Promotion Effect of Xylose Co-substrate on Stability of Catalytic System for Asymmetric Redox of (R,S)-1- Phenyl-1,2-ethanediol to Its (S)-Enantiomer by Candida parapsilosis

Immobilization of Cofactor Self-Sufficient Recombinant Escherichia coli for Enantioselective Biosynthesis of (R)-1-Phenyl-1,2-Ethanediol

(R)-1-phenyl-1,2-ethanediol is an important synthon for the preparation of β-adrenergic blocking agents. This study identified a (2R,3R)-butanediol dehydrogenase (KgBDH) from Kurthia gibsonii SC0312, which showed high enantioselectivity for production of (R)-1-phenyl-1,2-ethanediol by reduction of 2-hydroxyacetophenone. KgBDH was expressed in a recombinant engineered strain, purified, and characterized. It showed good catalytic activity at pH 6–8 and better stability in alkaline (pH 7.5–8) than an acidic environment (pH 6.0–7.0), providing approximately 73 and 88% of residual activity after 96 h at pH 7.5 and 8.0, respectively. The maximum catalytic activity was obtained at 45°C; nevertheless, poor thermal stability was observed at >30°C. Additionally, the examined metal ions did not activate the catalytic activity of KgBDH. A recombinant Escherichia coli strain coexpressing KgBDH and glucose dehydrogenase (GHD) was constructed and immobilized via entrapment with a mixture of activated carbon and calcium alginate via entrapment. The immobilized cells had 1.8-fold higher catalytic activity than that of cells immobilized by calcium alginate alone. The maximum catalytic activity of the immobilized cells was achieved at pH 7.5, and favorable pH stability was observed at pH 6.0–9.0. Moreover, the immobilized cells showed favorable thermal stability at 25–30°C and better operational stability than free cells, retaining approximately 55% of the initial catalytic activity after four cycles. Finally, 81% yields (195 mM product) and >99% enantiomeric excess (ee) of (R)-1-phenyl-1,2-ethanediol were produced within 12 h through a fed-batch strategy with the immobilized cells (25 mg/ml wet cells) at 35°C and 180 rpm, with a productivity of approximately 54 g/L per day.

Highly enantioselective resolution of racemic 1-phenyl-1,2-ethanediol to (S)-1-phenyl-1,2-ethanediol by Kurthia gibsonii SC0312 in a biphasic system

The asymmetric resolution of racemic 1-phenyl-1,2-ethanediol (PED) to (S)-PED by Kurthia gibsonii SC0312 (K. gibsonii SC0312) was conducted in a biphasic system comprised of an organic solvent and aqueous phosphate buffer. The impacts of organic solvents on the whole cell catalytic activity, metabolic activity, membrane integrity, and material distribution were first evaluated. The results showed that all organic solvents, except for dibutyl phthalate, showed a detrimental effect on the metabolic activity of the cells, especially for those with low log P values. All organic solvents were capable of changing the membrane permeability and membrane integrity of the cells. Moreover, some organic solvents showed a good extraction of the oxidation product. Finally, a high yield of 47.7 % of (S)-PED was obtained by the asymmetric resolution of racemic PED using K. gibsonii SC0312 in a biphasic system under the optimal conditions: racemic PED 120 mM, temperature 35 °C, reaction time 6 h, 180 rpm, and a volume ratio of dibutyl phthalate to aqueous phosphate buffer of 1:1. The optical purity of (S)-PED increased from 51.3 % to >99 %. This work described an efficient approach to improve reaction efficiency, and constructed a highly effective biphasic reaction system for the fabrication of (S)-PED via K. gibsonii SC0312.

Enhancement of substrate concentration in microbial stereoinversion through one-pot oxidation and reduction by aqueous two-phase system

An extractive biocatalytic method of aqueous two-phase system was employed for stereoinversing (R)-1-phenyl-1,2-ethanediol into (S)-1-phenyl-1,2-ethanediol by Candida parapsilosis CCTCC M203011. It was observed that substrate and product inhibitions in microbial stereoinversion through one-pot oxidation and reduction were removed efficiently by extractive biocatalysis in aqueous two-phase system with PEG 4000/phosphate potassium system, and that the substrate concentration was enhanced from 15 to 30 g/L with product optical purity of 99.02% e.e. and yield of 90% after 60 h. Simultaneously, it was observed that change in cell morphology impedes the further enhancement of substrate concentration in this system but can be reversibly changed after stereoinversion or cultivation in systems without PEG.