Structure-guided improvement in the enantioselectivity of an Aspergillus usamii epoxide hydrolase for the gram-scale kinetic resolution of ortho-trifluoromethyl styrene oxide

Abstract

Microtuning the substrate-binding pocket (SBP) of EHs has emerged as an effective approach to manipulate their enantio- or regio-selectivities and activities towards target substrates. Here, the enantioselectivity (enantiomeric ratio, E) of AuEH2 towards a racemic (rac-) ortho-trifluoromethyl styrene oxide (o-TFMSO) was improved via microtuning its SBP. Based on the analysis on the crystal structure of AuEH2, its specific residues I192, Y216, R322 and L344 lining the SBP in close to the catalytic triad were identified for site-saturation mutagenesis. After screening, five single-site mutants were selected with E values elevated from 8 to 12―25 towards rac-o-TFMSO. To further improve E, four double-site mutants were constructed by combinatorial mutagenesis of AuEH2R322V separately with AuEH2I192V, AuEH2Y216F, AuEH2L344A and AuEH2L344C. Among all the mutants, AuEH2R322V/L344C possessed the largest E of 83 with activity of 67 U/g wet cell. The kinetic resolution of 200 mM rac-o-TFMSO was conducted at 0 °C for 5.5 h using 80 mg/mL wet cells of E. coli/Aueh2R322V/L344C, a transformant expressing AuEH2R322V/L344C, retaining (S)-o-TFMSO with 98.4 % ees and 49.3 % yields. Furthermore, the molecular docking simulation analysis indicated that AuEH2R322V/L344C more enantiopreferentially attacks the terminal carbon (Cβ) in the oxirane ring of (R)-o-TFMSO than AuEH2.