Biocatalytic anti-Prelog reduction of prochiral ketones with whole cells of Acetobacter pasteurianus GIM1.158
© Du et al.; licensee BioMed Central Ltd. 2014
Received: 22 February 2014
Accepted: 5 June 2014
Published: 10 June 2014
Enantiomerically pure alcohols are important building blocks for production of chiral pharmaceuticals, flavors, agrochemicals and functional materials and appropriate whole-cell biocatalysts offer a highly enantioselective, minimally polluting route to these valuable compounds. At present, most of these biocatalysts follow Prelog’s rule, and thus the (S)-alcohols are usually obtained when the smaller substituent of the ketone has the lower CIP priority. Only a few anti-Prelog (R)-specific whole cell biocatalysts have been reported. In this paper, the biocatalytic anti-Prelog reduction of 2-octanone to (R)-2-octanol was successfully conducted with high enantioselectivity using whole cells of Acetobacter pasteurianus GIM1.158.
Compared with other microorganisms investigated, Acetobacter pasteurianus GIM1.158 was shown to be more effective for the reduction reaction, affording much higher yield, product enantiomeric excess (e.e.) and initial reaction rate. The optimal temperature, buffer pH, co-substrate and its concentration, substrate concentration, cell concentration and shaking rate were 35°C, 5.0, 500 mmol/L isopropanol, 40 mmol/L, 25 mg/mL and 120 r/min, respectively. Under the optimized conditions, the maximum yield and the product e.e. were 89.5% and >99.9%, respectively, in 70 minutes. Compared with the best available data in aqueous system (yield of 55%), the yield of (R)-2-octanol was greatly increased. Additionally, the efficient whole-cell biocatalytic process was feasible on a 200-mL preparative scale and the chemical yield increased to 95.0% with the product e.e. being >99.9%. Moreover, Acetobacter pasteurianus GIM1.158 cells were proved to be capable of catalyzing the anti-Prelog bioreduction of other prochiral carbonyl compounds with high efficiency.
Via an effective increase in the maximum yield and the product e.e. with Acetobacter pasteurianus GIM1.158 cells, these results open the way to use of whole cells of this microorganism for challenging enantioselective reduction reactions on laboratory and commercial scales.
Enantiomerically pure chemicals are important building blocks for production of chiral pharmaceuticals, flavors, agrochemicals and functional materials . For instance, (R)-2-octanol is a versatile intermediate for the synthesis of FLCD (ferroelectric liquid crystals FLCs) and several optically active pharmaceuticals such as steroid and insecticidal ectohormone. Asymmetric reduction of prochiral carbobyl compounds is an efficient method to produce chiral alcohols [2–5]. Whole-cell based biocatalytic reduction has attracted great attention and has been extensively investigated in recent years for the unique advantages such as outstanding enantioselectivity, mild reaction conditions, environmental friendliness and regeneration of cofactors in situ[6–8]. So far, yeasts, bacteria, fungi, and even plant tissues have been extensively researched as biocatalysts for bio-reduction processes [9, 10], and many excellent biocatalytic reaction processes have been developed. However, most of these biocatalysts follow Prelog’s rule , and thus the (S)-alcohols are usually obtained when the smaller substituent of the ketone has the lower CIP priority. Only a few anti-Prelog (R)-specific whole cell biocatalysts have been reported [12–17]. As far as we know, most of the previously reported anti-Prelog microorganisms have not been used for industrial preparation of chiral alcohols for their relatively low catalytic activity and stereoselectivity. Take the substrate 2-octanone as example, the best reported result was given by a recombinant Escherichia coli overexpressing the genes of a Lactobacillus brevis alcohol dehydrogenase (LB-ADH) and a Candida boidinii formate dehydrogenase (CB-FDH) for cofactor regeneration . In buffer system, the recombinant E. coli gave a very low yield of just 2% on a 1.4 mL scale, which increased to 55% when the bioreduction was on a 200 mL scale. Using ionic liquids (ILs) or organic solvents as a second phase could significantly increase the product yield [14, 17–20]. For industrial application, the discovery of more efficient microorganisms would be of great significance.
