Microsomal epoxide hydrolase

{{infobox enzyme

| Name = Microsomal epoxide hydrolase

| image = Microsomal Epoxide Hydrolase.png

| EC_number = 3.3.2.9

| CAS_number =

| GO_code =

| width =

| caption = A cartoon depiction of microsomal epoxide hydrolase from Aspergillus niger

}}

In enzymology, a microsomal epoxide hydrolase (mEH) ({{EC number|3.3.2.9}}) is an enzyme that catalyzes the hydrolysis reaction between an epoxide and water to form a diol.

File:Epoxide Hydrolysis.png

This enzyme plays a role in the uptake of bile salts within the large intestine. It functions as a Na+ dependent transporter. This enzyme participates in metabolism of xenobiotics by cytochrome p450. mEH has been identified as playing a large role in the detoxification and bioactivation of a wide variety of substrates, such as polycyclic aromatic hydrocarbons (PAHs), which are known for their carcinogenic properties.{{cite journal | vauthors = Kiyohara C, Yoshimasu K, Takayama K, Nakanishi Y | title = EPHX1 polymorphisms and the risk of lung cancer: a HuGE review | journal = Epidemiology | volume = 17 | issue = 1 | pages = 89–99 | date = January 2006 | pmid = 16357600 | doi = 10.1097/01.ede.0000187627.70026.23 | doi-access = free }}

The human homolog of microsomal epoxide hydrolase is EPHX1 and is located on chromosome 1.{{cite journal|vauthors=Jackson MR, Craft JA, Burchell B|date=September 1987|title=Nucleotide and deduced amino acid sequence of human liver microsomal epoxide hydrolase|journal=Nucleic Acids Research|volume=15|issue=17|pages=7188|doi=10.1093/nar/15.17.7188|pmc=306212|pmid=3502697}}

Nomenclature

This enzyme belongs to the family of hydrolases, specifically those acting on ether bonds (ether hydrolases). The systematic name of this enzyme class is cis-stilbene-oxide hydrolase. Other names in common use include epoxide hydratase (ambiguous), microsomal epoxide hydratase (ambiguous), epoxide hydrase, microsomal epoxide hydrase, arene-oxide hydratase (ambiguous), benzo[a]pyrene-4,5-oxide hydratase, benzo(a)pyrene-4,5-epoxide hydratase, aryl epoxide hydrase (ambiguous), cis-epoxide hydrolase, and mEH.

