Cloning, expression, purification, and characterization of a novel epoxide hydrolase from Aspergillus niger SQ-6

A novel epoxide hydrolase from Aspergillus niger SQ-6 has now been cloned by inverse PCR. Its gene shows eight exons including a non-coding exon at its 5′-terminal (GenBank Accession No. AY966486). Phylogenetic analysis using deduced amino acid sequence (395 aa) confirms it as an epoxide hydrolase and shares 58.3% identity with that of A. niger LCP521 (GenBank Accession No. AF238460). The predicted catalytic triad is composed of Asp191, His369 and Glu343. Active recombinant epoxide hydrolase has been successfully expressed in Escherichia coli as protein fusions with a poly-His tail. Scale-up fermentation can yield 2.5 g/L of recombinant protein. The electrophoretic pure recombinant protein, which shows similar characterization as natural enzyme purified from A. niger SQ-6, can be easily purified by Ni2+-chelated affinity and gel-filtration chromatography. Optimal pH and temperature for purified enzyme are pH 7.5 and 37 °C, respectively. The Km, kcat and maximal velocity (Vmax) for p-nitrostyrene oxide are determined to be 1.02 mM, 172 s−1 and 231 μmol  min−1 mg−1, respectively. The enzyme can be inhibited by oxidant (H2O2), solvent (Tetrahydrofuran) and several metal ions including Hg2+, Fe2+ and Co2+. This (R)-stereospecific epoxide hydrolase exhibits high enantioselectivity (enantiomeric excess value, 99%) for the less hindered carbon atom of epoxide. It may be an industrial biocatalyst for the preparation of enantiopure epoxides or vicinal diols.