Rational mutagenesis of an epoxide hydrolase and its structural mechanism for the enantioselectivity improvement toward chiral ortho-fluorostyrene oxide.

Chiral (S)-o-fluorostyrene oxide (oFSO) and vicinal diol (R)-o-fluorophenylethane-1,2-diol (oFPED) are important intermediates for synthesizing treatments for neuropathic diseases. This study aimed to engineer Aspergillus usamii epoxide hydrolase (AuEH2) through a rational mutagenesis strategy to customize high enantioselectivity mutant for rac-oFSO. Out of 181 single-site mutants screened, six showed elevated enantiomeric ratio (E value) ranging from 32 to 108 according to E value and activity mutability landscapes. By combinatorial mutagenesis of A250I with other five single-site mutants, we constructed five double-site mutants, with the best-performing mutant, D5 (A250I/L344V), achieving an E value of 180. This mutant enabled the efficient kinetic resolution of 400 mM rac-oFSO in pure water system using E. coli/Aueh2A250I/L344V, yielding (S)-oFSO (>99 % ees, 50 % yields) and (R)-oFPED (>99 % eep, 50 % yieldp) with space-time yields (STYs) of 331.5 and 376.1 g/L/d, respectively. Combining crystal structure resolution with theoretical computations clarified the enantioselectivity mechanism of D5, demonstrating that A250I reduced the funnel-shaped substrate binding pocket (SBP) while L344V extended its bottom, enhancing specific recognition of (R)-oFSO and inhibiting (S)-oFSO hydrolysis. These findings provide valuable insights for designing highly enantioselective enzyme mutants, advancing the field of asymmetric synthesis of chiral compounds using green biocatalytic processes.

Chemoenzymatic Asymmetric Synthesis of Chiral Triazole Fungicide (R)-Tebuconazole in High Optical Purity Mediated by an Epoxide Hydrolase from Rhodotorula paludigensis.

Tebuconazole is a chiral triazole fungicide used globally in agriculture as a racemic mixture, but its enantiomers exhibit significant enantioselective dissimilarities in bioactivity and environmental behaviors. The steric hindrance caused by the tert-butyl group makes it a great challenge to synthesize tebuconazole enantiomers. Here, we designed a simple chemoenzymatic approach for the asymmetric synthesis of (R)-tebuconazole, which includes the biocatalytic resolution of racemic epoxy-precursor (2-tert-butyl-2-[2-(4-chlorophenyl)ethyl] oxirane, rac-1a) by Escherichia coli/Rpeh whole cells expressed epoxide hydrolase from Rhodotorula paludigensis (RpEH), followed by a one-step chemocatalytic synthesis of (R)-tebuconazole. It was observed that (S)-1a was preferentially hydrolyzed by E. coli/Rpeh, whereas (R)-1a was retained with a specific activity of 103.8 U/g wet cells and a moderate enantiomeric ratio (E value) of 13.4, which was remarkably improved to 43.8 after optimizing the reaction conditions. Additionally, a gram-scale resolution of 200 mM rac-1a was performed using 150 mg/mL E. coli/Rpeh wet cells, resulting in the retention of (R)-1a in a 97.0% ees, a 42.5% yields, and a 40.5 g/L/d space-time yield. Subsequently, the synthesis of highly optical purity (R)-tebuconazole (>99% ee) was easily achieved through the chemocatalytic ring-opening of the epoxy-precursor (R)-1a with 1,2,4-triazole. To elucidate insight into the enantioselectivity, molecular docking simulations revealed that the unique L-shaped substrate-binding pocket of RpEH plays a crucial role in the enantioselective recognition of bulky 2,2-disubstituted oxirane 1a.