De novo construction of multi-enzyme system for one-pot deracemization of (R,S)-1-phenyl-1,2-ethanediol by stereoinversion of (S)-enantiomer to the corresponding counterpart

Multi-Enzymatic Cascade for Efficient Deracemization of dl-Pantolactone into d-Pantolactone

d-pantolactone is an intermediate in the synthesis of d-pantothenic acid, which is known as vitamin B₅. The commercial synthesis of d-pantolactone is carried out through the selective resolution of dl-pantolactone catalyzed by lactone hydrolase. In contrast to a kinetic resolution approach, the deracemization of dl-pantolactone is a simpler, greener, and more sustainable way to obtain d-pantolactone with high optical purity. Herein, an efficient three-enzyme cascade was developed for the deracemization of dl-pantolactone, using l-pantolactone dehydrogenase from Amycolatopsis methanolica (AmeLPLDH), conjugated polyketone reductase from Zygosaccharomyces parabailii (ZpaCPR), and glucose dehydrogenase from Bacillus subtilis (BsGDH). The AmeLPLDH was used to catalyze the dehydrogenated l-pantolactone into ketopantolactone; the ZpaCPR was used to further catalyze the ketopantolactone into d-pantolactone; and glucose dehydrogenase together with glucose fulfilled the function of coenzyme regeneration. All three enzymes were co-expressed in E. coli strain BL21(DE3), which served as the whole-cell biocatalyst. Under optimized conditions, 36 h deracemization of 1.25 M dl-pantolactone d-pantolactone led to an e.e._p value of 98.6%, corresponding to productivity of 107.7 g/(l·d).

Highly enantioselective synthesis of (R)-1,3-butanediol via deracemization of the corresponding racemate by a whole-cell stereoinverting cascade system

Background

Deracemization, the transformation of the racemate into a single stereoisomeric product in 100% theoretical yield, is an appealing but challenging option for the asymmetric synthesis of optically pure chiral compounds as important pharmaceutical intermediates. To enhance the synthesis of (R)-1,3-butanediol from the corresponding low-cost racemate with minimal substrate waste, we designed a stereoinverting cascade deracemization route and constructed the cascade reaction for the total conversion of racemic 1,3-butanediol into its (R)-enantiomer. This cascade reaction consisted of the absolutely enantioselective oxidation of (S)-1,3-butanediol by Candida parapsilosis QC-76 and the subsequent asymmetric reduction of the intermediate 4-hydroxy-2-butanone to (R)-1,3-butanediol by Pichia kudriavzevii QC-1.

Results

The key reaction conditions including choice of cosubstrate, pH, temperature, and rotation speed were optimized systematically and determined as follows: adding acetone as the cosubstrate at pH 8.0, a temperature of 30 °C, and rotation speed of 250 rpm for the first oxidation process; in the next reduction process, the optimal conditions were: adding glucose as the cosubstrate at pH 8.0, a temperature of 35 °C, and rotation speed of 200 rpm. By investigating the feasibility of the step-by-step method with one-pot experiment as a natural extension for performing the oxidation–reduction cascade, the step-by-step approach exhibited high efficiency for this cascade process from racemate to (R)-1,3-butanediol. Under optimal conditions, 20 g/L of the racemate transformed into 16.67 g/L of (R)-1,3-butanediol with 99.5% enantiomeric excess by the oxidation–reduction cascade system in a 200-mL bioreactor.

Conclusions

The step-by-step cascade reaction efficiently produced (R)-1,3-butanediol from the racemate by biosynthesis and shows promising application prospects.

Simultaneous cyclic deracemisation and stereoinversion of alcohols using orthogonal biocatalytic oxidation and reduction reactions

We developed a concurrent cyclic deracemisation approach for secondary alcohols that combines a non-stereospecific oxidation step and a stereoselective reduction step using two mutants of TeSADH that exhibit various extents of stereoselectivities.

Synthesis of enantiomerically pure alcohols and amines via biocatalytic deracemisation methods

Deracemisation via chemo-enzymatic or multi-enzymatic approaches is the optimum substitute for kinetic resolution, which suffers from the limitation of a theoretical maximum 50% yield albeit high enantiomeric excess is attainable. This review covers the recent progress in various deracemisation approaches applied to the synthesis of enantiomerically pure alcohols and amines, such as (1) dynamic kinetic resolution, (2) cyclic deracemisation, (3) linear deracemisation (including stereoinversion) and (4) enantioconvergent methods.

Sortase A-mediated crosslinked short-chain dehydrogenases/reductases as novel biocatalysts with improved thermostability and catalytic efficiency

(S)-carbonyl reductase II (SCRII) from Candida parapsilosis is a short-chain alcohol dehydrogenase/reductase. It catalyses the conversion of 2-hydroxyacetophenone to (S)-1-phenyl-1,2-ethanediol with low efficiency. Sortase was reported as a molecular “stapler” for site-specific protein conjugation to strengthen or add protein functionality. Here, we describe Staphylococcus aureus sortase A-mediated crosslinking of SCRII to produce stable catalysts for efficient biotransformation. Via a native N-terminal glycine and an added GGGGSLPETGG peptide at C-terminus of SCRII, SCRII subunits were conjugated by sortase A to form crosslinked SCRII, mainly dimers and trimers. The crosslinked SCRII showed over 6-fold and 4-fold increases, respectively, in activity and k_cat/K_m values toward 2-hydroxyacetophenone compared with wild-type SCRII. Moreover, crosslinked SCRII was much more thermostable with its denaturation temperature (T_m) increased to 60 °C. Biotransformation result showed that crosslinked SCRII gave a product optical purity of 100% and a yield of >99.9% within 3 h, a 16-fold decrease in transformation duration with respect to Escherichia coli/pET-SCRII. Sortase A-catalysed ligation also obviously improved T_ms and product yields of eight other short-chain alcohol dehydrogenases/reductases. This work demonstrates a generic technology to improve enzyme function and thermostability through sortase A-mediated crosslinking of oxidoreductases.