CPR/cyb5/P450 systems employed by most eukaryotic microsomal (i.e., not mitochondrial) CYPs involve the reduction of cytochrome P450 reductase (variously CPR, POR, or CYPOR) by NADPH, and the transfer of reducing power as electrons to the CYP. Cytochrome b5 (cyb5) can also contribute reducing power to this system after being reduced by cytochrome b5 reductase (CYB5R).
- FR/Fd/P450 systems, which are employed by mitochondrial and some bacterial CYPs.
- 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 CYP5 (thromboxane synthase), CYP8, prostacyclin synthase, and CYP74A (allene oxide synthase).
P450s in humans
Human 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 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.[10]
Drug metabolism
Most drugs undergo biotransformation and are eventually excreted from the body; and many require bioactivation to form the active compound. CYPs are the major enzymes involved in drug metabolism and bioactivation, accounting for ~75% of the total metabolism.[12] (Metabolism in this context is the chemical modification or degradation of drugs.)
Drug interaction
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 decrease because of enzyme induction.
Interaction of other substances
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 paradisin-A, have been found to inhibit CYP3A4-mediated metabolism of certain medications, leading to increased bioavailability and, thus, the strong possibility of overdosing.[13] Because of this risk, avoiding grapefruit juice and fresh grapefruits entirely while on drugs is usually advised.[14]
Other examples:
- Saint-John's wort, a common herbal remedy induces CYP3A4, but also inhibits CYP1A1, CYP1B1, CYP2D6, and CYP3A4.[15][16]
- Tobacco smoking induces CYP1A2 (example CYP1A2 substrates are clozapine, olanzapine, and fluvoxamine)[17]
- At relatively high concentrations, starfruit juice has also been shown to inhibit CYP2A6 and other CYPs.[18] Watercress is also a known inhibitor of the Cytochrome P450 CYP2E1, which may result in altered drug metabolism for individuals on certain medications (ex., chlorzoxazone).[19]
Other specific CYP functions
A subset of cytochrome P450 enzymes play important roles in the synthesis of steroid hormones (steroidogenesis) by the adrenals, gonads, and peripheral tissue:
- CYP11A1 (also known as P450scc or P450c11a1) in adrenal mitochondria effects “the activity formerly known as 20,22-desmolase” (steroid 20α-hydroxylase, steroid 22-hydroxylase, cholesterol side-chain scission).
- CYP11B1 (encoding the protein P450c11β) found in the inner mitochondrial membrane of adrenal cortex has steroid 11β-hydroxylase, steroid 18-hydroxylase, and steroid 18-methyloxidase activities.
- CYP11B2 (encoding the protein P450c11AS), found only in the mitochondria of the adrenal zona glomerulosa, has steroid 11β-hydroxylase, steroid 18-hydroxylase, and steroid 18-methyloxidase activities.
- CYP17A1, in endoplasmic reticulum of adrenal cortex has steroid 17α-hydroxylase and 17,20-lyase activities.
- CYP21A1 (P450c21) in adrenal cortex conducts 21-hydroxylase activity.
- CYP19A (P450arom, aromatase) in endoplasmic reticulum of gonads, brain, adipose tissue, and elsewhere catalyzes aromatization of androgens to estrogens.
CYP families in humans
Humans have 57 genes and more than 59 pseudogenes divided among 18 families of cytochrome P450 genes and 43 subfamilies.[20] 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.[10]
| 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) |
P450s in other species
Animals
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).[21] 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. CYP6AM1 is involved in caste differentiation in the dampwood termite Hodotermopsis sjostedti.
Bacteria
Bacterial 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.
- Cytochrome P450cam (CYP101) originally from Pseudomonas putida has been used as a model for many cytochromes P450 and was the first cytochrome P450 three-dimensional protein structure solved by X-ray crystallography. This enzyme is part of a camphor-hydroxylating catalytic cycle consisting of two electron transfer steps from putidaredoxin, a 2Fe-2S cluster-containing protein cofactor.
