Novel anti-Prelog stereospecific carbonyl reductases from Candida parapsilosis for asymmetric reduction of prochiral ketones

Copper-Catalyzed Transamidation of Unactivated Secondary Amides via C-H and C-N Bond Simultaneous Activations

Here, we demonstrate that α-C-H and C-N bonds of unactivated secondary amides can be activated simultaneously by the copper catalyst to synthesize α-ketoamides or α-ketoesters in one step, which is a challenging and underdeveloped transformation. Using copper as a catalyst and air as an oxidant, the reaction is compatible with a broad range of acetoamides, amines, and alcohols. The preliminary mechanism studies and density functional theory calculation indicated that the reaction process may undergo first radical α-oxygenation and then transamidation with the help of the resonant six-membered N,O-chelation and molecular oxygen plays a role as an initiator to trigger the transamidation process. The combination of chelation assistance and dioxygen selective oxygenation strategy would substantially extend the modern mild synthetic amide cleavage toolbox, and we envision that this broadly applicable method will be of great interest in the biopharmaceutical industry, synthetic chemistry, and agrochemical industry.

Oligomeric interactions maintain active-site structure in a noncooperative enzyme family

The evolutionary benefit accounting for widespread conservation of oligomeric structures in proteins lacking evidence of intersubunit cooperativity remains unclear. Here, crystal and cryo-EM structures, and enzymological data, demonstrate that a conserved tetramer interface maintains the active-site structure in one such class of proteins, the short-chain dehydrogenase/reductase (SDR) superfamily. Phylogenetic comparisons support a significantly longer polypeptide being required to maintain an equivalent active-site structure in the context of a single subunit. Oligomerization therefore enhances evolutionary fitness by reducing the metabolic cost of enzyme biosynthesis. The large surface area of the structure-stabilizing oligomeric interface yields a synergistic gain in fitness by increasing tolerance to activity-enhancing yet destabilizing mutations. We demonstrate that two paralogous SDR superfamily enzymes with different specificities can form mixed heterotetramers that combine their individual enzymological properties. This suggests that oligomerization can also diversify the functions generated by a given metabolic investment, enhancing the fitness advantage provided by this architectural strategy.

Alcohol Dehydrogenases with anti-Prelog Stereopreference in Synthesis of Enantiopure Alcohols

Biocatalytic production of both enantiomers of optically active alcohols with high enantiopurities is of great interest in industry. Alcohol dehydrogenases (ADHs) represent an important class of enzymes that could be used as catalysts to produce optically active alcohols from their corresponding prochiral ketones. This review covers examples of the synthesis of optically active alcohols using ADHs that exhibit anti-Prelog stereopreference. Both wild-type and engineered ADHs that exhibit anti-Prelog stereopreference are highlighted.

Metal-free visible-light-induced aerobic oxidation of α-diazoesters leading to α-ketoesters in air

A metal-free and visible-light-induced strategy has been established for the construction of α-ketoesters via aerobic oxidation of α-diazoesters with dioxygen in air at room temperature.

Introducing an α-Keto Ester Functional Group through Pt-Catalyzed Direct C-H Acylation with Ethyl Chlorooxoacetate

Platinum-catalyzed selective C-H acylation of 2-aryloxypyridines with ethyl chlorooxoacetate provides an efficient way of introducing an α-keto ester functional group. The reaction is oxidant-free, additive-free, and, more significantly, free of any decarbonylative side reactions. The reaction tolerates a variety of substituents from strongly electron-donating to strongly electron-withdrawing groups. Double acylation is feasible for 2-phenoxypyridine and its derivatives with only one substituent at the para position. Although the reaction of 2-(2-methylphenoxy)pyridine with ethyl malonyl chloride did not produce the desired β-keto ester, the reaction with ethyl succinyl chloride proceeded smoothly to give the γ-keto ester. Ethyl chlorooxoacetate is much more reactive than ethyl succinyl chloride in this Pt-catalyzed C-H acylation reaction.

New insights into the stereospecific reduction by an (S) specific carbonyl reductase from Candida parapsilosis ATCC 7330: experimental and QM/MM studies

The quantum mechanics/molecular mechanics study of an (S) specific carbonyl reductase from C. parapsilosis ATCC 7330 showing a dual kinetic response for the reduction of ketones and α-ketoesters suggests different reaction mechanisms for the same.