On the other hand, Acetobacter is a kind of bacteria that is widely used for production of acetic acid owing to its large bioavailibility, easiness of treatment, less environmental pollution, low cost, and mild cultivation conditions . Recently, Acetobacter sp. has been found to produce carbonyl reductase and shown catalytic activity for reduction of carbonyl compounds . However, to our best knowledge, the use of Acetobacter sp. as a biocatalyst for the asymmetric reduction of prochiral ketones remains unexplored, with only few accounts published by us [16, 20, 23, 24]. In these cases, a new acetic acid bacterium, Acetobacter sp. CCTCC M209061, was isolated from China kefir grains, and was capable of effectively catalyzing anti-Prelog asymmetric reduction of a number of carbonyl compounds with excellent enantioselectivity. Therefore, it can be well recognized that Acetobacter has the tremendous potential for asymmetric synthesis of valuable enantiopure alcohols.
In the present study, a number of microorganisms including acetic acid bacteria were tested for their potential for biocatalytic anti-Prelog asymmetric reduction of 2-octanone to (R)-2-octanol. Another Acetobacter pasteurianus GIM1.158 was found to be more active and enantioselective in catalyzing the bioreduction of 2-octanone and, for the first time, was applied as the biocatalyst for the asymmetric reduction of prochiral ketones. The effects of several crucial variables on the bioreduction of 2-octanone with whole cells of Acetobacter pasteurianus GIM1.158 were explored systematically. Also, the efficient biocatalytic process was evaluated on a preparative scale, and the applicability of the promising Acetobacter pasteurianus GIM1.158 was examined for the bioreduction of other prochiral ketones.
Results and discussion
Comparison of the biocatalytic enantioselective reduction of 2-octanone with Acetobacter pasteurianus GIM1.158 and other potential microorganisms
Biocatalytic asymmetric reduction of 2-octanone with various strains
V0(×10−1 μ mol/min)
Y a (%)
Acetobacter pasteurianus GIM1.158
Acetobacter sp. CCTCC M209061
Bacillus cereus AS1.126
Pseudomonas putica GIM1.193
Candida parapsilosis CCTCCM203011
Candia tropicalis CICC1316
Saccharomyces cerevisiae GIM 2.34
Rhodotorula sp. AS2.2241
Pseudomonas oleovorans GIM1.304
Effects of several key variables on the biocatalytic reduction of 2-octanone to (R)-2-octanol with Acetobacter pasteurianus GIM1.158 cells
To gain a deeper insight into the bioreduction and improve the results with respect to the initial reaction rate, the yield and the product e.e., a systematic investigation was made of the effects of several important variables such as reaction temperature, buffer pH, different co-substrates and their concentration, substrate concentration, cell concentration and shaking rate on the reaction.
Effect of substrate concentration on the bioreduction of 2-octanone to ( R )-2-octanol with Acetobacter pasteurianus GIM1.158 cells
V0(×10−2 μ mol/min)
Under the optimized reaction conditions described above (the optimal reaction temperature, buffer pH, co-substrate concentration, substrate concentration, cell concentration and shaking rate were 35°C, 5.0, 500 mmol/L isopropanol, 40 mmol/L, 25 mg/mL and 120 r/min, respectively), the biocatalytic asymmetric reduction of 2-octanone to (R)-2-octanol with Acetobacter pasteurianus GIM1.158 cells gave an encouraging result, with a yield of 89.5% and a product e.e. above 99.9% at a reaction time of 70 min.
Biocatalytic anti-Prelog stereoselective reduction of various prochiral carbonyl compounds
Biocatalytic anti-Prelog stereoselective reduction of various prochiral carbobyl compounds with Acetobacter pasteurianus GIM1.158 cells
V0(×10−1 μ mol/min)
Preparative scale bioreduction of 2-octanone to (R)-2-octanol
Biotransformation on a 200-mL scale was performed to determine scalability of biocatalytic asymmetric reduction of 2-octanone to (R)-2-octanol with Acetobacter pasteurianus GIM1.158 cells. The reaction process was monitored by GC analysis and the product was extracted from the reaction mixture with acetic ether when no more substrate was converted to the product. A final chemical yield of 95.0% was achieved and the product e.e. was above 99.9% in 70 min. To our best knowledge, the reported maximum yield of biotransformation of 2-octanone to (R)-2-octanol in buffer was only 55% while the highest substrate concentration was much lower .
