Stereoselective Reduction of Ethyl 4-Chloro-3-Oxobutanoate by a Microbial Aldehyde Reductase in an Organic Solvent-Water Diphasic System

Self-sufficient asymmetric reduction of β-ketoesters catalysed by a novel and robust thermophilic alcohol dehydrogenase co-immobilised with NADH

β-Hydroxyesters are essential building blocks utilised by the pharmaceutical and food industries in the synthesis of functional products. Beyond the conventional production methods based on chemical catalysis or whole-cell synthesis, the asymmetric reduction of β-ketoesters with cell-free enzymes is gaining relevance. To this end, a novel thermophilic (S)-3-hydroxybutyryl-CoA dehydrogenase from Thermus thermophilus HB27 (Tt27-HBDH) has been expressed, purified and biochemically characterised, determining its substrate specificity towards β-ketoesters and its dependence on NADH as a cofactor. The immobilization of Tt27-HBDH on agarose macroporous beads and its subsequent coating with polyethyleneimine has been found the best strategy to increase the stability and workability of the heterogeneous biocatalyst. Furthermore, we have embedded NADH in the cationic layer attached to the porous surface of the carrier. Since Tt27-HBDH catalyses cofactor recycling through 2-propanol oxidation, we achieve a self-sufficient heterogeneous biocatalyst where NADH is available for the immobilised enzymes but its lixiviation to the reaction bulk is avoided. Taking advantage of the autofluorescence of NADH, we demonstrate the activity of the enzyme towards the immobilised cofactor through single-particle analysis. Finally, we tested the operational stability in the asymmetric reduction of β-ketoesters in batch, succeeding in the reuse of both the enzyme and the co-immobilised cofactor up to 10 reaction cycles.

Magnetic combined cross-linked enzyme aggregates (Combi-CLEAs) for cofactor regeneration in the synthesis of chiral alcohol

Magnetic Fe₃O₄ nanoparticles were prepared and embedded into the Combi-CLEAs to produce the magnetic Combi-CLEAs in this work. The process for magnetic Combi-CLEAs preparation was optimized, and its properties were investigated. The optimum temperature, thermal stability and optimum pH of magnetic Combi-CLEAs were similar to those of Combi-CLEAs. The catalytic performance of magnetic Combi-CLEAs was tested with the biosynthesis of (S)-ethyl 4-chloro-3-hydroxybutyrate ((S)-CHBE). Magnetic Combi-CLEAs could tolerate higher substrate concentration in the biphasic system. The catalytic efficiency and long-term operational stability of magnetic Combi-CLEAs were obviously superior to those of Combi-CLEAs in both aqueous and biphasic systems. Embedding of magnetic Fe₃O₄ nanoparticles endowing rigidity contributed to these improvements. Furthermore, the preparation of magnetic Combi-CLEAs was easy, and its recovery during multiple batches of reactions could be fulfilled by magnetic field. Aforementioned advantages make the magnetic Combi-CLEAs hold obvious potential for industrial application.

Rules for biocatalyst and reaction engineering to implement effective, NAD(P)H-dependent, whole cell bioreductions

Access to chiral alcohols of high optical purity is today frequently provided by the enzymatic reduction of precursor ketones. However, bioreductions are complicated by the need for reducing equivalents in the form of NAD(P)H. The high price and molecular weight of NAD(P)H necessitate in situ recycling of catalytic quantities, which is mostly accomplished by enzymatic oxidation of a cheap co-substrate. The coupled oxidoreduction can be either performed by free enzymes in solution or by whole cells. Reductase selection, the decision between cell-free and whole cell reduction system, coenzyme recycling mode and reaction conditions represent design options that strongly affect bioreduction efficiency. In this paper, each option was critically scrutinized and decision rules formulated based on well-described literature examples. The development chain was visualized as a decision-tree that can be used to identify the most promising route towards the production of a specific chiral alcohol. General methods, applications and bottlenecks in the set-up are presented and key experiments required to "test" for decision-making attributes are defined. The reduction of o-chloroacetophenone to (S)-1-(2-chlorophenyl)ethanol was used as one example to demonstrate all the development steps. Detailed analysis of reported large scale bioreductions identified product isolation as a major bottleneck in process design.