A novel carbonyl reductase with anti-Prelog stereospecificity for the production of t-butyl 6-cyano-(3R, 5R)-dihydroxyhexanoate

A novel gene (crc1) from Candida boidinii was cloned and then overexpressed in a recombinant strain BL21(DE3)/pET30a-crc1 of Escherichia coli. The resulting carbonyl reductase was prepared through fermentations using the recombinant strain. The purified enzyme showed an NADPH-dependent activity and specific activity was 4.65 U/mg using t-butyl 6-cyano-(5R)-hydroxy-3-oxohexanoate (ATS-6) as substrate. The enzyme was optimally active at 35 °C and pH 7, respectively. The apparent K _m and V _max of the enzyme for ATS-6 are 1.5 mM and 21.1 μmol/min mg, respectively, indicating excellent anti-Prelog stereospecificity. Under the optimum condition, t-butyl 6-cyano-(3R,5R)-dihydroxyhexanoate (ATS-7) was prepared with the enzyme with high d.e. value (99.9%) and good conversion (94%) in 4 h, indicating high stereoselectivity and conversion efficiency in biotransformation of ATS-6 to ATS-7.

Immobilized and Free Cells of Geotrichum candidum for Asymmetric Reduction of Ketones: Stability and Recyclability

Marine-derived fungus Geotrichum candidum AS 2.361 was previously reported by our group as an active strain for the enantioselective reduction of ketones. Although some other Geotrichum strains were also found from the terrestrial sources, information on their stability and reusability is scarce. Herein, the stabilities—in terms of pH tolerance, thermostability, and storage stability, and reusability—of G. candidum AS 2.361 were described for the asymmetric reduction of a series of aromatic ketones. Two differently immobilized cells (agar immobilization and calcium alginate immobilization) as well as free cells were prepared. For three substrates (1-(3-bromophenyl) ethan-1-one (1b), 1-(2-chlorophenyl) ethan-1-one (1d), and acetophenone (1g)) immobilized cells on agar showed a great improvement in the bioreduction activities compared to the free cells, increasing yields up to 97% with ee values of 99%. Cells immobilized on agar/calcium alginate could maintain more than 90% of the original activities within the assayed pH ranges of 3.5–11, while free cells were highly sensitive to alkaline and acidic conditions. Concerning thermostability, immobilized cells on agar kept 99% of their original activities after incubation at 60 °C for 1 h, while almost no activity was detected for the free cells under the same condition. Immobilized cells were stable at 4 °C for 80 days without any activity loss, while free cells started to decrease the activity after storage at 4 °C for six days. The immobilized cells retained almost 99% activity after four reuse cycles, while free cells lost almost all the activities at on the third cycle.

Synthesis of 2,3-Difunctionalized Benzofuran Derivatives through Palladium-Catalyzed Double Isocyanide Insertion Reaction

A novel palladium-catalyzed tandem cyclization of 1-(allyloxy)-2-ethynylbenzene derivatives with isocyanides in the presence of water has been developed. The key intermediates, benzofuran-3-α-carbonyl aldehydes, were obtained through a simple acid hydrolysis process and could serve as precursors for structurally diverse 2,3-difunctionalized benzofuran derivatives such as important 2-benzofurylquinoxalines, benzofuran-3-α-ketoesters and benzofuryl ynediones. This transformation features convenient operation, simple and commercially available starting materials, broad functional-group compatibility, and moderate to good reaction yields.

Genome hunting of carbonyl reductases from Candida glabrata for efficient preparation of chiral secondary alcohols

In this work, genome hunting strategy was adopted in screening for reductases from Candida glabrata. A total of 37 putative reductases were successfully expressed in E. coli BL21(DE3). A substrate library containing 32 substrates was established for characterization of each reductase by average specific activity and Shannon-Wiener index. Among them, Cg26 was identified with the highest efficiency and wider substrate spectrum in the reduction of prochiral ketones, with average activity and Shannon-Wiener index of 8.95U·mg^-1 and 2.82. Cg26 is a member of 'extended' short chain dehydrogenase/reductase superfamily. Ni²⁺ could improve its activity. As much as 150g·L^-1 ethyl 2-oxo-4-phenylbutyrate could be completely converted by 10g·L^-1 Cg26. This study provides evidence for this newly identified Cg26 in the preparation of chiral secondary alcohols.

