Occurrence of multiple ethyl 4-chloro-3-oxobutanoate-reducing enzymes in Candida magnoliae

Efficient biocatalytic stereoselective reduction of methyl acetoacetate catalyzed by whole cells of engineered E. coli

Asymmetric synthesis of chiral β-hydroxy esters, the key building blocks for many functional materials, is currently of great interest. In this study, the biocatalytic anti-Prelog reduction of methyl acetoacetate (MAA) to methyl-(R)-3-hydroxybutyrate ((R)-HBME) was successfully carried out with high enantioselectivity using the whole cell of engineered E. coli, which harbored an AcCR (carbonyl reductase) gene from Acetobacter sp. CCTCC M209061 and a GDH (glucose dehydrogenase) gene from Bacillus subtilis 168 for the in situ regeneration of the coenzyme. Compared with the corresponding wild strain, the engineered E. coli cells were proved to be more effective for the bio-reduction of MAA, and afforded much higher productivity. Under the optimized conditions, the product e.e. was >99.9% and the maximum yield was 85.3% after a reaction time of 10 h, which were much higher than those reported previously. In addition, the production of (R)-HBME increased significantly by using a fed-batch strategy of tuning pH, with a space-time yield of approximately 265 g L-1 d-1, thus the issue in previous research of relatively low substrate concentrations appears to be solved. Besides, the established bio-catalytic system was proved to be feasible up to a 150 mL scale with a large-scale relatively high substrate concentration and selectivity. For further industrial application, these results open a way to use of whole cells of engineered E. coli for challenging higher substrate concentrations of β-ketone esters enantioselective reduction reactions.

A carbonyl reductase from Candida parapsilosis ATCC 7330: substrate selectivity and enantiospecificity

A purified carbonyl reductase (SRED) asymmetrically reduces ketones and α-ketoesters to (S)-alcohols with a potential application in the synthesis of industrially important chiral molecules.

Efficient synthesis of (R)-3-hydroxypentanenitrile in high enantiomeric excess by enzymatic reduction of 3-oxopentanenitrile

(R)-3-Hydroxypentanenitrile (HPN) is an important intermediate in the synthesis of an immunosuppressive inosine 5′-monophosphate dehydrogenase inhibitor. An efficient enzymatic procedure for the synthesis of (R)-HPN with over 99 % enantiomeric excess using a novel acetoacetyl-CoA reductase (AdKR) from Achromobacter denitrificans was successfully established. Many microorganisms are known to reduce 3-oxopentannitrile (KPN) to (R)-HPN. An enzyme from A. denitrificans partially purified using ion exchange chromatography reduced KPN to (R)-HPN with high enantioselectivity. The AdKR gene was cloned and sequenced and found to comprise 738 bp and encode a polypeptide of 26,399 Da. The deduced amino acid sequence showed a high degree of similarity to those of other putative acetoacetyl-CoA reductases and putative 3-ketoacyl-ACP reductases. The AdKR gene was singly expressed and coexpressed together with a glucose dehydrogenase (GDH) as a coenzyme regenerator in Escherichia coli under the control of the lac promoter. (R)-HPN was synthesized with over 99 % e.e. using a cell-free extract of recombinant E. coli cells coexpressing AdKR and GDH.

Diversity of 4-chloroacetoacetate ethyl ester-reducing enzymes in yeasts and their application to chiral alcohol synthesis

Enzymes which reduce 4-chloroacetoacetate ethyl ester (CAAE) to (R)- or (S)-4-chloro-3-hydroxybutanoate ethyl ester (CHBE) were investigated. Several microorganisms which can reduce CAAE with high yields were discovered. An NADPH-dependent aldehyde reductase, ARI, and an NADPH-dependent carbonyl reductase, S1, were isolated from Sporobolomyces salmonicolor and Candida magnoliae, respectively, and enzymatic synthesis of chiral CHBE was performed through the reduction of CAAE. When ARI-overproducing Escherichia coli transformant cells or C. magnoliae cells were incubated in an organic solvent-water diphasic system. CAAE was stoichiometrically converted to (R)- or (S)-CHBE (> 92% enantiomeric excess), respectively. Multiple CAAE-reducing enzymes were present in S. salmonicolor, C. magnoliae and bakers' yeast. Comparison of the primary structures of these CAAE-reducing enzymes with other protein sequences showed that CAAE-reducing enzymes are widely distributed in various protein families, and various physiological roles of these enzymes in the cell were speculated.

Purification and characterization of NADPH-dependent aldo?keto reductase specific for ?-keto esters from Penicillium citrinum, and production of methyl (S)-4-bromo-3-hydroxybutyrate

A novel beta-keto ester reductase (KER) was purified to homogeneity from recombinant Escherichia coli (pTrcKER) cells, which efficiently expressed the ker gene cloned from Penicillium citrinum IFO4631. The enzyme was monomeric and had a molecular mass of 37 kDa. It catalyzed the reduction of some beta-keto esters, especially alkyl 4-halo-3-oxobutyrates. However, it did not catalyze the reverse reaction, the dehydrogenation of alkyl 4-halo-3-hydroxybutyrates and other alcohols. The enzyme required NADPH as a cofactor and showed no activity with NADH. Therefore, it was defined as a NADPH-dependent aldo-keto reductase (AKR3E1), belonging to the AKR superfamily. The enzyme stereospecifically produced methyl (S)-4-bromo-3-hydroxybutyrate from its keto derivative with high stereospecificity (97.9% enantiomer excess). E. coli cells expressing KER and glucose dehydrogenase in the water/butyl acetate two-phase system achieved a high productivity of (S)-4-bromo-3-hydroxybutyrate (277 mM, 54 mg/ml) in the organic solvent layer.