It should be emphasized that the bioreduction process described above suffered from the drawback of low substrate concentration and overall productivity for the large-scale industrial application because of the low solubility of 2-cotanone in buffer system. We believe that the reaction efficiency could be further improved by employing a biphasic system containing an organic solvent or preferably a biocompatible ionic liquid to relieve the restriction of low solubility of substrate in buffer as the concentration of 2-octanone in the second phase could reach up to 1.5 mol/L.
The preparation of enantiopure (R)-2-octanol on a 200-mL preparative scale can be successfully conducted through anti-Prelog asymmetric bioreduction of 2-octanone with Acetobacter pasteurianus GIM1.158 cells. Under the optimal conditions (35°C, pH 5.5, 500 mmol/L isopropanol as co-substrate, substrate concentration 40 mmol/L, cell concentration 25 mg/L, shaking rate 120 r/min), the maximum yield and the product e.e. were 95.0% and above 99.9% respectively in 70 min. Furthermore, Acetobacter pasteurianus cells exhibited high catalytic activity for highly enantioselective reduction of various kinds of carbonyl compounds.
Material and methods
Biological and chemical materials
Acetobacter pasteurianus GIM1.158 was purchased from Guangdong Culture Collection Center. Other strains (Acetobacter sp. CCTCC M209061, Bacillus cereus AS1.126, Pseudomonas putica GIM1.193, Candida parapsilosis CCTCCM203011, Candia tropicalis CICC1316, Saccharomyces cerevisiae GIM 2.34, Rhodotorula sp. AS2.2241, Pseudomonas oleovorans GIM1.304) used in this work were kept in our laboratory (Lab of Applied Biocatalysis, South China University of Technology, China).
2-Octanone (99% purity) and ethyl acetoacetate were purchased from Alfa Aesar (USA). (R)-2-Octanol (98% purity) and (S)-2-octanol (98% purity) were from Sigma-Aldrich (USA). Other prochiral ketones and the corresponding alcohols were obtained from Aldrich-Fluka and were all over 97% purity. All other chemicals were from commercial sources and were of analytical grade.
Acetobacter pasteurianus GIM1.158 cells were cultivated on medium containing 6 g/L yeast extract, 6 g/L peptone, 10 g/L sodium lactate solution, 0.75 g/L K2HPO4, 0.5 g/L NaH2PO4, 0.1 g/L MnSO4, 0.2 g/L MgSO4, 0.1 g/L CaCl2. Other bacteria cells were grown in Nutrient Broth Medium (NB). Yeast cells were cultivated in Yeast Extract Peptone Dextrose Medium (YPD).
General procedure for biocatalytic asymmetric reductions of prochiral ketones
In a typical experiment, 2.0 mL of TEA-HCL buffer (pHs 3.0-7.0, 50 mmol/L) containing wet cells (10–50 mg/mL) and a predetermined quantity of co-substrate (50–700 mmol/L) were added to a10-mL Erlenmeyer flask capped with a septum, and pre-incubated in a water-bath shaker at a specified shaking rate (60–180 r/min) and an appropriate temperature (20–50°C) for 15 min. The reaction was initiated by adding various prochial ketones (5–40 mmol/L) to the mixture. Aliquots (20 μL) were withdrawn at specified time intervals. The product and the residual substrate were extracted with ethyl acetate (40 μL) for twice containing 5.0 mmol/L n-decane (as internal standard) prior to GC analysis. Details about reaction temperature, buffer pH, substrate concentration, co-substrate concentration, cell concentration and shaking rate are specified for each case.
Preparative scale biocatalytic reduction of 2-octanone to (R)-2-octanol
The preparative scale biocatalytic reduction of 2-octanone with whole cells of Acetobacter pasteurianus GIM1.158 was performed under the optimized reaction conditions. The bioreduction reaction was conducted by adding 5 g wet cells of Acetobacter pasteurianus (25 mg/mL) and 8 mmol of 2-octanone (40 mmol/L) to 200 mL of TEA-HCl buffer (50 mmol/L, pH 5.0) containing 500 mmol/L isopropanol as co-substrate at 35°C and 120 r/min. The reaction was terminated when no substrate was transformed to product any more. Then the product and the residual substrate were extracted with ethyl acetate (2 × 200 mL). The yield and e.e. of (R)-2-octanol were determined by GC analysis.
