Enantioselective reduction of prochiral ketones by engineered bifunctional fusion proteins

Stairway to Stereoisomers: Engineering Short- and Medium-Chain Ketoreductases To Produce Chiral Alcohols

The short- and medium-chain dehydrogenase/reductase superfamilies are responsible for most chiral alcohol production in laboratories and industries. In nature, they participate in diverse roles such as detoxification, housekeeping, secondary metabolite production, and catalysis of several chemicals with commercial and environmental significance. As a result, they are used in industries to create biopolymers, active pharmaceutical intermediates (APIs), and are also used as components of modular enzymes like polyketide synthases for fabricating bioactive molecules. Consequently, random, semi-rational and rational engineering have helped transform these enzymes into product-oriented efficient catalysts. The rise of newer synthetic chemicals and their enantiopure counterparts has proved challenging, and engineering them has been the subject of numerous studies. However, they are frequently limited to the synthesis of a single chiral alcohol. The study attempts to defragment and describe hotspots of engineering short- and medium-chain dehydrogenases/reductases for the production of chiral synthons.

BioWF: A naturally-fused, di-domain biocatalyst from biotin biosynthesis displays an unexpectedly broad substrate scope

The carbon backbone of biotin is constructed from the C₇ di‐acid pimelate, which is converted to an acyl‐CoA thioester by an ATP‐dependent, pimeloyl‐CoA synthetase (PCAS, encoded by BioW). The acyl‐thioester is condensed with ʟ‐alanine in a decarboxylative, Claisen‐like reaction to form an aminoketone (8‐amino‐7‐oxononanoic acid, AON). This step is catalysed by the pyridoxal 5’‐phosphate (PLP)‐dependent enzyme (AON synthase, AONS, encoded by BioF). Distinct versions of Bacillus subtilis BioW (BsBioW) and E. coli BioF (EcBioF) display strict substrate specificity. In contrast, a BioW‐BioF fusion from Corynebacterium amycolatum (CaBioWF) accepts a wider range of mono‐ and di‐fatty acids. Analysis of the active site of the BsBioW : pimeloyl‐adenylate complex suggested a key role for a Phe (F192) residue in the CaBioW domain; a F192Y mutant restored the substrate specificity to pimelate. This surprising substrate flexibility also extends to the CaBioF domain, which accepts ʟ‐alanine, ʟ‐serine and glycine. Structural models of the CaBioWF fusion provide insight into how both domains interact with each other and suggest the presence of an intra‐domain tunnel. The CaBioWF fusion catalyses conversion of various fatty acids and amino acids to a range of AON derivatives. Such unexpected, natural broad substrate scope suggests that the CaBioWF fusion is a versatile biocatalyst that can be used to prepare a number of aminoketone analogues.

Genetic fusion of P450 BM3 and formate dehydrogenase towards self-sufficient biocatalysts with enhanced activity

Fusion of multiple enzymes to multifunctional constructs has been recognized as a viable strategy to improve enzymatic properties at various levels such as stability, activity and handling. In this study, the genes coding for cytochrome P450 BM3 from B. megaterium and formate dehydrogenase from Pseudomonas sp. were fused to enable both substrate oxidation catalyzed by P450 BM3 and continuous cofactor regeneration by formate dehydrogenase within one construct. The order of the genes in the fusion as well as the linkers that bridge the enzymes were varied. The resulting constructs were compared to individual enzymes regarding substrate conversion, stability and kinetic parameters to examine whether fusion led to any substantial improvements of enzymatic properties. Most noticeably, an activity increase of up to threefold was observed for the fusion constructs with various substrates which were partly attributed to the increased diflavin reductase activity of the P450 BM3. We suggest that P450 BM3 undergoes conformational changes upon fusion which resulted in altered properties, however, no NADPH channeling was detected for the fusion constructs.

Construction and characterization of novel bifunctional fusion proteins composed of alcohol dehydrogenase and NADH oxidase with efficient oxidized cofactor regeneration