High-yield production of vanillin from ferulic acid by a coenzyme-independent decarboxylase/oxygenase two-stage process

Vanillin is one of the world's most important flavor and fragrance compounds in foods and cosmetics. Recently, we demonstrated that vanillin could be produced from ferulic acid via 4-vinylguaiacol in a coenzyme-independent manner using the decarboxylase Fdc and the oxygenase Cso2. In this study, we investigated a new two-pot bioprocess for vanillin production using the whole-cell catalyst of Escherichia coli expressing Fdc in the first stage and that of E. coli expressing Cso2 in the second stage. We first optimized the second-step Cso2 reaction from 4-vinylguaiacol to vanillin, a rate-determining step for the production of vanillin. Addition of FeCl2 to the cultivation medium enhanced the activity of the resulting E. coli cells expressing Cso2, an iron protein belonging to the carotenoid cleavage oxygenase family. Furthermore, a butyl acetate-water biphasic system was effective in improving the production of vanillin. Under the optimized conditions, we attempted to produce vanillin from ferulic acid by a two-pot bioprocess on a flask scale. In the first stage, E. coli cells expressing Fdc rapidly decarboxylated ferulic acid and completely converted 75 mM of this substrate to 4-vinylguaiacol within 2 h at pH 9.0. After the first-stage reaction, cells were removed from the reaction mixture by centrifugation, and the pH of the resulting supernatant was adjusted to 10.5, the optimal pH for Cso2. This solution was subjected to the second-stage reaction. In the second stage, E. coli cells expressing Cso2 efficiently oxidized 4-vinylguaiacol to vanillin. The concentration of vanillin reached 52 mM (7.8 g L(-1)) in 24 h, which is the highest level attained to date for the biotechnological production of vanillin using recombinant cells.

Characterization and identification of three novel aldo-keto reductases from Lodderomyces elongisporus for reducing ethyl 4-chloroacetoacetate

Lodderomyces elongisporus LH703 isolated from soil samples contained three novel aldo-keto reductases (AKRs) (LEAKR 48, LEAKR 49, and LEAKR 50). The three enzymes were cloned, expressed, and purified to homogeneity for characterization. These three AKRs shared <40% amino acid identity with each other. LEAKR 50 was identified as a member of AKR3 family, whereas the other two LEAKRs were identified as members of two novel AKR families, respectively. All the three AKRs required nicotinamide adenine dinucleotide phosphate as a cofactor. However, they showed diverse characteristics, including optimum catalyzing conditions, resistance to adverse reaction conditions, and substrate specificity. LEAKR 50 was estimated to be a promising biocatalyst that could reduce ethyl 4-chloroacetoacetate with high enantiomeric excess (98% e. e.) and high activity residue under adverse conditions.

Combined cross-linked enzyme aggregates (combi-CLEAs) for efficient integration of a ketoreductase and a cofactor regeneration system

An alternative strategy for cofactor regeneration in the synthesis of valuable chiral alcohols catalyzed by ketoreductases was developed. combi-CLEAs of ketoreductase and d-glucose dehydrogenase enabled the repeated and effective conversion of substrate ethyl 4-chloro-3-oxobutanoate (COBE) with several superiorities. Wide application of this strategy in production of various chiral alcohols could be expected in the future for its high efficiency with low cost.

