Nh

Figure 6 The structure of cetrorelix, an N- and C-protected decapeptide containing D- and artificial residues in addition to natural amino acids. The arrows indicate the sites of attack by peptidases leading to the four identified metabolites.25

Figure 7 A simplified model showing that the nucleophilic attack of the substrate is mediated by a carboxylate group in the catalytic site to form an ester intermediate. Only in the second step is the intermediate hydrolyzed by an activated water molecule, leading to enzyme reactivation and product liberation. (Reproduced from Testa, B.; Mayer, J. M. Hydrolysis in Drug and Prodrug Metabolism - Chemistry, Biochemistry and Enzymology; Wiley-Verlag Helvetica Chimica Acta: Zurich, Switzerland, 2003, with the kind permission of the copyright owner, Verlag Helvetica Chimica Acta in Zurich.)

located in many organs and tissues. They play essential physiological roles, e.g., vitamin K1 oxide reductase, and are also major modifiers of biological activity in the metabolism of xenobiotics. In mammals, epoxide hydrolases (EH) of broad and complementary substrate specificity are the microsomal EH and the soluble EH.

The microsomal epoxide hydrolases (mEH) are predominantly found in the endoplasmic reticulum. They catalyze the hydration of both alkene and arene oxides, including oxides of polycyclic aromatic hydrocarbons, and do so with regio- and stereoselectivity. The human mEH contains 455 amino acids (52.5 kDa) and is the product of the EPHX1 gene. The human soluble epoxide hydrolase (sEH, also known as cytosolic EH, cEH) has 554 amino acids (62.3 kDa) and is the product of the EPHX2 gene. Its specific substrate is trans-stilbene oxide, and it appears unable to hydrate epoxides of bulky steroids or polycyclic aromatic hydrocarbons.5

The overall reaction catalyzed by epoxide hydrolases is the addition of a water molecule to an epoxide. Alkene oxides thus yield diols whereas arene oxides yield dihydrodiols. As shown in Figure 7, the nucleophilic attack of the substrate is recognized to be mediated by a catalytic carboxylate group to form an ester intermediate. In a second step, this intermediate is hydrolyzed by an activated water molecule, leading to enzyme reactivation and product liberation. This mechanism involves a catalytic triad consisting of a nucleophile, a general base, and a charge relay acid, in close analogy with many other hydrolases (see Section 5.06.2.1).

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