Epoxide Hydrolases and their Application in Organic Synthesis

Abstract

c h a p t e r 8 INTRODUCTIONOrganic chemists have become interested in enzymes as catalysts due to their high efficiencies and specificities. Moreover, recent progress in molecular biology and enzyme-related research areas enabled and simplified the production and purifica tion of recombinant enzymes in large quantities and their engineering toward tailormade biocatalysts using straightforward mutagenesis and screening techniques. This is also true for epoxide hydrolases (EHs), as evidenced by the many published research papers about the synthetic applications of naturally occurring or engi neered EHs. EHs catalyze the opening of oxirane rings, generating a vicinal diol as the final product ( Figure 8.1). From a synthetic chemistry point of view, the most valuable EHs are those with either (i) high enantioselectivities or (ii) a combination of low enantio preference with a high level of enantioconvergence. While the former generate enan tiopure epoxides with a maximum yield of 50% in a kinetic resolution process, the latter produce ideally an enantiopure diol product with a theoretical yield of 100%. Thus, EHs provide convenient access to enantiopure epoxides or diols from racemic epoxides. Furthermore, the enantiopure diols can then often be chemically trans formed back to the corresponding epoxides with no effect on the enantiomeric excess (ee). High values of enantioselectivity or enantioconvergence are a consequence of particular enzyme-substrate interactions, which can be modulated by specific reac tion conditions or through the exchange of specific amino acids of the biocatalyst. The final stereochemical outcome of an EH-catalyzed reaction depends solely on the regioselectivity coefficients, which determine the absolute configuration and the ee of the diol product when the reaction reaches 100% conversion. During the reaction, the oxirane ring of each enantiomer is often attacked at either carbon atom, resulting in a mixture of diol enantiomers (Figure 8.2). Unfortunately, several authors used the product-derived E-value, E P (calculated from c and ee P using Sih's equation; see Ref.[1]) for characterizing the stereochemistry of the diol formation of an EH-mediated hydrolysis reaction [2,3]. This is wrong and misleading for epoxide-opening reac tions since there are two possible positions of attack for each oxirane enantiomer. The ideal enantioconvergent EH leads to deracemization of a racemic mixture of an epox ide and exhibits reversed regioselectivity for either substrate enantiomer, that is,

Computational modelling of enzymes: predicting and understanding the selectivity of an epoxide hydrolase

“…Stereoselective enzymes are highly desired within the biochemical and pharmaceutical industries for the production of fine chemicals and drugs [6,12]. Epoxide hydrolases (EH) [13] in particular are a family of enzymes that have attracted much attention from synthetic organic chemists for their ability to selectively produce enantiopure epoxides and corresponding vicinal diols [14]. EHs are ubiquitous in nature [15] and are part of the α/β -hydrolase super-family, whose members are characterised by a shared structural fold and catalytic triad [13].…”

“…Epoxide hydrolases (EH) [13] in particular are a family of enzymes that have attracted much attention from synthetic organic chemists for their ability to selectively produce enantiopure epoxides and corresponding vicinal diols [14]. EHs are ubiquitous in nature [15] and are part of the α/β -hydrolase super-family, whose members are characterised by a shared structural fold and catalytic triad [13]. A wealth of structural and biochemical data has been generated for EHs [1,[16][17][18][19][20][21][22][23], however the origin(s) of their enantio-and regioselective (Figure 1.1) properties remains uncertain [24].…”

“…Of particular interest are epoxide hydrolases (EHs) [169,13]. EHs are capable of producing optically pure epoxides and diols, which are valuable intermediates for synthesising many pharmaceuticals [13].…”

“…Of particular interest are epoxide hydrolases (EHs) [169,13]. EHs are capable of producing optically pure epoxides and diols, which are valuable intermediates for synthesising many pharmaceuticals [13]. For example, they have been used in the production of β -adrenergic receptor blocking drugs (S)-propranolol, (S)-alprenolol and (R)-nifénalol [276][277][278].…”

“…Given the industrial applications of EHs, considerable effort has gone into expanding their functional capabilities [279, 277, 278, 280-285, 116, 286, 287]. Most EHs are members of the α/β -hydrolase superfamily, characterised by a shared structural fold and catalytic triad (Figures 7.1 and 7.2) [13]. In the case of EHs the catalytic triad consists of an aspartate nucleophile, a histidine and charge-relaying aspartic acid [288].…”