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.

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.

Microfluidic synthesis of α-ketoesters via oxidative coupling of acetophenones with alcohols under metal-free conditions

An efficient and novel method for the synthesis of α-ketoesters has been developed via oxidative coupling of acetophenones with alcohols under TBHP/I₂/DBU conditions in a microfluidic chip reactor, which has a wide substrate scope, uses a lower dosage of iodine and affords higher product yields in only a few seconds.

Copper(II)-Catalyzed Benzylic C(sp3)-H Aerobic Oxidation of (Hetero)Aryl Acetimidates: Synthesis of Aryl-α-ketoesters

A straightforward method is developed in this paper for the synthesis of α-ketoesters through copper-catalyzed aerobic oxidation of (hetero)aryl acetimidates using molecular oxygen as a sustainable oxidant. The reaction represents the first example of the direct synthesis of aryl-α-ketoesters from arylacetimidates through the aerobic oxidation of a benzylic C(sp3)-H (C═O) bond in moderate to good yield. This transformation occurs under mild reaction conditions with a wide range of substrates and utilizes a readily available oxidant and catalyst. The synthetic utility of this transformation is demonstrated through scaled-up synthesis. A plausible reaction mechanism is also proposed.

Efficient biosynthesis of ethyl (R)-4-chloro-3-hydroxybutyrate using a stereoselective carbonyl reductase from Burkholderia gladioli

Background

Ethyl (R)-4-chloro-3-hydroxybutyrate ((R)-CHBE) is a versatile chiral precursor for many pharmaceuticals. Although several biosynthesis strategies have been documented to convert ethyl 4-chloro-3-oxobutanoate (COBE) to (R)-CHBE, the catalytic efficiency and stereoselectivity are still too low to be scaled up for industrial applications. Due to the increasing demand of (R)-CHBE, it is essential to explore more robust biocatalyst capable of preparing (R)-CHBE efficiently.

Results

A stereoselective carbonyl reductase toolbox was constructed and employed into the asymmetric reduction of COBE to (R)-CHBE. A robust enzyme designed as BgADH3 from Burkholderia gladioli CCTCC M 2012379 exhibited excellent activity and enantioselectivity, and was further characterized and investigated in the asymmetric synthesis of (R)-CHBE. An economical and satisfactory enzyme-coupled cofactor recycling system was created using recombinant Escherichia coli cells co-expressing BgADH3 and glucose dehydrogenase genes to regenerate NADPH in situ. In an aqueous/octanol biphasic system, as much as 1200 mmol COBE was completely converted by using substrate fed-batch strategy to afford (R)-CHBE with 99.9 % ee at a space-time yield per gram of biomass of 4.47 mmol∙L⁻¹∙h⁻¹∙g DCW⁻¹.

Conclusions

These data demonstrate the promising of BgADH3 in practical synthesis of (R)-CHBE as a valuable chiral synthon. This study allows for the further application of BgADH3 in the biosynthesis of chiral alcohols, and establishes a preparative scale process for producing (R)-CHBE with excellent enantiopurity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-016-0301-x) contains supplementary material, which is available to authorized users.

In situ expression of (R)-carbonyl reductase rebalancing an asymmetric pathway improves stereoconversion efficiency of racemic mixture to (S)-phenyl-1,2-ethanediol in Candida parapsilosis CCTCC M203011

Background

Candida parapsilosis (R)-carbonyl reductase (RCR) and (S)-carbonyl reductase (SCR) are involved in the stereoconversion of racemic (R,S)-1-phenyl-1,2-ethanediol (PED) to its (S)-isomer. RCR catalyzes (R)-PED to 2-hydroxyacetophenone (2-HAP), and SCR catalyzes 2-HAP to (S)-PED. However, the stereoconversion efficiency of racemic mixture to (S)-PED is not high because of an activity imbalance between RCR and SCR, with RCR performing at a lower rate than SCR. To realize the efficient preparation of racemic mixture to (S)-PED, an in situ expression of RCR and a two-stage control strategy were introduced to rebalance the RCR- and SCR-mediated pathways.