- Cytochrome P450 eryF (CYP107A1) originally from the actinomycete bacterium Saccharopolyspora erythraea is responsible for the biosynthesis of the antibiotic erythromycin by C6-hydroxylation of the macrolide 6-deoxyerythronolide B.
- Cytochrome P450 BM3 (CYP102A1) from the soil bacterium Bacillus megaterium catalyzes the NADPH-dependent hydroxylation of several long-chain fatty acids at the ω–1 through ω–3 positions. Unlike almost every other known CYP (except CYP505A1, cytochrome P450 foxy), it constitutes a natural fusion protein between the CYP domain and an electron donating cofactor. Thus, BM3 is potentially very useful in biotechnological applications.[22][23]
Fungi
The commonly used imidazole and triazole-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.
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
InterPro subfamilies:
- Cytochrome P450, B-class IPR002397
- Cytochrome P450, mitochondrial IPR002399
- Cytochrome P450, E-class, group I IPR002401
- Cytochrome P450, E-class, group II IPR002402
- Cytochrome P450, E-class, group IV IPR002403
References
- ↑ Roland Sigel; Sigel, Astrid; Sigel, Helmut (2007). The Ubiquitous Roles of Cytochrome P450 Proteins: Metal Ions in Life Sciences. New York: Wiley. ISBN 0-470-01672-8.
- ↑ Danielson PB (December 2002). "The cytochrome P450 superfamily: biochemistry, evolution and drug metabolism in humans". Curr. Drug Metab. 3 (6): 561–97. doi:10.2174/1389200023337054. PMID 12369887.
- ↑ Nelson D. "Cytochrome P450 Homepage". University of Tennessee. http://drnelson.uthsc.edu/CytochromeP450.html. Retrieved 2010-05-04.
- ↑ "NCBI sequence viewer". http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?db=protein&id=3915660. Retrieved 2007-11-19.
- ↑ PROSITE consensus pattern for P450
- ↑ 6.0 6.1 6.2 Meunier B, de Visser SP, Shaik S (September 2004). "Mechanism of oxidation reactions catalyzed by cytochrome p450 enzymes". Chem. Rev. 104 (9): 3947–80. doi:10.1021/cr020443g. PMID 15352783.
- ↑ Poulos TL, Finzel BC, Howard AJ (June 1987). "High-resolution crystal structure of cytochrome P450cam". J. Mol. Biol. 195 (3): 687–700. doi:10.1016/0022-2836(87)90190-2. PMID 3656428.
- ↑ 8.0 8.1 Ortiz de Montellano, Paul R.; Paul R. Ortiz de Montellano (2005). Cytochrome P450: structure, mechanism, and biochemistry (3rd ed.). New York: Kluwer Academic/Plenum Publishers. ISBN 0-306-48324-6.
- ↑ Sligar SG, Cinti DL, Gibson GG, Schenkman JB (October 1979). "Spin state control of the hepatic cytochrome P450 redox potential". Biochem. Biophys. Res. Commun. 90 (3): 925–32. doi:10.1016/0006-291X(79)91916-8. PMID 228675.
- ↑ 10.0 10.1 "P450 Table". http://drnelson.utmem.edu/human.P450.table.html.
- ↑ doctorfungus > Antifungal Drug Interactions Content Director: Russell E. Lewis, Pharm.D. Retrieved on Jan 23, 2010
- ↑ Guengerich FP (January 2008). "Cytochrome p450 and chemical toxicology". Chem. Res. Toxicol. 21 (1): 70–83. doi:10.1021/tx700079z. PMID 18052394.
- ↑ Bailey DG, Dresser GK (2004). "Interactions between grapefruit juice and cardiovascular drugs". Am J Cardiovasc Drugs 4 (5): 281–97. doi:10.2165/00129784-200404050-00002. PMID 15449971.
- ↑ Zeratsky K (2008-11-06). "Grapefruit juice: Can it cause drug interactions?". Ask a food & nutrition specialist. MayoClinic.com. http://www.mayoclinic.com/health/food-and-nutrition/AN00413. Retrieved 2009-02-09.