Effect of ribose, xylose, aspartic acid, glutamine and nicotinic acid on ethyl (S)-4-chloro-3-hydroxybutanoate synthesis by recombinant Escherichia coli

Most reductases which belong to the short chain dehydrogenase/reductase (SDR) superfamily require NAD (P) H for activity. Addition of this cofactor was still necessary for the production of ethyl (S)-4-chloro-3-hydroxybutanoate by Escherichia coli even when a cofactor regeneration system was constructed by co-expressing carbonyl reductase from Pichia stipitis (PsCRI) and glucose dehydrogenase from Bacillus megaterium (BmGDH). In an attempt to reduce dependence on the expensive cofactor, compounds directly or indirectly involved in NADP synthesis were added to the medium. Only glutamine and xylose enhanced the content of intracellular NADP (H) and the concentration of product. The concentration and yield of (S)-CHBE reached 730 mM and 48.7%, with 30 g/L of glutamine and 40 g/L of xylose, a 2.6-fold increase over the control without the addition of the two compounds.

Engineering of formate dehydrogenase: synergistic effect of mutations affecting cofactor specificity and chemical stability

Formate dehydrogenases (FDHs) are frequently used for the regeneration of cofactors in biotransformations employing NAD(P)H-dependent oxidoreductases. Major drawbacks of most native FDHs are their strong preference for NAD(+) and their low operational stability in the presence of reactive organic compounds such as α-haloketones. In this study, the FDH from Mycobacterium vaccae N10 (MycFDH) was engineered in order to obtain an enzyme that is not only capable of regenerating NADPH but also stable toward the α-haloketone ethyl 4-chloroacetoacetate (ECAA). To change the cofactor specificity, amino acids in the conserved NAD(+) binding motif were mutated. Among these mutants, MycFDH A198G/D221Q had the highest catalytic efficiency (k cat/K m) with NADP(+). The additional replacement of two cysteines (C145S/C255V) not only conferred a high resistance to ECAA but also enhanced the catalytic efficiency 6-fold. The resulting quadruple mutant MycFDH C145S/A198G/D221Q/C255V had a specific activity of 4.00 ± 0.13 U mg(-1) and a K m, NADP(+) of 0.147 ± 0.020 mM at 30 °C, pH 7. The A198G replacement had a major impact on the kinetic constants of the enzyme. The corresponding triple mutant, MycFDH C145S/D221Q/C255V, showed the highest specific activity reported to date for a NADP(+)-accepting FDH (v max, 10.25 ± 1.63 U mg(-1)). However, the half-saturation constant for NADP(+) (K m, NADP(+) , 0.92 ± 0.10 mM) was about one order of magnitude higher than the one of the quadruple mutant. Depending on the reaction setup, both novel MycFDH variants could be useful for the production of the chiral synthon ethyl (S)-4-chloro-3-hydroxybutyrate [(S)-ECHB] by asymmetric reduction of ECAA with NADPH-dependent ketoreductases.

Highly efficient synthesis of chiral alcohols with a novel NADH-dependent reductase from Streptomyces coelicolor

An NADH-dependent reductase (ScCR) from Streptomyces coelicolor was discovered by genome mining for carbonyl reductases. ScCR was overexpressed in Escherichia coli BL21, purified to homogeneity and its catalytic properties were studied. This enzyme catalyzed the asymmetric reduction of a broad range of prochiral ketones including aryl ketones, α- and β-ketoesters, with high activity and excellent enantioselectivity (>99% ee) towards β-ketoesters. Among them, ethyl 4-chloro-3-oxobutanoate (COBE) was efficiently converted to ethyl (S)-4-chloro-3-hydroxybutanoate ((S)-CHBE), an important pharmaceutical intermediate, in water/toluene biphasic system. As much as 600 g/L (3.6M) of COBE was asymmetrically reduced within 22 h using 2-propanol as a co-substrate for NADH regeneration, resulting in a yield of 93%, an enantioselectivity of >99% ee, and a total turnover number (TTN) of 12,100. These results indicate the potential of ScCR for the industrial production of valuable chiral alcohols.

