Multi-path asymmetric reactions of racemic epoxides mediated by an engineered Escherichia coli overexpressing SfEH2, a newfound epoxide hydrolase from Streptomyces fradiae SF-2

Abstract

To enrich ‘epoxide hydrolase (EH) tool cabinet’ for preparing enantiopure epoxides and 1,2-diols, a novel microbial EH, SfEH2, was identified from Streptomyces fradiae SF-2. Bioinformatics analysis indicated that SfEH2 has typical structural characteristics of α/β-hydrolase fold superfamily. Its coding gene, sfeh2, was then codon-optimized and amplified. A recombinant (re) SfEH2-expressing E. coli BL21(DE3) strain (E. coli/sfeh2) was constructed through DNA manipulations. Substrate spectrum assay including fifteen common racemic (rac-) epoxides by E. coli/sfeh2 showed that reSfEH2 displayed the highest and best complementary regioselectivities (regioselectivity coefficients, αR = 89.7% and βS = 91.4%) towards rac-14a, as well as the highest enantioselectivity (enantiomeric ratio, E = 15.3) in S-type towards rac-15a. Molecular docking simulation revealed that SfEH2 preferentially attacked Cα of (R)-14a and Cβ of (S)-14a geometrically. In kinetic parameter analysis, the KmR (5.74 mM) of purified reSfEH2 for (R)-15a was 1.76-fold lower than KmS (10.09 mM) for (S)-15a. At last, using 50 mg dry cell weight/mL E. coli/sfeh2, the scale-up enantioconvergent hydrolysis of 100 mM rac-14a at 30 °C for 6.0 h afforded (S)-14b with 80.9% eep and 90.1% yield, while the kinetic resolution of 150 mM rac-15a retained (S)-15a with >99.0% ees and 42.2% yield.