Catalytic triad of microsomal epoxide hydrolase: replacement of Glu404 with Asp leads to a strongly increased turnover rate

Abstract

Microsomal epoxide hydrolase (mEH) belongs to the superfamily of alpha/beta-hydrolase fold enzymes. A catalytic triad in the active centre of the enzyme hydrolyses the substrate molecules in a two-step reaction via the intermediate formation of an enzyme-substrate ester. Here we show that the mEH catalytic triad is composed of Asp226, Glu404 and His431. Replacing either of these residues with non-functional amino acids results in a complete loss of activity of the enzyme recombinantly expressed in Saccharomyces cerevisiae. For Glu404 and His431 mutants, their structural integrity was demonstrated by their retained ability to form the substrate ester intermediate, indicating that the lack of enzymic activity is due to an indispensable function of either residue in the hydrolytic step of the enzymic reaction. The role of Asp226 as the catalytic nucleophile driving the formation of the ester intermediate was substantiated by the isolation of a peptide fraction carrying the 14C-labelled substrate after cleavage of the ester intermediate with cyanogen bromide. Sequence analysis revealed that one of the two peptides within this sample harboured Asp226. Surprisingly, the replacement of Glu404 with Asp greatly increased the Vmax of the enzyme with styrene 7,8-oxide (23-fold) and 9, 10-epoxystearic acid (39-fold). The increase in Vmax was paralleled by an increase in Km with both substrates, in line with a selective enhancement of the second, rate-limiting step of the enzymic reaction. Owing to its enhanced catalytic properties, the Glu404-->Asp mutant might represent a versatile tool for the enantioselective bio-organic synthesis of chiral fine chemicals. The question of why all native mEHs analysed so far have a Glu in place of the acidic charge relay residue is discussed.

Non-synonymous single nucleotide alterations in the microsomal epoxide hydrolase gene and their functional effects

“…Moreover, it has been reported that Asp226, Glu404 and His431 constitute the EPHX1 catalytic triad. The Asp226 residue attacks the oxirane ring to yield an alkylenzyme intermediate, which is followed by subsequent hydrolysis with water activated through proton abstraction by Glu404 and His431 residues (Arand et al 1999). Together with these latter residues, the non-synonymous change at the residue 43 might exert an influence on the catalytic activity or the conformational structure of the active site.…”

Comparative homology modeling-inspired protein engineering for improvement of catalytic activity of Mugil cephalus epoxide hydrolase

“…On the contrary, M. cephalus EH has glutamic acid (E378). Based on this feature, we can expect that the hydrolysis rate can be increased by changing the E378 to aspartate because the pK a value of aspartic acid is smaller than that of glutamic acid (Arand et al 1999).…”

Epoxide hydrolase-mediated enantioconvergent bioconversions to prepare chiral epoxides and alcohols

“…Because the activation energy required for the hydrolysis of ester intermediate is higher than that for ester formation, the second step is the slow rate-limiting step (Arand et al 2003). Based on this feature, the hydrolysis rate could be increased by replacing the Glu to Asp because the pK a value of Asp is smaller than that of Glu (Arand et al 1999). …”