Purification and characterization of a novel NADH-dependent carbonyl reductase from Pichia stipitis involved in biosynthesis of optically pure ethyl (S)-4-chloro-3-hydroxybutanoate

A novel NADH-dependent dehydrogenases/reductases (SDRs) superfamily reductase (PsCRII) was isolated from Pichia stipitis. It produced ethyl (S)-4-chloro-3-hydroxybutanoate [(S)-CHBE] in greater than 99% enantiomeric excess. This enzyme was purified to homogeneity by ammonium sulfate precipitation followed by Q-Sepharose chromatography. Compared to similar known reductases producing (S)-CHBE, PsCR II was more suitable for production since the purified PsCRII preferred the inexpensive cofactor NADH to NADPH as the electron donor. Furthermore, the Km of PsCRII for ethyl 4-chloro-3-oxobutanoate (COBE) was 3.3 mM, and the corresponding Vmax was 224 μmol/mg protein/min. The catalytic efficiency is the highest value ever reported for NADH-dependent reductases from yeasts that produce CHBE with high enantioselectivity. In addition, this enzyme exhibited broad substrate specificity for several β-keto esters using NADH as the coenzyme. The properties of PsCRII with those of other carbonyl reductases from yeasts were also compared in this study.

Construction and co-expression of a polycistronic plasmid encoding carbonyl reductase and glucose dehydrogenase for production of ethyl (S)-4-chloro-3-hydroxybutanoate

Biocatalysis of ethyl 4-chloro-3-oxobutanoate (COBE) to ethyl (S)-4-chloro-3-hydroxybutanoate [(S)-CHBE] was carried out using Escherichia coli co-expressing a carbonyl reductase gene from Pichia stipitis and a glucose dehydrogenase gene from Bacillus megaterium. An efficient polycistronic plasmid with a high-level of enzyme co-expression was constructed by changing the order of the genes, altering the Shine-Dalgarno (SD) regions, and aligned spacing (AS) between the SD sequence and the translation initiation codon. The optimal SD sequence was 5-TAAGGAGG-3, and the optimal AS distance was eight nucleotides. Asymmetric reduction of COBE to (S)-CHBE with more than 99% enantiomeric excess was demonstrated by transformants, using a water/ethyl caprylate system. The recombinant cells produced 1260 mM product in the organic phase, and the total turnover number, defined as moles (S)-CHBE formed per mole NADP(+), was 12,600, which was more than 10-fold higher than in aqueous systems.

Regeneration of nicotinamide coenzymes: principles and applications for the synthesis of chiral compounds

Dehydrogenases which depend on nicotinamide coenzymes are of increasing interest for the preparation of chiral compounds, either by reduction of a prochiral precursor or by oxidative resolution of their racemate. The regeneration of oxidized and reduced nicotinamide cofactors is a very crucial step because the use of these cofactors in stoichiometric amounts is too expensive for application. There are several possibilities to regenerate nicotinamide cofactors: established methods such as formate/formate dehydrogenase (FDH) for the regeneration of NADH, recently developed electrochemical methods based on new mediator structures, or the application of gene cloning methods for the construction of "designed" cells by heterologous expression of appropriate genes.A very promising approach is enzymatic cofactor regeneration. Only a few enzymes are suitable for the regeneration of oxidized nicotinamide cofactors. Glutamate dehydrogenase can be used for the oxidation of NADH as well as NADPH while L: -lactate dehydrogenase is able to oxidize NADH only. The reduction of NAD(+) is carried out by formate and FDH. Glucose-6-phosphate dehydrogenase and glucose dehydrogenase are able to reduce both NAD(+) and NADP(+). Alcohol dehydrogenases (ADHs) are either NAD(+)- or NADP(+)-specific. ADH from horse liver, for example, reduces NAD(+) while ADHs from Lactobacillus strains catalyze the reduction of NADP(+). These enzymes can be applied by their inclusion in whole cell biotransformations with an NAD(P)(+)-dependent primary reaction to achieve in situ the regeneration of the consumed cofactor.Another efficient method for the regeneration of nicotinamide cofactors is the electrochemical approach. Cofactors can be regenerated directly, for example at a carbon anode, or indirectly involving mediators such as redox catalysts based on transition-metal complexes.An increasing number of examples in technical scale applications are known where nicotinamide dependent enzymes were used together with cofactor regenerating enzymes.