Structure

Microsomal epoxide hydrolase is a single polypeptide chain composed of 455 amino acids with a molecular weight of 52.96 kilodaltons. It is known that the N-terminal region of the enzyme is responsible for anchoring the protein to the cell membrane,{{cite journal | vauthors = Craft JA, Baird S, Lamont M, Burchell B | title = Membrane topology of epoxide hydrolase | journal = Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism | volume = 1046 | issue = 1 | pages = 32–9 | date = August 1990 | pmid = 2397243 | doi = 10.1016/0005-2760(90)90091-B }} while the C-terminal region of the enzyme contains catalytic residues.{{PDB|3G0I}}; {{cite journal|vauthors=Zou J, Hallberg BM, Bergfors T, Oesch F, Arand M, Mowbray SL, Jones TA|date=February 2000|title=Structure of Aspergillus niger epoxide hydrolase at 1.8 A resolution: implications for the structure and function of the mammalian microsomal class of epoxide hydrolases|journal=Structure|volume=8|issue=2|pages=111–22|doi=10.1016/S0969-2126(00)00087-3|pmid=10673439|doi-access=free}} Microsomal epoxide hydrolase belongs to the superfamily α/β-hydrolase fold enzymes.{{cite journal|vauthors=Ollis DL, Cheah E, Cygler M, Dijkstra B, Frolow F, Franken SM, Harel M, Remington SJ, Silman I, Schrag J, Sussman JL|date=April 1992|title=The α/β hydrolase fold. Protein Engineering, Design and Selection.|volume=5|issue=3|pages=197–211|doi=10.1093/protein/5.3.197|pmid=1409539|journal=Protein Eng.|url=https://pure.rug.nl/ws/files/3321987/1992ProteinEngOllis.pdf|hdl=11370/2d4c057d-1a67-437d-ad10-701f7a60f1e6|hdl-access=free}} The center of all α/β-hydrolase fold enzymes is an alpha/beta-sheet consisted of 8 beta strands connected by 6 alpha helices.{{cite journal | vauthors = Ollis DL, Cheah E, Cygler M, Dijkstra B, Frolow F, Franken SM, Harel M, Remington SJ, Silman I, Schrag J | title = The alpha/beta hydrolase fold | journal = Protein Engineering | volume = 5 | issue = 3 | pages = 197–211 | date = April 1992 | pmid = 1409539 | doi=10.1093/protein/5.3.197| url = https://pure.rug.nl/ws/files/3321987/1992ProteinEngOllis.pdf | hdl = 11370/2d4c057d-1a67-437d-ad10-701f7a60f1e6 | hdl-access = free }}{{cite journal | vauthors = Carr PD, Ollis DL | title = Alpha/beta hydrolase fold: an update | journal = Protein and Peptide Letters | volume = 16 | issue = 10 | pages = 1137–48 | date = 2009 | pmid = 19508187 | doi = 10.2174/092986609789071298 }} The three dimensional structure of mEH has been elucidated from Aspergillus niger. Although no 3D modeling has been solved for the mammalian mEH enzyme (EPHX1), the overall homology between fungal and mammalian mEH is relatively high.{{cite book |last1=Arand|first1=Michael |last2=Oesch|first2=Franz | name-list-style = vanc |title=Mammalian Xenobiotic Epoxide Hydrolases|date=2002-02-14|doi=10.1002/0470846305.ch12|work=Enzyme Systems that Metabolise Drugs and Other Xenobiotics|pages=459–483|publisher=John Wiley & Sons, Ltd|isbn=9780470846308 }}{{cite journal | vauthors = Arand M, Hemmer H, Dürk H, Baratti J, Archelas A, Furstoss R, Oesch F | title = Cloning and molecular characterization of a soluble epoxide hydrolase from Aspergillus niger that is related to mammalian microsomal epoxide hydrolase | journal = The Biochemical Journal | volume = 344 | issue = 1 | pages = 273–80 | date = November 1999 | pmid = 10548561 | doi = 10.1042/0264-6021:3440273 | pmc = 1220641 }} This high homology has allowed for the elucidation overall general structure and subsequent catalytic mechanism of EPHX1 in humans by comparisons to existing structures of fungal mEH.

