Enzymatic resolution of epichlorohydrin catalyzed by whole cells in an organic solvent/buffer biphasic system

Preparation of (S)-epichlorohydrin using a novel halohydrin dehalogenase by selective conformation adjustment

Chiral epichlorohydrin (ECH) is an attractive intermediate for chiral pharmaceuticals and chemicals preparation. The asymmetric synthesis of chiral ECH using 1,3-dicholoro-2-propanol (1,3-DCP) catalyzed by a haloalcohol dehalogenase (HHDH) was considered as a feasible approach. However, the reverse ring opening reaction caused low optical purity of chiral ECH, thus severely restricts the industrial application of HHDHs. In the present study, a novel selective conformation adjustment strategy was developed with an engineered HheCPS to regulate the kinetic parameters of the forward and reverse reactions, based on site saturation mutation and molecular simulation analysis. The HheCPS mutant E85P was constructed with a markable change in the conformation of (S)-ECH in the substrate pocket and a slight impact on the interaction between 1,3-DCP and the enzyme, which resulted in the kinetic deceleration of the reverse reactions. Compared with HheCPS, the catalytic efficiency (kcat(S)-ECH/Km(S)-ECH) of the reversed reaction dropped to 0.23-fold (from 0.13 to 0.03 mM-1 s-1), while the catalytic efficiency (kcat(1,3-DCP)/Km(1,3-DCP)) of the forward reaction only reduced from 0.83 to 0.71 mM-1 s-1. With 40 mM 1,3-DCP as substrate, HheCPS E85P catalyzed the synthesis of (S)-ECH with the yield up to 55.35% and the e.e. increased from 92.54 to >99%. Our work provided an effective approach for understanding the stereoselective catalytic mechanism as well as the green manufacturing of chiral epoxides.

Gram-Scale Synthesis of (R)-P-Chlorophenyl-1,2-Ethanediol at High Concentration by a Pair of Epoxide Hydrolases

(R)-p-chlorophenyl-1,2-ethanediol (pCPED) is an important intermediate for the synthesis of (R)-eliprodil that is widely applied in the treatment of ischemic stroke. To prepare (R)-pCPED with high enantiomeric excess (eep) and yield via the enantioconvergent hydrolysis of racemic styrene oxide (rac-pCSO) at high concentration, the bi-enzymatic catalysis was designed and investigated by a pair of epoxide hydrolases, a mutant (PvEH1Z4X4-59) of Phaseolus vulgaris EH1 and a mutant (RpEHF361V) of Rhodotorula paludigena RpEH. Firstly, the maximum allowable concentration of rac-pCSO was confirmed. Subsequently, the addition mode and the weight ratio of two Escherichia coli cells were optimized. Finally, under the optimized reaction conditions—the cell weight ratio 20:1 of E. coli/pveh1z4x4-59 to E. coli/rpehF361V, a simultaneous addition mode, and reaction temperature at 25°C—300 mM rac-pCSO in the 100 ml 4% (v/v) Tween-20/phosphate buffer system (100 mM, pH 7.0) was completely hydrolyzed within 5 h, affording (R)-pCPED with 87.8% eep, 93.4% yield, and 8.63 g/L/h space–time yield (STY). This work would be an efficient technical strategy for the preparation of chiral vicinal diols at industrial scale.

A integrated process for nitrilase-catalyzed asymmetric hydrolysis and easy biocatalyst recycling by introducing biocompatible biphasic system

The whole-cell nitrilase-catalyzed asymmetric hydrolysis of nitriles is a green and efficient preparation approach for chiral carboxylic acids, but often suffers from toxicity and cell lysis from organic substrates. In this work, a novel integrated process for whole-cell nitrilase-catalyzed asymmetric hydrolysis was developed for the first time by introducing a biocompatible ionic liquid (IL)-based biphasic system. The whole-cell nitrilases displayed an outstanding stability and recyclability in the biphasic system and still retained > 85% activity even after 7 cycles reaction. A preparative-scale fed-batch hydrolysis of o-chloromandelonitrile to (R)-o-chloromandelic acid (R-CMA) was performed using the integrated process. The results revealed a yield of 91.3% and a space-time yield of 746.4 g·L^-1·d^-1, which are currently the highest reported values for R-CMA biosynthesis. The proposed integrated process avoids substrate inhibition, facilitates the reusability of whole-cell nitrilases, and thus shows great potential for the sustainable production of chiral carboxylic acids.