Efficient Production of Recombinant Aldehyde Reductase and its Application for Asymmetric Reduction of Ethyl 4‐Chloro‐3‐oxobutanoate to Ethyl (R)‐4‐Chloro‐3‐hydroxybutanoate

An NADPH-dependent aldehyde reductase (ALR, EC1.1.1.2) gene is cloned from Sporobolomyces salmonicolor ZJUB 105, and inserted into plasmid pQE30 to construct the expression plasmid (pQE30-ALR). A variety of E. coli strains were employed as hosts to obtain transformants with pQE30-ALR, respectively. Among these different types of transformants, the highest enzyme activity of ALR can be produced with E. coli M15 (pQE30-ALR). The bioactivity of ALR could be further improved significantly by the optimization of induction conditions. The results showed that the enzyme activity of ALR reached 6.48 U/mg protein, which is fifteen times higher than that of S. salmonicolor ZJUB 105. This recombinant strain was applied to the asymmetric reduction of ethyl 4-chloro-3-oxobutanoate (COBE) to ethyl (R)-4-chloro-3- hydroxybutanoate (CHBE). The results showed that the yield and optical purity of (R)-CHBE reached 98.5% and 99% e.e. (enantiomeric excess), respectively.

Asymmetric reduction of o-chloroacetophenone with Candida pseudotropicalis 104

The asymmetric reduction of o-chloroacetophenone 1 with Candida pseudotropicalis 104 produced the corresponding (S)-1-(2-chloro-phenyl)-ethanol 2 with the enantiomeric excess (ee >99%) without addition of any cosolvent. The cell could tolerate high ketone 1 concentration of 233.8 mmol/L (i.e., 36 g/L) with considerable reduction activity in this method. The product 2 concentration achieved 38.9 and 58.4 mmol/L with cells of 40 and 60 g(DCW) (dry cell weight)/L, respectively, in 24 h. The optimum reaction time, the effect of substrate concentration, cosubstrate type and concentration, and cell concentration in the reaction were investigated in this paper.

Bioconversion of ethyl 4-chloro-3-oxobutanoate by permeabilized fresh brewer's yeast cells in the presence of allyl bromide

Ethyl(R)-4-chloro-3-hydroxybutanoate ((R)-CHBE) are obtained by cetyltrimetylammonium bromide (CTAB) permeabilized fresh brewer's yeast whole cells bioconversion of ethyl 4-chloro-3-oxobutanoate (COBE ) in the presence of allyl bromide. The results showed that the activities of alcohol dehydrogenase (ADH) and glucose-6-phosphate dehydrogenase (G6PDH) in CTAB permeabilized brewer's yeast cells increased 525 and 7.9-fold, respectively, compared with that in the nonpermeabilized cells and had high enantioselectivity to convert COBE to (R)-CHBE. As one of co-substrates, glucose-6-phosphate was preprepared using glucose phosphorylation by hexokinase-catalyzed of CTAB permeabilized brewer's yeast cells. In a two phase reaction system with n-butyl acetate as organic solvent and with 2-propanol and glucose-6-phosphate as co-substrates, the highest (R)-CHBE concentration of 447 mM was obtained with 110-130 g/l of the CTAB permeabilized cells at optimized pH, temperature, feeding rate and the shake speed of 125 r/min. The yield and enantiomeric excess (ee) of (R)-CHBE reached 99.5 and 99%, respectively, within 6 h.

