Epoxide hydrolase

Epoxides are reactive electrophiles by nature of their ring strain and change localization they vary considerably in stability and reactivity (Guengerich, 

P450s convert many olefins and aromatic compounds to epoxides that, unless hydrolyzed or conjugated, can react with tissue nucleophiles to initiate injury. 

Epoxide hydrolases catalyze the simple addition of H2O to epoxides  

In this way, the chemistry is more related to the enzymes that catalyze conjugations than to those that oxidize and reduce. The major epoxide hydrolase is a microsomal enzyme, abundant at a relatively high concentration in the liver and many other tissues. This is often referred to simply as the microsomal epoxide hydrolase. At least three other epoxide hydrolases are also found in the endoplasmic reticulum, but these all have specialized functions and do not act on xenobiotic chemicals (Hammock et al., 1997). 

The epoxide hydrolases are very important enzymes that render the epoxides formed by CYP enzymes harmless. Mammals have three distinct epoxide hydrolases. One microsomal form is specific for cholesterol-5,6a-oxide. It is induced by the same substances that induce the CYP enzymes. Another, less specific epoxide hydrolase is located in the endoplasmatic reticulum close to the CYP enzymes and is more important for xenobiotic substances. A third type, of some importance, is located in the cytosol. A typical substrate is trans-stilbene oxide, which also is an inducer. Some epoxides, such as diel-drin, are not detoxicated by these enzymes due to steric hindrance, but stilbene epoxide is a good substrate that is frequently used in experimental studies of these enzymes. 

The epoxidation of trans-stilbene to the epoxide and the formation of the diol by epoxide hydrolase 

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Epoxide-hydrolases as asymmetric catalysts for ring opening of oxiranes 97T15617. 

For a review on epoxide hydrolases and related enzymes in the context of organic synthesis, see Faber, K. Biotransformations in Organic Chemistry, Springer New York 2004. 

Scheme 10.32 Examples of reactions catalyzed by different classes of dehalogenases. HD haloalcohol dehalogenase EH epoxide hydrolase CL p-chlorobenzoyl-CoA ligase CBD p-chlorobenzoyl-CoA dehalogenase.

The metabolism of foreign compounds (xenobiotics) often takes place in two consecutive reactions, classically referred to as phases one and two. Phase I is a functionalization of the lipophilic compound that can be used to attach a conjugate in Phase II. The conjugated product is usually sufficiently water-soluble to be excretable into the urine. The most important biotransformations of Phase I are aromatic and aliphatic hydroxylations catalyzed by cytochromes P450. Other Phase I enzymes are for example epoxide hydrolases or carboxylesterases. Typical Phase II enzymes are UDP-glucuronosyltrans-ferases, sulfotransferases, N-acetyltransferases and methyltransferases e.g. thiopurin S-methyltransferase. 

Several reports regarding the directed evolution of enantioselective epoxide hydrolases (EHs) have appeared [23,57-59]. These enzymes constitute important catalysts in synthetic organic chemistry [4,60]. The first two reported studies concern the Aspergillus niger epoxide hydrolase (ANEH) [57,58]. Initial attempts were made to enhance the enantioselectivity of the AN E H -catalyzed hydrolytic kinetic resolution of glycidyl phenyl ether (rac-19). The WT leads to an Evalue of only 4.6 in favor of (S)-20 (see Scheme 2.4) [58]. 

Figure 2.14 CASTing of the epoxide hydrolase from A. niger (ANEH) based on the X-ray structure of the WT [61]. (a) Defined randomization sites A-E (b) top view of tunnel-like binding pocket showing sites A-E (blue) and the catalytically active D192 (red) [23].

Figure 2.15 Iterative CASTing in the evolution of enantioseiective epoxide hydrolases as catalysts in the hydrolytic kinetic resolution ofrac-19[23].

Furstoss et al. have reported their studies on the use of an epoxide hydrolase with four styrene oxide derivatives (Figure 5.26) [39]. The (R)-diol (43) was obtained in 91% ee at 100% conversion from racemic (42), demonstrating an enantioconvergent... 

Figure 5.25 Enantioconvergent hydrolysis of epoxides (35) to the corresponding diols (36) using mung bean epoxide hydrolase.

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

Epoxide hydrolases hydrate epoxides to yield transdihydrodiols without any requirement for cofactors. Examples are given in Figure 2.12. Epoxide hydrolases are... 

The microsomal fraction consists mainly of vesicles (microsomes) derived from the endoplasmic reticulum (smooth and rough). It contains cytochrome P450 and NADPH/cytochrome P450 reductase (collectively the microsomal monooxygenase system), carboxylesterases, A-esterases, epoxide hydrolases, glucuronyl transferases, and other enzymes that metabolize xenobiotics. The 105,000 g supernatant contains soluble enzymes such as glutathione-5-trans-ferases, sulfotransferases, and certain esterases. The 11,000 g supernatant contains all of the types of enzyme listed earlier. 

Emphasis is given to the critical role of metabolism, both detoxication and activation, in determining toxicity. The principal enzymes involved are described, including monooxygenases, esterases, epoxide hydrolases, glutathione-5 -transferases, and glucuronyl transferases. Attention is given to the influence of enzyme induction and enzyme inhibition on toxicity. 

In the rabbit, the nonplanar PCB 2,2, 5,5 -tetrachlorobiphenyl (2,2, 5,5 -TCB) is converted into the 3, 4 -epoxide by monooxygenase attack on the meta-para position, and rearrangement yields two monohydroxymetabolites with substitution in the meta and para positions (Sundstrom et al. 1976). The epoxide is also transformed into a dihydrodiol by epoxide hydrolase attack (see Chapter 2, Section 2.3.2.4). This latter conversion is inhibited by 3,3,3-trichloropropene-l,2-oxide (TCPO), thus providing strong confirmatory evidence for the formation of an unstable epoxide in the primary oxidative attack (Forgue et al. 1980). 

Epoxide hydrolase A type of enzyme that converts epoxides to diols by the addition of water. 

McElroy NR, Jurs PC, Morisseau C, Hammock BD. QSAR and classification of murine and human epoxide hydrolase inhibition by urea-like compounds. J Med Chem 2003 46 1066-80. 

Lewis DF, Lake BG, Bird MG. 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. Toxicol In Vitro 2005 19 517-22. 

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