Epoxide hydrolases catalytic mechanism

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. 

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. 

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. 

F. Pinot, D. F. Grant, J. K. Beetham, A. G. Parker, B. Borhan, S. Landt, A. D. Jones, B. D. Hammock, Molecular and Biochemical Evidence for the Involvement of the Asp333-His523 Pair in the Catalytic Mechanism of Soluble Epoxide Hydrolase , J. Biol. Chem. 1995, 270, 7968 - 7974. 

Enzyme-catalyzed epoxide ring opening including discussion of convergent families of epoxide hydrolases, the catalytic mechanism of microsomal epoxide hydrolases, epoxide hydrolases in metabolism, and the synthesis, structure, and mechanism of leukotriene A4 hydrolase, and glutathione transferase has been reviewed by Armstrong <1999CONAP(5)51>. 

Lacourciere GM, Armstrong RN. The catalytic mechanism of microsomal epoxide hydrolase involves an ester intermediate. J Am Chem Soc 1993 115 10466-10467. 

Hydrolysis of epoxides, esters, amides, and related structures is an important biotransformation reaction that limits the therapeutic activity of many drugs and generates therapeutically active drugs from prodmg structures. In a few cases, hydrolytic reactions can generate a toxic structure. Epoxide hydrolases and esterases are members of the a/(3 hydrolase-fold family of enzymes (Morisseau and Hammock, 2005 Satoh and Hosokawa, 2006). Although their substrate specificities are radically different (e.g., lipids, peptides, epoxides, esters, amides, haloalkanes), their catalytic mechanisms are similar. All of these enzymes have an active site catalytic triad composed of a nucleophilic serine or cysteine residue (esterases/amidases), or aspartate residue (epoxide hydrolases) to activate the substrate, and histidine residue and glutamate or aspartate residues that act cooperatively in an acid—base reaction to activate a water molecule for the hydrolytic step. 

Most EHs have a/ 3-hydrolase fold topology and consist of a core and a lid domain [65,66]. The lid domain is mainly a-helical and contains two tyrosine residues that point toward the catalytic triad and cover the core domain. Both tyrosine residues are involved in substrate binding, Uansition-state stabilization, and activation of the epoxide by protonation. The catalytic center is composed of two aspartate and one histidine residue. The first crystal structure of an epoxide hydrolase was solved for the enzyme from Agrobacterium radiobacter ADI (EchA) [67]. The reaction mechanism of EHs is depiaed in Scheme 9.9. First, a nucleophilic attack of the aspartic residue on the epoxide ring of the substrate 31 takes place and a covalently bound ester 32 is formed. This intermediate is subsequently hydrolyzed by a so-called charge relay system (general base catalysis) and the diol 33 is released from the active site. Key reaction parmers are a histidine residue and a water molecule. It is worth mentioning that a limonene epoxide hydrolase discovered by Arand et al. displayed a different crystal structure and catalytic cycle that is discussed elsewhere [68]. 

Rink, R., Kingma, J., Spelberg, H.L. and Janssen, D. (2000) Tyrosine residues serve as proton donor in the catalytic mechanism of epoxide hydrolase from Agrobacterium radiobacter. Biochemistry, 39, 5600-5613. 

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