Enantioconvergent hydrolysis of m-nitrostyrene oxide at an elevated concentration by Phaseolus vulgaris epoxide hydrolase in the organic/aqueous two-phase system

(R)-m-Nitrophenyl-1,2-ethanediol (m-NPED) is a versatile and highly value-added chiral building block for the synthesis of some bioactive compounds, such as (R)-Nifenalol. To efficiently produce (R)-m-NPED through the enantioconvergent hydrolysis of racemic (rac-) m-nitrostyrene oxide (m-NSO) using the whole resting cells of Escherichia coli/pCold-pveh2 intracellularly expressing PvEH2, an epoxide hydrolase from Phaseolus vulgaris, two reaction systems were investigated. In the Na₂ HPO₄ -NaH₂ PO₄ buffer (50 mmol l^-1 , pH 7·0) system, merely 15 mmol l^-1 rac-m-NSO was successfully subjected to enantioconvergent hydrolysis, producing (R)-m-NPED with 86·0% enantiomeric excess (ee_p ) and 177·6 mg l^-1  h^-1 space-time yield (STY). The experimental result indicated that there is inhibitory effect of rac-m-NSO at high concentration on PvEH2. To efficiently increase the concentration of rac-m-NSO and the STY of (R)-m-NPED, petroleum ether was first selected to construct an organic/aqueous two-phase system. Then, both the volume ratio (v_o /v_b ) of petroleum ether to phosphate buffer and the weight ratio (w_c /w_s ) of E. coli/pCold-pveh2 dry cells to rac-m-NSO were optimized as 2 : 8 and 5 : 1, respectively. In the optimized petroleum ether/phosphate buffer two-phase system, the enantioconvergent hydrolysis of rac-m-NSO at 40 mmol l^-1 (6·6 mg ml^-1 ) was carried out at 25°C for 12 h using 33·0 mg ml^-1 vacuum freeze-dried cells of E. coli/pCold-pveh2, producing (R)-m-NPED with 87·4% ee_p , 82·3% yield and 502·4 mg l^-1  h^-1 STY. SIGNIFICANCE AND IMPACT OF THE STUDY: Epoxide hydrolases play a crucial role in producing enantiopure epoxides and/or vicinal diols. However, numerous biocatalytic reactions of organic compounds, such as epoxides, in aqueous phase suffered various restrictions. Herein, the enantioconvergent hydrolysis of rac-m-NSO in two reaction systems was investigated using the whole cells of Escherichia coli/pCold-pveh2. As a result, the concentration of rac-m-NSO and the space-time yield of (R)-m-NPED in organic/aqueous two-phase system were significantly increased, when compared with those in aqueous phase. To our knowledge, this is the first report about the production of (R)-m-NPED from rac-m-NSO at an elevated concentration by PvEH2 in the two-phase system.

Molecular modification of a halohydrin dehalogenase for kinetic regulation to synthesize optically pure (S)-epichlorohydrin

Asymmetric synthesis of chiral epichlorohydrin (ECH) from 1,3-dichloro-2-propanol (1,3-DCP) using halohydrin dehalogenases (HHDHs) is of great value due to the 100% theoretical yield and high enantioselectivity. The vital problem in the asymmetric synthesis is to prepare optically pure ECH. In this study, key amino acid residues located at halide ion channels of HheC (P175S/W249P) (HheC_PS) were modified to regulate the kinetic parameters. HheC_PS I81W, F86N and V94R were constructed with the corresponding halide ion channels destroyed. The catalytically efficiencies (k_cat/K_m) of the three mutants exhibited 0.38-, 0.23- and 0.23-fold decrease toward (S)-ECH and the reverse reaction was significantly inhibited. As the results, (S)-ECH was synthesized with >99% enantiomeric excess (e.e.) and 63.42%, 67.08% and 57.01% yields, respectively, under 20 mM 1,3-DCP as substrate. To our knowledge, this is the first investigation of the molecule kinetic modification of HHDHs and also the first report for the biosynthesis of optically pure (S)-ECH from 1,3-DCP using HHDHs.

