Structure-Guided Regulation in the Enantioselectivity of an Epoxide Hydrolase to Produce Enantiomeric Monosubstituted Epoxides and Vicinal Diols via Kinetic Resolution

Peunipyrone A, an Unexpected Highly Oxygenated γ-Pyrone with a 6/6/6/6/6 Pentacyclic Ring System from a Sponge-Derived Fungus Aspergillus peuniceus

A highly oxygenated γ-pyrone with an unprecedented carbon skeleton, peunipyrone A (1), was isolated from the sponge-derived fungus Aspergillus peuniceus. Compound 1 features a unique fused 6/6/6/6/6 pentacyclic ring system. The chemical structure of compound 1 was elucidated by comprehensive spectroscopic techniques and single-crystal X-ray diffraction. The plausible biosynthetic pathway of compound 1 was proposed. Compound 1 promoted leukemic cell apoptosis by increasing p53 expression.

Significantly enhanced specific activity of Bacillus subtilis (2,3)-butanediol dehydrogenase through computer-aided refinement of its substrate-binding pocket

(2,3)-Butanediol dehydrogenases (BDHs) are widely utilized for the stereoselective interconversion between α-hydroxy ketones and vicinal diols to produce various functional building blocks. In this study, to enhance the specific activity towards (R)-phenyl-1,2-ethanediol (1a) for 2-hydroxyacetophenone (1b), the substrate-binding pocket of a Bacillus subtilis BDH (BsBDHA) was refined through site-directed mutagenesis. Based on molecular docking simulations, 14 residues were identified and subjected to alanine scanning mutagenesis. After screening, two residues, His42 and Gly292, were singled out for partial site-saturation mutagenesis. The results revealed that BsBDHAH42A and BsBDHAG292A displayed high activities of 3.21 and 1.97 U/mg, respectively. Employing combinatorial mutagenesis, a superior mutant, BsBDHAI49L/V266L/G292A, was developed, exhibiting significantly enhanced specific activity and catalytic efficiency towards (R)-1a, achieving 14.81 U/mg and 4.47 mM-1 s-1, respectively, which were 27.4- and 55.9-fold higher than those of BsBDHA. Further substrate spectrum analysis revealed that the superior mutant displayed increased specific activities for (R)-2a-6a by 1.4- to 10.3-fold. The integration of BsBDHAI49L/V266L/G292A into a three-enzymatic cascade for the synthesis of 1b effectively elevated the yield from 58.1 to 82.4%. Molecular mechanism analysis indicated that the mutation-induced changes in intermolecular forces resulted in a higher frequency of reactive conformations for (R)-1a in BsBDHAI49L/V266L/G292A compared to BsBDHA.

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.

Engineering an Epoxide Hydrolase for Chemoenzymatic Asymmetric Synthesis of Chiral Triazole Fungicide (S)- and (R)-Flutriafol

Flutriafol, a globally utilized triazole fungicide in agriculture, is typically applied as a racemic mixture, but its enantiomers differ in bioactivity and environmental impact. The synthesis of flutriafol enantiomers is critically dependent on chiral precursors: 2,2-bisaryl-substituted oxirane [(2-fluorophenyl)-2-(4-fluorophenyl)oxirane, 1a] and 1,2-diol [1-(2-fluorophenyl)-1-(4-fluorophenyl)ethane-1,2-diol, 1b]. Here, we engineered a Rhodotorula paludigensis epoxide hydrolase (RpEH), obtaining mutant Escherichia coli/RpehH336W/L360F with a 6.4-fold enhanced enantiomeric ratio (E) from 5.5 to 35.4. This enabled a gram-scale resolution of rac-1a by E. coli/RpehH336W/L360F, producing (S)-1a (98.2% ees) and (R)-1b (75.0% eep) with 44.3 and 55.7% analytical yields, respectively. As follows, chiral (S)-flutriafol (98.2% ee) and (R)-flutriafol (75.0% ee) were easily synthesized by a one-step chemocatalytic process from (S)-1a and a two-step chemocatalytic process from (R)-1b, respectively. This chemoenzymatic approach offers a superior alternative for the asymmetric synthesis of flutriafol enantiomers. Furthermore, molecular dynamics simulations revealed insight into the enantioselectivity improvement of RpEH toward bulky 2,2-bisaryl-substituted oxirane 1a.

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.

Epoxide Hydrolases: Multipotential Biocatalysts

Epoxide hydrolases are attractive and industrially important biocatalysts. They can catalyze the enantioselective hydrolysis of epoxides to the corresponding diols as chiral building blocks for bioactive compounds and drugs. In this review article, we discuss the state of the art and development potential of epoxide hydrolases as biocatalysts based on the most recent approaches and techniques. The review covers new approaches to discover epoxide hydrolases using genome mining and enzyme metagenomics, as well as improving enzyme activity, enantioselectivity, enantioconvergence, and thermostability by directed evolution and a rational design. Further improvements in operational and storage stabilization, reusability, pH stabilization, and thermal stabilization by immobilization techniques are discussed in this study. New possibilities for expanding the synthetic capabilities of epoxide hydrolases by their involvement in non-natural enzyme cascade reactions are described.