- ↑ Chaudhary A, Willett KL (January 2006). "Inhibition of human cytochrome CYP 1 enzymes by flavonoids of St. John's wort". Toxicology 217 (2-3): 194–205. doi:10.1016/j.tox.2005.09.010. PMID 16271822.
- ↑ Strandell J, Neil A, Carlin G (February 2004). "An approach to the in vitro evaluation of potential for cytochrome P450 enzyme inhibition from herbals and other natural remedies". Phytomedicine 11 (2-3): 98–104. doi:10.1078/0944-7113-00379. PMID 15070158.
- ↑ Kroon LA (September 2007). "Drug interactions with smoking". Am J Health Syst Pharm 64 (18): 1917–21. doi:10.2146/ajhp060414. PMID 17823102.
- ↑ Zhang JW, Liu Y, Cheng J, Li W, Ma H, Liu HT, Sun J, Wang LM, He YQ, Wang Y, Wang ZT, Yang L (2007). "Inhibition of human liver cytochrome P450 by star fruit juice". J Pharm Pharm Sci 10 (4): 496–503. PMID 18261370.
- ↑ Leclercq I, Desager JP, Horsmans Y (August 1998). "Inhibition of chlorzoxazone metabolism, a clinical probe for CYP2E1, by a single ingestion of watercress". Clin Pharmacol Ther. 64 (2): 144–9. doi:10.1016/S0009-9236(98)90147-3. PMID 9728894.
- ↑ Nelson D (2003). Cytochromes P450 in humans. Retrieved May 9, 2005.
- ↑ Goldstone JV, Hamdoun A, Cole BJ, Howard-Ashby M, Nebert DW, Scally M, Dean M, Epel D, Hahn ME, Stegeman JJ (December 2006). "The chemical defensome: environmental sensing and response genes in the Strongylocentrotus purpuratus genome". Dev. Biol. 300 (1): 366–84. doi:10.1016/j.ydbio.2006.08.066. PMID 17097629.
- ↑ Narhi L, Fulco A (5 June 1986). "Characterization of a catalytically self-sufficient 119,000-dalton cytochrome P-450 monooxygenase induced by barbiturates in Bacillus megaterium". J Biol Chem 261 (16): 7160–9. PMID 3086309. http://www.jbc.org/cgi/pmidlookup?view=long&pmid=3086309.
- ↑ Girvan H, Waltham T, Neeli R, Collins H, McLean K, Scrutton N, Leys D, Munro A (2006). "Flavocytochrome P450 BM3 and the origin of CYP102 fusion species". Biochem Soc Trans 34 (Pt 6): 1173–7. doi:10.1042/BST0341173. PMID 17073779.
External links
- Degtyarenko K (2009-01-09). "Directory of P450-containing Systems". International Centre for Genetic Engineering and Biotechnology. http://www.icgeb.org/~p450srv/. Retrieved 2009-02-10.
- Estabrook R (2003). "A passion for P450s (rememberances of the early history of research on cytochrome P450)". Drug Metab Dispos 31 (12): 1461–73. doi:10.1124/dmd.31.12.1461. PMID 14625342.
- Feyereisen R (2005-12-19). "The Insect P450 Site". Institut National de la Recherche Agronomique. http://p450.sophia.inra.fr/. Retrieved 2009-02-10.
- Flockhart DA (2007). "Cytochrome P450 drug interaction table". Indiana University-Purdue University Indianapolis. http://medicine.iupui.edu/flockhart/. Retrieved 2009-02-10.
- Fowler L, Mercer A. "Cytochrome P450 Animated Tutorial". School of Pharmacy, London. http://elearn.pharmacy.ac.uk/flash/view/Cytochrome_P450.html. Retrieved 2009-02-10.
- Nelson D (2005-04-08). "Cytochrome P450 Homepage". University of Tennessee Health Science Center. http://drnelson.uthsc.edu/CytochromeP450.html. Retrieved 2009-02-10.
- Sim SC (2008-09-04). "Human Cytochrome P450 (CYP) Allele Nomenclature Committee". Karolinska Institutet. http://www.cypalleles.ki.se/. Retrieved 2009-02-10.
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