Mechanism

α/β-hydrolase fold enzymes use a catalytic triad in their active site. The catalytic triad present in microsomal epoxide hydrolase is composed of glutamine, histidine and aspartic acid.{{cite journal | vauthors = Arand M, Müller F, Mecky A, Hinz W, Urban P, Pompon D, Kellner R, Oesch F | title = Catalytic triad of microsomal epoxide hydrolase: replacement of Glu404 with Asp leads to a strongly increased turnover rate | journal = The Biochemical Journal | volume = 337 | issue = 1 | pages = 37–43 | date = January 1999 | pmid = 9854022 | doi = 10.1042/0264-6021:3370037 | pmc = 1219933 }} The substrate is positioned in an orientation poised for nucleophilic attack through hydrogen bonding stabilization from two nearby tyrosine residues {{cite journal | vauthors = Lewis DF, Lake BG, Bird MG | title = Molecular modelling of human microsomal epoxide hydrolase (EH) by homology with a fungal (Aspergillus niger) EH crystal structure of 1.8 A resolution: structure-activity relationships in epoxides inhibiting EH activity | journal = Toxicology in Vitro | volume = 19 | issue = 4 | pages = 517–22 | date = June 2005 | pmid = 15826809 | doi = 10.1016/j.tiv.2004.07.001 }}{{cite journal | vauthors = Saenz-Méndez P, Katz A, Pérez-Kempner ML, Ventura ON, Vázquez M | title = Structural insights into human microsomal epoxide hydrolase by combined homology modeling, molecular dynamics simulations, and molecular docking calculations | journal = Proteins | volume = 85 | issue = 4 | pages = 720–730 | date = April 2017 | pmid = 28120429 | doi = 10.1002/prot.25251 | s2cid = 9772104 }} The proposed mechanism for the mEH-catalyzed reaction first involves a nucleophilic attack on the oxirane ring of the substrate from the aspartic acid residue near the active site, which forms an ester intermediate.{{Cite journal| vauthors = Lacourciere GM, Armstrong RN |date=November 1993 |title=The catalytic mechanism of microsomal epoxide hydrolase involves an ester intermediate |journal=Journal of the American Chemical Society |volume=115 |issue=22 |pages=10466–10467 |doi=10.1021/ja00075a115 }} The second step in this mechanism is hydrolysis of the ester that occurs by an activated water molecule.{{cite journal | vauthors = McCall PM, Srivastava S, Perry SL, Kovar DR, Gardel ML, Tirrell MV | title = Partitioning and Enhanced Self-Assembly of Actin in Polypeptide Coacervates | journal = Biophysical Journal | volume = 114 | issue = 7 | pages = 1636–1645 | date = April 2018 | pmid = 29642033 | pmc = 5954293 | doi = 10.1016/j.bpj.2018.02.020 | bibcode = 2018BpJ...114.1636M }} The activation of water is facilitated by proton abstraction via the catalytic triad between a water molecule, glutamine, and histidine.{{cite journal | vauthors = Oesch F, Herrero ME, Hengstler JG, Lohmann M, Arand M | title = Metabolic detoxification: implications for thresholds | journal = Toxicologic Pathology | volume = 28 | issue = 3 | pages = 382–7 | date = May 2000 | pmid = 10862554 | doi = 10.1177/019262330002800305 | doi-access = free }} After hydrolysis, the substrate is then released from its bond to the aspartic acid residue, liberating the diol product from the enzyme active site.{{cite journal | vauthors = Reetz MT, Bocola M, Wang LW, Sanchis J, Cronin A, Arand M, Zou J, Archelas A, Bottalla AL, Naworyta A, Mowbray SL | title = Directed evolution of an enantioselective epoxide hydrolase: uncovering the source of enantioselectivity at each evolutionary stage | journal = Journal of the American Chemical Society | volume = 131 | issue = 21 | pages = 7334–43 | date = June 2009 | pmid = 19469578 | doi = 10.1021/ja809673d | url = http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-130672 }}

File:MEH-Mechanism.pdf

The active site of this enzyme lies within a hydrophobic pocket in the enzyme, which in turn leads to the enzyme's preferential reactivity with molecules with hydrophobic side-chains.{{cite journal | vauthors = Václavíková R, Hughes DJ, Souček P | title = Microsomal epoxide hydrolase 1 (EPHX1): Gene, structure, function, and role in human disease | journal = Gene | volume = 571 | issue = 1 | pages = 1–8 | date = October 2015 | pmid = 26216302 | pmc = 4544754 | doi = 10.1016/j.gene.2015.07.071 }} The mEH enzyme typically binds to small organic epoxides, such as styrene epoxide and cis-stillbene-oxide. mEH does not catalyze the hydrolysis of bulkier molecules, as their large side-chains may sterically disrupt the charge relay system responsible for water activation.

File:MEH Active Site.png bound to small molecule 2-propoylpentanamide.|alt=|408x408px]]

Function

In humans, mEH has been found in the ovary, lung, kidney, lymphocytes, epithelial cells, and liver.{{cite book|title=Pharmacology|last=Bachmann|first=Kenneth|date=2009|publisher=Elsevier|isbn=978-0-12-369521-5|pages=131–173|chapter=Chapter 8: Drug Metabolism|doi=10.1016/b978-0-12-369521-5.00008-7|name-list-style=vanc}} Microsomal epoxide hydrolase serves as a protective enzyme against potentially harmful small molecules derived from the external environment.{{cite journal | vauthors = Oesch F | title = Mammalian epoxide hydrases: inducible enzymes catalysing the inactivation of carcinogenic and cytotoxic metabolites derived from aromatic and olefinic compounds | journal = Xenobiotica; the Fate of Foreign Compounds in Biological Systems | volume = 3 | issue = 5 | pages = 305–40 | date = May 1973 | pmid = 4584115 | doi = 10.3109/00498257309151525 }} This hydrolysis of genotoxic epoxides causes subsequent effects in several signal transduction pathways, rendering this enzyme important to metabolism.{{cite journal | vauthors = Samuelsson B, Dahlén SE, Lindgren JA, Rouzer CA, Serhan CN | title = Leukotrienes and lipoxins: structures, biosynthesis, and biological effects | journal = Science | volume = 237 | issue = 4819 | pages = 1171–6 | date = September 1987 | pmid = 2820055 | doi = 10.1126/science.2820055 | bibcode = 1987Sci...237.1171S }}{{cite journal | vauthors = Moghaddam MF, Grant DF, Cheek JM, Greene JF, Williamson KC, Hammock BD | title = Bioactivation of leukotoxins to their toxic diols by epoxide hydrolase | journal = Nature Medicine | volume = 3 | issue = 5 | pages = 562–6 | date = May 1997 | pmid = 9142128 | doi = 10.1038/nm0597-562 | pmc = 7095900 }}