Results

An in situ expression plasmid pCP was designed and RCR was successfully expressed in C. parapsilosis. With respect to wild-type, recombinant C. parapsilosis/pCP-RCR exhibited over four-fold higher activity for catalyzing racemic (R,S)-PED to 2-HAP, while maintained the activity for catalyzing 2-HAP to (S)-PED. The ratio of k_cat/K_M for SCR catalyzing (R)-PED and RCR catalyzing 2-HAP was about 1.0, showing the good balance between the functions of SCR and RCR. Based on pH and temperature preferences of RCR and SCR, a two-stage control strategy was devised, where pH and temperature were initially set at 5.0 and 30 °C for RCR rapidly catalyzing racemic PED to 2-HAP, and then adjusted to 4.5 and 35 °C for SCR transforming 2-HAP to (S)-PED. Using these strategies, the recombinant C. parapsilosis/pCP-RCR catalyzed racemic PED to its (S)-isomer with an optical purity of 98.8 % and a yield of 48.4 %. Most notably, the biotransformation duration was reduced from 48 to 13 h.

Conclusions

We established an in situ expression system for RCR in C. parapsilosis to rebalance the functions between RCR and SCR. Then we designed a two-stage control strategy based on pH and temperature preferences of RCR and SCR, better rebalancing RCR and SCR-mediated chiral biosynthesis pathways. This work demonstrates a method to improve chiral biosyntheses via in situ expression of rate-limiting enzyme and a multi-stage control strategy to rebalance asymmetric pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0539-y) contains supplementary material, which is available to authorized users.

Metal-free oxidative esterification of acetophenones with alcohols: a facile one-pot approach to α-ketoesters

A novel oxidative esterification method for the synthesis of α-ketoesters has been developed, which is a facile one-pot approach to α-ketoesters from acetophenones with alcohols, especially under metal-free conditions with broad substrate scope.

Highly Efficient Synthesis of Optically Pure (S)-1-phenyl-1,2-ethanediol by a Self-Sufficient Whole Cell Biocatalyst

Terminal vicinal diols are important chiral building blocks and intermediates in organic synthesis. Reduction of α-hydroxy ketones provides a straightforward approach to access these important compounds. In this study, it has been found that asymmetric reduction of a series of α-hydroxy aromatic ketones and 1-hydroxy-2-pentanone, catalyzed by Candida magnolia carbonyl reductase (CMCR) with glucose dehydrogenase (GDH) from Bacillus subtilis for cofactor regeneration, afforded 1-aryl-1,2-ethanediols and pentane-1,2-diol, respectively, in up to 99 % ee. In order to evaluate the efficiency of the bioreduction, lyophilized recombinant Escherichia coli whole cells coexpressing CMCR and GDH genes were used as the biocatalyst and α-hydroxy acetophenone as the model substrate, and the reaction conditions, such as pH, cosolvent, the amount of biocatalyst and the presences of a cofactor (i.e., NADP⁺), were optimized. Under the optimized conditions (pH 6, 16 h), the bioreduction proceeded smoothly at 1.0 m substrate concentration without the external addition of cofactor, and the product (S)-1-phenyl-1,2-ethanediol was isolated with 90 % yield and 99 % ee. This offers a practical biocatalytic method for the preparation of these important vicinal diols.

Cu-catalyzed aerobic oxidative esterification of acetophenones with alcohols to α-ketoesters

Copper-catalyzed aerobic oxidative esterification of acetophenones with alcohols using molecular oxygen has been developed to form a broad range of α-ketoesters in good yields. In addition to reporting scope and limitations of our new method, mechanism studies are reported that reveal that the carbonyl oxygen in the ester mainly originated from dioxygen.

2-Oxo promoted hydrophosphonylation & aerobic intramolecular nucleophilic displacement reaction

Highly efficient catalyst free methods for the synthesis of α-hydroxy-β-oxo phosphonates (HOP) and α-oxoesters (OE) have been described for the first time. The existence of a 2-oxo group in α-oxoaldehydes (OA) was a key factor in promoting the reaction of the tervalent phosphite form towards activated aldehydes (OA) in the synthesis of HOP.

Asymmetric synthesis of optically active methyl-2-benzamido-methyl-3-hydroxy-butyrate by robust short-chain alcohol dehydrogenases from Burkholderia gladioli

Specific short-chain alcohol dehydrogenases were discovered and used in the dynamic kinetic asymmetric transformation of methyl 2-benzamido-methyl-3-oxobutanoate with excellent stereo-selectivity.