To tune the efficiency of oxidized cofactor recycling between alcohol dehydrogenase (ADH) and NADH oxidase (NOX) for the production of aromatic chiral alcohols, we designed and constructed four novel bifunctional fusion proteins composed of thermostable ADH and NOX from Thermococcus kodakarensis KOD1. ADH was linked to the N- or C-terminus of NOX with a typical rigid linker (EAAAK)₃ and a flexible linker (GGGGS)₃ , respectively. Compared with the parental enzymes, the NOX moieties in the four fusion proteins exhibited higher specific activities (141%-282%), while the ADH moieties exhibited varying levels of specific activity (69%-167%). All fusion proteins showed decreased affinities toward the cofactors, with increased K_m values toward NADH (159%-406%) and NAD⁺ (202%-372%). In the enantioselective oxidation of (RS)-1-phenylethanol coupled with cofactor regeneration, the four fusion proteins displayed different positive and negative effects on the recycling efficiency of the oxidized cofactor. The two fusion proteins composed of NOX at the N-terminus exhibited higher total turnover numbers than the corresponding mixtures of individual enzymes with equal activities, particularly at low cofactor concentrations. These findings suggest high cofactor recycling efficiencies of the fusion proteins with appropriate design and their potential application in the biosynthesis of chiral alcohols.

Construction and characterization of a novel glucose dehydrogenase-leucine dehydrogenase fusion enzyme for the biosynthesis of L-tert-leucine

Background

Biosynthesis of l-tert-leucine (l-tle), a significant pharmaceutical intermediate, by a cofactor regeneration system friendly and efficiently is a worthful goal all the time. The cofactor regeneration system of leucine dehydrogenase (LeuDH) and glucose dehydrogenase (GDH) has showed great coupling catalytic efficiency in the synthesis of l-tle, however the multi-enzyme complex of GDH and LeuDH has never been constructed successfully.

Results

In this work, a novel fusion enzyme (GDH–R3–LeuDH) for the efficient biosynthesis of l-tle was constructed by the fusion of LeuDH and GDH mediated with a rigid peptide linker. Compared with the free enzymes, both the environmental tolerance and thermal stability of GDH–R3–LeuDH had a great improved since the fusion structure. The fusion structure also accelerated the cofactor regeneration rate and maintained the enzyme activity, so the productivity and yield of l-tle by GDH–R3–LeuDH was all enhanced by twofold. Finally, the space–time yield of l-tle catalyzing by GDH–R3–LeuDH whole cells could achieve 2136 g/L/day in a 200 mL scale system under the optimal catalysis conditions (pH 9.0, 30 °C, 0.4 mM of NAD⁺ and 500 mM of a substrate including trimethylpyruvic acid and glucose).

Conclusions

It is the first report about the fusion of GDH and LeuDH as the multi-enzyme complex to synthesize l-tle and reach the highest space–time yield up to now. These results demonstrated the great potential of the GDH–R3–LeuDH fusion enzyme for the efficient biosynthesis of l-tle.

Engineering bi-functional enzyme complex of formate dehydrogenase and leucine dehydrogenase by peptide linker mediated fusion for accelerating cofactor regeneration

This study reports the application of peptide linker in the construction of bi-functional formate dehydrogenase (FDH) and leucine dehydrogenase (LeuDH) enzymatic complex for efficient cofactor regeneration and L-tert leucine (L-tle) biotransformation. Seven FDH-LeuDH fusion enzymes with different peptide linker were successfully developed and displayed both parental enzyme activities. The incorporation order of FDH and LeuDH was investigated by predicting three-dimensional structures of LeuDH-FDH and FDH-LeuDH models using the I-TASSER server. The enzymatic characterization showed that insertion of rigid peptide linker obtained better activity and thermal stability in comparison with flexible peptide linker. The production rate of fusion enzymatic complex with suitable flexible peptide linker was increased by 1.2 times compared with free enzyme mixture. Moreover, structural analysis of FDH and LeuDH suggested the secondary structure of the N-, C-terminal domain and their relative positions to functional domains was also greatly relevant to the catalytic properties of the fusion enzymatic complex. The results show that rigid peptide linker could ensure the independent folding of moieties and stabilized enzyme structure, while the flexible peptide linker was likely to bring enzyme moieties in close proximity for superior cofactor channeling.

Spatial organization of multi-enzyme biocatalytic cascades

Multi-enzyme cascades provide a wealth of valuable chemicals. Efficiency of reaction schemes can be improved by spatial organization of biocatalysts. This review will highlight various methods of spatial organization of biocatalysts: fusion, immobilization, scaffolding and encapsulation.

