Molecular and Biochemical Evidence for the Involvement of the Asp-333–His-523 Pair in the Catalytic Mechanism of Soluble Epoxide Hydrolase

Abstract

In order to investigate the involvement of amino acids in the catalytic mechanism of the soluble epoxide hydrolase, different mutants of the murine enzyme were produced using the baculovirus expression system. Our results are consistent with the involvement of Asp-333 and His-523 in a catalytic mechanism similar to that of other alpha/beta hydrolase fold enzymes. Mutation of His-263 to asparagine led to the loss of approximately half the specific activity compared to wild-type enzyme. When His-332 was replaced by asparagine, 96.7% of the specific activity was lost and mutation of the conserved His-523 to glutamine led to a more dramatic loss of 99.9% of the specific activity. No activity was detectable after the replacement of Asp-333 by serine. However, more than 20% of the wild-type activity was retained in an Asp-333-->Asn mutant produced in Spodoptera frugiperda cells. We purified, by affinity chromatography, the wild-type and the Asp-333-->Asn mutant enzymes produced in Trichoplusia ni cells. We labeled these enzymes by incubating them with the epoxide containing radiolabeled substrate juvenile hormone III (JH III). The purified Asp-333-->Asn mutant bound 6% of the substrate compared to the wild-type soluble epoxide hydrolase. The mutant also showed 8% of the specific activity of the wild-type. Preincubation of the purified Asp-333-->Asn mutant at 37 degrees C (pH 8), however, led to a complete recovery of activity and to a change of isoelectric point (pI), both of which are consistent with hydrolysis of Asn-333 to aspartic acid. This intramolecular hydrolysis of asparagine to aspartic acid may explain the activity observed in this mutant. Wild-type enzyme that had been radiolabeled with the substrate was digested with trypsin. Using reverse phase-high pressure liquid chromatography, we isolated four radiolabeled peptides of similar polarity. These peptides were not radiolabeled if the enzyme was preincubated with a selective competitive inhibitor of soluble epoxide hydrolase 4-fluorochalcone oxide. This strongly suggested that these peptides contained a catalytic amino acid. Each peptide was characterized with N-terminal amino acid sequencing and electrospray mass spectrometry. All four radiolabeled peptides contained overlapping sequences. The only aspartic acid present in all four peptides and conserved in all epoxide hydrolases was Asp-333. These peptides resulted from cleavage at different trypsin sites and the mass of each was consistent with the covalent linkage of Asp-333 to the substrate.

Epoxide hydrolases: biochemistry and molecular biology

“…The alignment characterization also lead to identification of a proposed catalytic triad of amino acids within sEH. Mutagenesis experiments implicated Asp 333 and His 523 in the catalytic mechanism for sEH, similar to other a/b hydrolase enzymes [67]. The results of the latter investigation indicated that since His 523 was conserved in all epoxide hydrolase across species, and mutation of His 523 in murine sEH resulted in activity of 0.1% of wild-type activity, this residue was likely a key participant in the catalytic mechanism for the hydrolase.…”

Catalysis of potato epoxide hydrolase, StEH1

“…A suggested physiological role of the plant enzymes is through their involvement in the epoxide fatty acid metabolism and biosynthetic pathway of cutin [9]. The reaction mechanism of epoxide hydrolases from mammalian [10][11][12] and bacterial [13,14] sources have been thoroughly investigated through mutagenesis studies, kinetic analyses and solved threedimensional structures of enzyme representatives from mouse [15], human [16], Agrobacterium radiobacter AD1 [17] and Aspergillus niger [18] (see [19] for a recent review). The canonical epoxide hydrolases belong to the structural class of α/β hydrolase fold enzymes.…”

Visualization of a Covalent Intermediate between Microsomal Epoxide Hydrolase, but not Cholesterol Epoxide Hydrolase, and their Substrates

“…Theoretical considerations and experimental data suggest that the enzymatic hydrolysis of epoxides proceeds via intermediate formation of a covalent bond between enzyme and substrate. While the complete catalytic triad of soluble EH has been resolved (Arand et al, 1996;Pinot et al, 1995), only the water-activating histidine of the microsomal EH has been experimentally identified (Bell and Kasper, 1993). In an elegant study, Lacourciere and Armstrong (1993) demonstrated transfer of ''0 from microsomal EH to its substrate l,l0-phenanthroline 5,6-oxide under single-turnover conditions with a stoichiometry that is best explained by the formation of the postulated enzyme-substrate ester.…”