Biosynthesis of (R)-2-hydroxy-3-phenylpropionic acid using whole recombinant Escherichia coli cells in an aqueous/n-octane biphasic system

(R)-2-hydroxy-3-phenylpropionic acid (PLA) is an ideal antimicrobial compound with broad-spectrum activity against a wide range of Gram-positive bacteria, some Gram-negative bacteria, and fungi. We studied the bioconversion of phenylpyruvate (PPA) to PLA using whole recombinant Escherichia coli cells in a series of buffer/organic solvent systems. Octane was found to be the best organic solvent. The optimum volume ratio of the water phase to the n-octane phase, conversion temperature, substrate concentration, and cell concentration were 6:4, 40 °C, 12.5 g/L, and 30 g/L wet cells, respectively. Under the optimized conditions, the average PLA productivity in the aqueous/ n-octane system was 30.69% higher than that in the aqueous system, and 32.31 g/L PLA was obtained with the use of a stirred reactor (2-L scale). Taken together, our findings indicated that PLA biosynthesis was more efficient in an aqueous/n-octane biphasic system than in a monophasic aqueous system. The proposed biphasic system is an effective strategy for enhancing PLA yield and the biosynthesis of its analogues.

Biosynthesis of chiral epichlorohydrin using an immobilized halohydrin dehalogenase in aqueous and non-aqueous phase

Asymmetric synthesis of chiral epichlorohydrin (ECH) from 1,3-dichloro-2-propanol (1,3-DCP) using halohydrin dehalogenase (HHDH) is of great value due to the 100% theoretical yield and high enantioselectivity. In this study, HheC (P175S/W249P) was immobilized on an A502Ps resin and used for the preparation of (S)-ECH. In aqueous system, the immobilized HheC catalyzed the biosynthesis of (S)-ECH with 83.78% yield and 92.53% enantiomeric excess (ee) at 1,3-DCP concentration of 20 mM. The non-aqueous system was further developed using water saturated ethyl acetate as solvent and reaction phase. The non-aqueous bioconversion system showed higher enantioselectivity (>98% ee) toward (S)-ECH production with modest conversion (52.34%) compared with ever reported aqueous reactions. Batch reactions were performed in a packed-bed bioreactor for 45 batches in aqueous phase and 24 batches in non-aqueous phase. The present work demonstrated the potential of immobilized HheC (P175S/W249P) in aqueous and non-aqueous phase biosynthesis of chiral ECH.

Bioresolution of racemic phenyl glycidyl ether by a putative recombinant epoxide hydrolase from Streptomyces griseus NBRC 13350

In order to produce enantiomerically pure epoxides for the synthesis of value-added chemicals, a novel putative epoxide hydrolase (EH) sgeh was cloned and overexpressed in pET28a/Escherichia coli BL21(DE3). The 1047 bp sgeh gene was mined from Streptomyces griseus NBRC 13350 genome sequence. The recombinant hexahistidyl-tagged SGEH was purified (16.6-fold) by immobilized metal-affinity chromatography, with 90% yield as a homodimer of 100 kDa. The recombinant E. coli whole cells overexpressing SGEH could kinetically resolve racemic phenyl glycidyl ether (PGE) into (R)-PGE with 98% ee, 40% yield, and enantiomeric ratio (E) of 20. This was achieved under the optimized reaction conditions i.e. cell/substrate ratio of 20:1 (w/w) at pH 7.5 and 20 °C in 10% (v/v) dimethylformamide (DMF) in a 10 h reaction. 99% enantiopure (R)-PGE was obtained when the reaction time was prolonged to 12 h with a yield of 34%. In conclusion, an economically viable and environment friendly green process for the production of enantiopure (R)-PGE was developed by using wet cells of E. coli expressing recombinant SGEH.