Epoxide hydrolase mechanism

Morisseau C, Hammock BD. Epoxide hydrolases mechanisms, inhibitor designs, and biological roles. Annu Rev Pharmacol Toxicol 2005 45 311-333. 

Rink R, M Eennema, M Smids, U Dehmel, DB Janssen (1997) Primary structure and catalytic mechanism of the epoxide hydrolase from Agrobacterium radiobacter ADI. J Biol Chem 272 14650-14657. 

Arand, M., Cronin, A., Adamska, M. and Oesch, F. (2005) Epoxide hydrolases structure, function, mechanism, and assay, Methods in Enzymology, 400, 569-588. 

FIGURE 6.10 Mechanism of epoxide hydrolase-catalyzed hydrolytic opening of an epoxide. 

From the above, it is clear that the epoxide hydrolases of greatest significance in drug and xenobiotic metabolism are the microsomal and soluble ones. Their catalytic mechanism, which we now examine, is different from that of cholesterol epoxide hydrolase and LTA4 hydrolase (e.g., [57][58]). 

A critical input in unraveling the catalytic mechanism of epoxide hydrolases has come from the identification of essential residues by a variety of techniques such as analysis of amino acid sequence relationships with other hydrolases, functional studies of site-directed mutated enzymes, and X-ray protein crystallography (e.g., [48][53][68 - 74]). As schematized in Fig. 10.6, the reaction mechanism of microsomal EH and cytosolic EH involves a catalytic triad consisting of a nucleophile, a general base, and a charge relay acid, in close analogy to many other hydrolases (see Chapt. 3). 

R. N. Armstrong, C. S. Cassidy, New Structural and Chemical Insight into the Catalytic Mechanism of Epoxide Hydrolases , Drug Metab. Rev. 2000, 32, 327 - 338. 

B. Borhan, A. D. Jones, F. Knot, D. F. Grant, M. J. Kurth, B. D. Hammock, Mechanism of Soluble Epoxide Hydrolase. Formation of an a-Hydroxy Ester-Enzyme Intermediate through Asp-333 , J. Biol. Chem. 1995, 270, 26923 - 26930. 

C. Morisseau, G. Du, J. W. Newman, B. D. Hammock, Mechanism of Mammalian Soluble Epoxide Hydrolase by Chalcone Oxide Derivatives , Arch. Biochem. Biophys. 1998, 356, 214 - 228 C. Morisseau, M. H. Goodrow, D. Dowdy, J. Zheng, J. F. Greene, J. R. Sanborn, B. D. Hammock, Potent Urea and Carbamate Inhibitors of Soluble Epoxide Hydrolases , Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 8849 - 8854. 

H. F. Tzeng, L. T. Laughlin, S. Lin, R. N. Armstrong, The Catalytic Mechanism of Microsomal Epoxide Hydrolase Involves Reversible Formation and Rate-Limiting Hydrolysis of the Alkyl-Enzyme Intermediate , J. Am. Chem. Soc. 1996, 118, 9436 -9437. 

Scheme 6. Mechanism of microsomal epoxide hydrolase and of haloalkane dehalogenase...

The use of enzymes and whole cells as catalysts in organic chemistry is described. Emphasis is put on the chemical reactions and the importance of providing enantiopure synthons. In particular kinetics of resolution is in focus. Among the topics covered are enzyme classification, structure and mechanism of action of enzymes. Examples are given on the use of hydrolytic enzymes such as esterases, proteases, lipases, epoxide hydrolases, acylases and amidases both in aqueous and low-water media. Reductions and oxidations are treated both using whole cells and pure enzymes. Moreover, use of enzymes in sngar chemistiy and to prodnce amino acids and peptides are discnssed. 

Figure 2.16 The mechanism of soluble epoxide hydrolase starts with a...

Enzyme catalysed hydrolysis of racemic epoxides is interesting from a practical point of view. This reaction is catalysed by epoxide hydrolases (EHs, EC 3.3.2.3) (Archelas and Furstoss, 1998). Mammalian EHs are the most widely studied and they are divided into five groups among which the soluble (cytosolic) epoxide hydrolases (sEH) and microsomal epoxide hydrolases (mEH) are best charactelised. The mechanism of sEH from rat starts with a nucleophilic attack by Asp333 on a carbon of the epoxide (usually the least hindered one) to form a glycol monoester intermediate which is stabilised by an oxyanion hole. A water molecule activated by His523 releases the 1,2-diol product. An... 

The glycosidases act by two different mechanisms which is revealed by the stereochemistry at the anomeiic centre of the product (McCarter and Withers, 1994). In one type of glycosidases the anomeiic centre is directly attacked by a hydroxide to give a product with inverted stereochemistry at the anomeiic centre. In the other mechanism, the anomeric centre is attacked by the carboxylate group of a glutamic acid residue to form an intermediate in which the carbohydrate moiety is covalently bound to the enzyme similar to in epoxide hydrolases (Figure 2.16) and serine hydrolases. Attack on this intermediate by a nucleophile leads to the net result which is retention of the stereochemistry at the anomeric centre. 

Thomas H, Timms CW, Oesch F. Epoxide hydrolases molecular properties, induction, polymorphisms and function. In Ruckpaul K, Rein H, eds. Frontiers of Bio transformation. Vol. II. Principles, Mechanisms and Biological Consequences of Induction. London Taylor Francis, 1990. 

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