The reaction mixture was analyzed by a Shimadzu GC-2010 with a flame ionization detector and a HP-chiral CB column (30 m × 25 mm × 0.25 m) (USA). The split ratio was 50:1. The injector and the detector were both at 250°C. The carrier gas was nitrogen (>99.9). 2-Octanol was derived with trifluoroacetic anhydride before GC analysis. The column temperature was held at 110°C and the flow rate of nitrogen was 0.75 mL/min. The retention times for derived 2-octanol, n-decane and 2-octanone were 3.9, 4.1 and 4.7 min. For the determination of the product e.e., the column temperature was kept at 85°C for 15 min while the flow rate of nitrogen in the column was 0.5 mL/min. The retention times for (S)-2-octanol and (R)-2-octanol were 12.28 and 12.51 min, respectively. For other substrates and the corresponding products, the column temperature, the flow rate of nitrogen in the column and the retention times were as follows.
2-Pentanone/2-pentanol, 3,3-dimethyl-2-butanone/3,3-dimethyl-2-butanol: 80°C, 13.5 min, 60°C/min to 145°C, retention times: derivatized (S)-2-pentanol (11.1 min), derivatized (R)-2-pentanol (11.4 min), 2-pentanone (16.2 min); derivatized (S)-2-(3,3-dimethyl)butanol (9.6 min), derivatized (R)-2-(3,3-dimethyl)butanol (9.9 min), 3,3-dimethyl-2-butanone(15.7 min). 4'-Methoxyacetophenone/1-(4'-methoxyphenyl) ethanol, 4'-chloroacetophenone/1-(4'-chlorophenyl) ethanol, 4'-hydroxyacetophenone/1-(4'-hydroxyphenyl)ethanol: 140°C (10 min), 1°C /min to 145°C (4 min), retention times: 4-methoxyacetophen-one (11.6 min), (R)-1-(4'-methoxyphenyl)ethanol (15.2 min), (S)-1-(4'-methoxyphenyl)ethanol (15.5 min); 4'-chloroacetophenone (8.3 min), (R)-1-(4'-chlorophenyl)ethanol (12.2 min), (S)-1-(4'-chlorophenyl)ethanol (12.6 min); 4'-hydroxyacetophenone(10.1 min), (S)-1-(4'-hydroxyphenyl)ethanol (14.7 min), (R)-1-(4'-hydroxyphenyl)ethanol (15.1 min).
3-Chloropropiophenone/3-chloro-1-phenylpropanol: 140°C, 30 min, retention times: 3-chloropropiophenone (16.0 min), (R)-3-chloro-1-phenylpropanol (26.5 min), (S)-3-chloro-1-phenylpropanol (26.9 min). Ethyl 4-chloroacetoacetate/ethyl −4-chloro-3-hydroxybutyrate, ethyl acetylacetate/ethyl 3-hydroxybutyrate, methyl acetoacetate/methyl −3-hydroxybutyrate: 80°C (20 min), 50°C/min to 155°C (6 min), retention times: ethyl 4-chloroacetoacetate (24.2 min), derivatized ethyl (R)-4-chloro-3-hydroxybutyrate (19.1 min), derivatized ethyl (S)-4-chloro-3-hydroxybutyrate (19.4 min); ethyl acetylacetate (23.2 min), derivatized ethyl (R)-3-hydroxybutyrate (17.3 min), derivatized ethyl (R)-3-hydroxybutyrate (17.6 min); methyl acetoacetate (21.1 min), derivatized methyl (R)-3-hydroxybutyrate(15.2 min), derivatized methyl (R)-3-hydroxybutyrate (15.4 min).
We wish to thank the National Science Found for Excellent Young Scholars (21222606), the State Key Program of National Natural Science Foundation of China (21336002), the NSFC (21376096), the Key Program of Guangdong Natural Science Foundation (S2013020013049), the National Key Basic Research Program of China (2013CB733500) and the Fundamental Research Funds for SCUT (2013ZG0003) for partially funding this work.
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