Disease relevance

Microsomal epoxide hydrolase plays a large role in its effects on human health. Studies have shown that mutations EPHX1 in humans may be the cause of hypercholanemia,{{cite journal | vauthors = Zhu QS, Xing W, Qian B, von Dippe P, Shneider BL, Fox VL, Levy D | title = Inhibition of human m-epoxide hydrolase gene expression in a case of hypercholanemia | journal = Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease | volume = 1638 | issue = 3 | pages = 208–16 | date = July 2003 | pmid = 12878321 | doi = 10.1016/s0925-4439(03)00085-1 | doi-access = }} preeclampsia,{{cite journal | vauthors = Zusterzeel PL, Rütten H, Roelofs HM, Peters WH, Steegers EA | title = Protein carbonyls in decidua and placenta of pre-eclamptic women as markers for oxidative stress | journal = Placenta | volume = 22 | issue = 2–3 | pages = 213–9 | date = February 2001 | pmid = 11170826 | doi = 10.1053/plac.2000.0606 }}{{cite journal | vauthors = Laasanen J, Romppanen EL, Hiltunen M, Helisalmi S, Mannermaa A, Punnonen K, Heinonen S | title = Two exonic single nucleotide polymorphisms in the microsomal epoxide hydrolase gene are jointly associated with preeclampsia | journal = European Journal of Human Genetics | volume = 10 | issue = 9 | pages = 569–73 | date = September 2002 | pmid = 12173035 | doi = 10.1038/sj.ejhg.5200849 | doi-access = free }} and may contribute to fetal hydantoin syndrome.{{cite journal | vauthors = Buehler BA, Delimont D, van Waes M, Finnell RH | title = Prenatal prediction of risk of the fetal hydantoin syndrome | journal = The New England Journal of Medicine | volume = 322 | issue = 22 | pages = 1567–72 | date = May 1990 | pmid = 2336087 | doi = 10.1056/NEJM199005313222204 | doi-access = free }} Research also suggests that maternal polymorphisms in EPHX1 in pregnant women were related to facial malformations of children born from women taking phenytoin during their first trimester of pregnancy.{{cite journal | vauthors = Azzato EM, Chen RA, Wacholder S, Chanock SJ, Klebanoff MA, Caporaso NE | title = Maternal EPHX1 polymorphisms and risk of phenytoin-induced congenital malformations | journal = Pharmacogenetics and Genomics | volume = 20 | issue = 1 | pages = 58–63 | date = January 2010 | pmid = 19952982 | doi = 10.1097/fpc.0b013e328334b6a3 | s2cid = 29336596 }} While mEH participates in the protection of human health via detoxification of various environmental substances, it also has been found to facilitate the activation of carcinogens.