Construction and evaluation of a novel bifunctional phenylalanine–formate dehydrogenase fusion protein for bienzyme system with cofactor regeneration

Phenylalanine dehydrogenase (PheDH) plays an important role in enzymatic synthesis of l-phenylalanine for aspartame (sweetener) and detection of phenylketonuria (PKU), suggesting that it is important to obtain a PheDH with excellent characteristics. Gene fusion of PheDH and formate dehydrogenase (FDH) was constructed to form bifunctional multi-enzymes for bioconversion of l-phenylalanine coupled with coenzyme regeneration. Comparing with the PheDH monomer from Microbacterium sp., the bifunctional PheDH–FDH showed noteworthy stability under weakly acidic and alkaline conditions (pH 6.5–9.0). The bifunctional enzyme can produce 153.9 mM l-phenylalanine with remarkable performance of enantiomers choice by enzymatic conversion with high molecular conversion rate (99.87 %) in catalyzing phenylpyruvic acid to l-phenylalanine being 1.50-fold higher than that of the separate expression system. The results indicated the potential application of the PheDH and PheDH–FDH with coenzyme regeneration for phenylpyruvic acid analysis and l-phenylalanine biosynthesis in medical diagnosis and pharmaceutical field.

Enantioselective Synthesis of Vicinal (R,R)-Diols by Saccharomyces cerevisiae Butanediol Dehydrogenase

Butanediol dehydrogenase (Bdh1p) from Saccharomyces cerevisiae belongs to the superfamily of the medium-chain dehydrogenases and reductases and converts reversibly R-acetoin and S-acetoin to (2R,3R)-2,3-butanediol and meso-2,3-butanediol, respectively. It is specific for NAD(H) as a coenzyme, and it is the main enzyme involved in the last metabolic step leading to (2R,3R)-2,3-butanediol in yeast. In this study, we have used the activity of Bdh1p in different forms-purified enzyme, yeast extracts, permeabilized yeast cells, and as a fusion protein (with yeast formate dehydrogenase, Fdh1p)-to transform several vicinal diketones to the corresponding diols. We have also developed a new variant of the delitto perfetto methodology to place BDH1 under the control of the GAL1 promoter, resulting in a yeast strain that overexpresses butanediol dehydrogenase and formate dehydrogenase activities in the presence of galactose and regenerates NADH in the presence of formate. While the use of purified Bdh1p allows the synthesis of enantiopure (2R,3R)-2,3-butanediol, (2R,3R)-2,3-pentanediol, (2R,3R)-2,3-hexanediol, and (3R,4R)-3,4-hexanediol, the use of the engineered strain (as an extract or as permeabilized cells) yields mixtures of the diols. The production of pure diol stereoisomers has also been achieved by means of a chimeric fusion protein combining Fdh1p and Bdh1p. Finally, we have determined the selectivity of Bdh1p toward the oxidation/reduction of the hydroxyl/ketone groups from (2R,3R)-2,3-pentanediol/2,3-pentanedione and (2R,3R)-2,3-hexanediol/2,3-hexanedione. In conclusion, Bdh1p is an enzyme with biotechnological interest that can be used to synthesize chiral building blocks. A scheme of the favored pathway with the corresponding intermediates is proposed for the Bdh1p reaction.

Whole-cell biotransformation systems for reduction of prochiral carbonyl compounds to chiral alcohol in Escherichia coli

Lactobacillus brevis alcohol dehydrogenase (Lb-ADH) catalyzes reduction of prochiral carbonyl compounds to chiral alcohol and meanwhile consumes its cofactor NADH into NAD⁺, while the cofactor regeneration can be catalyzed by Candida boidinii formate dehydrogenase (Cb-FDH). This work presents three different Escherichia coli whole-cell biocatalyst systems expressing recombinant ADH/FDH, FDH-LIN1-ADH and FDH-LIN2-ADH, respectively, all of which display very high efficacies of prochiral carbonyl conversion with respect to conversion rates and enantiomeric excess values. ADH/FDH represents co-expression of Lb-ADH and Cb-FDH under different promoters in a single vector. Fusion of Lb-ADH and Cb-FDH by a linker peptide LIN1 (GGGGS)₂ or LIN2 (EAAAK)₂ generates the two bifunctional enzymes FDH-LIN1-ADH and FDH-LIN2-ADH, which enable efficient asymmetric reduction of prochiral ketones in whole-cell biotransformation.