Probing the substrate binding site of Candida tenuis xylose reductase (AKR2B5) with site-directed mutagenesis

Little is known about how substrates bind to CtXR (Candida tenuis xylose reductase; AKR2B5) and other members of the AKR (aldo-keto reductase) protein superfamily. Modelling of xylose into the active site of CtXR suggested that Trp23, Asp50 and Asn309 are the main components of pentose-specific substrate-binding recognition. Kinetic consequences of site-directed substitutions of these residues are reported. The mutants W23F and W23Y catalysed NADH-dependent reduction of xylose with only 4 and 1% of the wild-type efficiency (kcat/K(m)) respectively, but improved the wild-type selectivity for utilization of ketones, relative to xylose, by factors of 156 and 471 respectively. Comparison of multiple sequence alignment with reported specificities of AKR members emphasizes a conserved role of Trp23 in determining aldehyde-versus-ketone substrate selectivity. D50A showed 31 and 18% of the wild-type catalytic-centre activities for xylose reduction and xylitol oxidation respectively, consistent with a decrease in the rates of the chemical steps caused by the mutation, but no change in the apparent substrate binding constants and the pattern of substrate specificities. The 30-fold preference of the wild-type for D-galactose compared with 2-deoxy-D-galactose was lost completely in N309A and N309D mutants. Comparison of the 2.4 A (1 A=0.1 nm) X-ray crystal structure of mutant N309D bound to NAD+ with the previous structure of the wild-type holoenzyme reveals no major structural perturbations. The results suggest that replacement of Asn309 with alanine or aspartic acid disrupts the function of the original side chain in donating a hydrogen atom for bonding with the substrate C-2(R) hydroxy group, thus causing a loss of transition-state stabilization energy of 8-9 kJ/mol.

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.

Construction of a two-strain system for asymmetric reduction of ethyl 4-chloro-3-oxobutanoate to (S)-4-chloro-3-hydroxybutanoate ethyl ester

Escherichia coli M15 (pQE30-car0210) was constructed to express carbonyl reductase (CAR) by cloning the car gene from Candida magnoliae and inserting it into pQE30. By cultivating E. coli M15 (pQE30-car0210) and M15 (pQE30-gdh0310), 8.2-fold and 12.3-fold enhancements in specific enzymatic activity over the corresponding original strain were achieved, respectively. After separate cultivations, these two strains were then mixed together at appropriate ratio to construct a novel two-strain system, in which M15 (pQE30-car0210) expressed CAR for ethyl 4-chloro-3-oxobutanoate (COBE) bioreduction and M15 (pQE30-gdh0310) expressed glucose dehydrogenase (GDH) for nicotinamide adenine dinucleotide phosphate (NADPH) regeneration. In this complex system, the effects of substrate concentration, the biomass ratio between two strains as well as reaction temperature were investigated for efficient bioreduction. The results showed that the bioreduction reaction could be completed effectively without any addition of GDH or NADPH/NADP(+). An optical purity of 99% (enantiometric efficiency) was obtained, and the yield of (S)-4-chloro-3-hydroxybutanoate ethyl ester reached 96.6% when initial concentration of COBE was 36.9 mM. The coupling reactions between two different strains were further explored by determining the profile of NADPH in the reaction broth.

Production of a doubly chiral compound, (4R,6R)-4-hydroxy-2,2,6-trimethylcyclohexanone, by two-step enzymatic asymmetric reduction

A practical enzymatic synthesis of a doubly chiral key compound, (4R,6R)-4-hydroxy-2,2,6-trimethylcyclohexanone, starting from the readily available 2,6,6-trimethyl-2-cyclohexen-1,4-dione is described. Chirality is first introduced at the C-6 position by a stereoselective enzymatic hydrogenation of the double bond using old yellow enzyme 2 of Saccharomyces cerevisiae, expressed in Escherichia coli, as a biocatalyst. Thereafter, the carbonyl group at the C-4 position is reduced selectively and stereospecifically by levodione reductase of Corynebacterium aquaticum M-13, expressed in E. coli, to the corresponding alcohol. Commercially available glucose dehydrogenase was also used for cofactor regeneration in both steps. Using this two-step enzymatic asymmetric reduction system, 9.5 mg of (4R,6R)-4-hydroxy-2,2,6-trimethylcyclohexanone/ml was produced almost stoichiometrically, with 94% enantiomeric excess in the presence of glucose, NAD(+), and glucose dehydrogenase. To our knowledge, this is the first report of the application of S. cerevisiae old yellow enzyme for the production of a useful compound.