mEH detoxifies reactive epoxides that are commonly caused from cigarette smoke, and as such it is hypothesized that mutations in EPHX1 in humans may have an effect on an individual's susceptibility to COPD, emphysema and lung cancer. Some sources have demonstrated that individuals affected by COPD have a higher rate of containing an under-active variant of the EPHX1 gene, yet also demonstrated that the overactive variant of the gene was also found in higher frequencies in individuals affected by disease as well.{{cite journal | vauthors = Smith CA, Harrison DJ | title = Association between polymorphism in gene for microsomal epoxide hydrolase and susceptibility to emphysema | journal = Lancet | volume = 350 | issue = 9078 | pages = 630–3 | date = August 1997 | pmid = 9288046 | doi = 10.1016/s0140-6736(96)08061-0 | s2cid = 23974600 }}{{cite journal | vauthors = Brøgger J, Steen VM, Eiken HG, Gulsvik A, Bakke P | title = Genetic association between COPD and polymorphisms in TNF, ADRB2 and EPHX1 | journal = The European Respiratory Journal | volume = 27 | issue = 4 | pages = 682–8 | date = April 2006 | pmid = 16585076 | doi = 10.1183/09031936.06.00057005 | doi-access = free }} Other research has provided evidence supporting the idea that EPHX1 variants do not contribute to susceptibility of disease, but do contribute to disease severity. The role that mEH plays in lung cancer and COPD is still not fully elucidated, as the data on the topic in the literature is not completely unanimous.{{cite book |last1=Postma|first1=Dirkje S. |last2=Silverman|first2=Edwin K. | name-list-style = vanc | chapter = Chapter 4 - Genetics of Asthma and COPD | title=Genetics of Asthma and COPD|date=2009|doi = 10.1016/b978-0-12-374001-4.00004-3 |pages=37–51 |publisher=Elsevier |isbn=9780123740014 }}

There is some evidence that mEH variants may contribute to the occurrence of childhood asthma in combination with variants on the GSTP1 gene.{{cite journal | vauthors = Salam MT, Lin PC, Avol EL, Gauderman WJ, Gilliland FD | title = Microsomal epoxide hydrolase, glutathione S-transferase P1, traffic and childhood asthma | journal = Thorax | volume = 62 | issue = 12 | pages = 1050–7 | date = December 2007 | pmid = 17711870 | pmc = 2094290 | doi = 10.1136/thx.2007.080127 }}

Compared to soluble epoxide hydrolase, the contribution of mEH to metabolism of beneficial epoxy fatty acids such as Epoxyeicosatrienoic acid is considered minor since they are relatively poor mEH substrates in vitro. Yet, in vivo, it was found that mEH can play a considerable role in regulation of EET levels{{cite journal|author3-link=John R. Falck|vauthors=Marowsky A, Burgener J, Falck JR, Fritschy JM, Arand M|date=June 2009|title=Distribution of Soluble and Microsomal Epoxide Hydrolase in the Mouse Brain and Its Contribution to Cerebral Epoxyeicosatrienoic Acid Metabolism|journal=Neuroscience|volume=163|issue=2|pages=646–661|doi=10.1016/j.neuroscience.2009.06.033|pmid=19540314|s2cid=25808698 }}{{cite journal|vauthors=Edin ML, Hamedani BG, Gruzdev A, Graves JP, Lih FB, Arbes SJ, Singh R, Leon AO, Bradbury JA, DeGraff LM, Hoopes SL, Arand M, Zeldin DC|date=January 2018|title=Epoxide hydrolase 1 (EPHX1) hydrolyzes epoxyeicosanoids and impairs cardiac recovery after ischemia|journal=The Journal of Biological Chemistry|volume=293|issue=9 |pages=3281–3292|doi=10.1074/jbc.RA117.000298|pmc=5836130|pmid=29298899 |doi-access=free }} and hence inhibition of mEH or dual inhibition of mEH and sEH might have therapeutic potential. Amide, amine and urea based mEH inhibitors have been explored.{{cite journal|vauthors=Morisseau C, Newman JW, Dowdy DL, Goodrow MH, Hammock BD|date=April 2001|title=Inhibition of Microsomal Epoxide Hydrolases by Ureas, Amides, and Amines|journal=Chemical Research in Toxicology|volume=14|issue=4|pages=409–415|doi=10.1021/tx0001732|pmid=11304129}} Based on the most potent inhibitors characterized, an amide with a bulky alpha-substituent and a phenyl ring with lipophilic groups at meta-positions appear to be key pharmacophore units.{{cite journal|vauthors=Barnych B, Singh N, Negrel S, Zhang Y, Magis D, Roux C, Hua X, Ding Z, Morisseau C, Tantillo DJ, Siegel JB, Hammock BD|date=March 2020|title=Development of potent inhibitors of the human microsomal epoxide hydrolase|journal=European Journal of Medicinal Chemistry|volume=193|pages=112206|doi=10.1016/j.ejmech.2020.112206|pmc=7366823|pmid=32203787|doi-access=free}}

The overall effect that mEH has on human health is still debated, with some sources finding evidence that the overactive EPHX1 gene is the culprit for some diseases, while other evidence supports that the under active variant is the cause of others.