Multi-enzymatic one-pot reduction of dehydrocholic acid to 12-keto-ursodeoxycholic acid with whole-cell biocatalysts

Ursodeoxycholic acid (UDCA) is a bile acid of industrial interest as it is used as an agent for the treatment of primary sclerosing cholangitis and the medicamentous, non-surgical dissolution of gallstones. Currently, it is prepared industrially from cholic acid following a seven-step chemical procedure with an overall yield of <30%. In this study, we investigated the key enzymatic steps in the chemo-enzymatic preparation of UDCA-the two-step reduction of dehydrocholic acid (DHCA) to 12-keto-ursodeoxycholic acid using a mutant of 7β-hydroxysteroid dehydrogenase (7β-HSDH) from Collinsella aerofaciens and 3α-hydroxysteroid dehydrogenase (3α-HSDH) from Comamonas testosteroni. Three different one-pot reaction approaches were investigated using whole-cell biocatalysts in simple batch processes. We applied one-biocatalyst systems, where 3α-HSDH, 7β-HSDH, and either a mutant of formate dehydrogenase (FDH) from Mycobacterium vaccae N10 or a glucose dehydrogenase (GDH) from Bacillus subtilis were expressed in a Escherichia coli BL21(DE3) based host strain. We also investigated two-biocatalyst systems, where 3α-HSDH and 7β-HSDH were expressed separately together with FDH enzymes for cofactor regeneration in two distinct E. coli hosts that were simultaneously applied in the one-pot reaction. The best result was achieved by the one-biocatalyst system with GDH for cofactor regeneration, which was able to completely convert 100 mM DHCA to >99.5 mM 12-keto-UDCA within 4.5 h in a simple batch process on a liter scale.

One-step synthesis of 12-ketoursodeoxycholic acid from dehydrocholic acid using a multienzymatic system

12-ketoursodeoxycholic acid (12-keto-UDCA) is a key intermediate for the synthesis of ursodeoxycholic acid (UDCA), an important therapeutic agent for non-surgical treatment of human cholesterol gallstones and various liver diseases. The goal of this study is to develop a new enzymatic route for the synthesis 12-keto-UDCA based on a combination of NADPH-dependent 7β-hydroxysteroid dehydrogenase (7β-HSDH, EC 1.1.1.201) and NADH-dependent 3α-hydroxysteroid dehydrogenase (3α-HSDH, EC 1.1.1.50). In the presence of NADPH and NADH, the combination of these enzymes has the capacity to reduce the 3-carbonyl- and 7-carbonyl-groups of dehydrocholic acid (DHCA), forming 12-keto-UDCA in a single step. For cofactor regeneration, an engineered formate dehydrogenase, which is able to regenerate NADPH and NADH simultaneously, was used. All three enzymes were overexpressed in an engineered expression host Escherichia coli BL21(DE3)Δ7α-HSDH devoid of 7α-hydroxysteroid dehydrogenase, an enzyme indigenous to E. coli, in order to avoid formation of the undesired by-product 12-chenodeoxycholic acid in the reaction mixture. The stability of enzymes and reaction conditions such as pH value and substrate concentration were evaluated. No significant loss of activity was observed after 5 days under reaction condition. Under the optimal condition (10 mM of DHCA and pH 6), 99 % formation of 12-keto-UDCA with 91 % yield was observed.

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.

Novel whole-cell biocatalysts with recombinant hydroxysteroid dehydrogenases for the asymmetric reduction of dehydrocholic acid

Ursodeoxycholic acid is an important pharmaceutical so far chemically synthesized from cholic acid. Various biocatalytic alternatives have already been discussed with hydroxysteroid dehydrogenases (HSDH) playing a crucial role. Several whole-cell biocatalysts based on a 7α-HSDH-knockout strain of Escherichia coli overexpressing a recently identified 7β-HSDH from Collinsella aerofaciens and a NAD(P)-bispecific formate dehydrogenase mutant from Mycobacterium vaccae for internal cofactor regeneration were designed and characterized. A strong pH dependence of the whole-cell bioreduction of dehydrocholic acid to 3,12-diketo-ursodeoxycholic acid was observed with the selected recombinant E. coli strain. In the optimal, slightly acidic pH range dehydrocholic acid is partly undissolved and forms a suspension in the aqueous solution. The batch process was optimized making use of a second-order polynomial to estimate conversion as function of initial pH, initial dehydrocholic acid concentration, and initial formate concentration. Complete conversion of 72 mM dehydrocholic acid was thus made possible at pH 6.4 in a whole-cell batch process within a process time of 1 h without cofactor addition. Finally, a NADH-dependent 3α-HSDH from Comamonas testosteroni was expressed additionally in the E. coli production strain overexpressing the 7β-HSDH and the NAD(P)-bispecific formate dehydrogenase mutant. It was shown that this novel whole-cell biocatalyst was able to convert 50 mM dehydrocholic acid directly to 12-keto-ursodeoxycholic acid with the formation of only small amounts of intermediate products. This approach may be an efficient process alternative which avoids the costly chemical epimerization at C-7 in the production of ursodeoxycholic acid.