Enantioselective Reduction of α,β-Unsaturated Ketones and Aryl Ketones by Perakine Reductase

His126 Substitution Shifts Perakine Reductase Activity from Carbonyl Reduction to Ene Reduction

Perakine reductase (PR) is an aldo-keto reductase (AKR) carbonyl reductase. His126 is one of the AKR catalytic tetrads in PR. Substitution of His126 with any other amino acids except Gln switched PR to an ene reductase. Molecular simulation suggested that the substrate-binding pose and the properties of the 126 residue determined the chemoselectivity. Given the strict conservation of His126 in AKR superfamily carbonyl reductases, modification of the corresponding site is a feasible strategy to design AKR-derived novel ene reductases.

Advances in aldo-keto reductases immobilization for biocatalytic synthesis of chiral alcohols

Chiral alcohols are essential building blocks of numerous pharmaceuticals and fine chemicals. Aldo-keto reductases (AKRs) constitute a superfamily of oxidoreductases that catalyze the reduction of aldehydes and ketones to their corresponding alcohols using NAD(P)H as a coenzyme. Knowledge about the crucial roles of AKRs immobilization in the biocatalytic synthesis of chiral alcohols is expanding. Herein, we reviewed the characteristics of various AKRs immobilization approaches, the applications of different immobilization materials, and the prospects of continuous flow bioreactor construction by employing these immobilized biocatalysts for synthesizing chiral alcohols. Finally, the opportunities and ongoing challenges for AKR immobilization are discussed and the outlook for this emerging area is analyzed.

Green synthesis aspects of (R)-(-)-mandelic acid; a potent pharmaceutically active agent and its future prospects

(R)-(-)-mandelic acid is an important carboxylic acid known for its numerous potential applications in the pharmaceutical industry as it is an ideal starting material for the synthesis of antibiotics, antiobesity drugs and antitumor agents. In past few decades, the synthesis of (R)-(-)-mandelic acid has been undertaken mainly through the chemical route. However, chemical synthesis of optically pure (R)-(-)-mandelic acid is difficult to achieve at an industrial scale. Therefore, its microbe mediated production has gained considerable attention as it exhibits many merits over the chemical approaches. The present review focuses on various biotechnological strategies for the production of (R)-(-)-mandelic acid through microbial biotransformation and enzymatic catalysis; in particular, an analysis and comparison of the synthetic methods and different enzymes. The wild type as well as recombinant microbial strains for the production of (R)-(-)-mandelic acid have been elucidated. In addition, different microbial strategies used for maximum bioconversion of mandelonitrile into (R)-(-)-mandelic acid are discussed in detail with regard to higher substrate tolerance and maximum bioconversion.HighlightsMandelonitrile, mandelamide and o-chloromandelonitrile can be used as substrates to produce (R)-(-)-mandelic acid by enzymes.Three enzymes (nitrilase, nitrile hydratase and amidase) are systematically introduced for production of (R)-(-)-mandelic acid.Microbial transformation is able to produce optically pure (R)-(-)-mandelic acid with 100% productive yield.

Switching the chemoselectivity of perakine reductase for selective reduction of α,β-unsaturated ketones by Arg127 mutation

The chemoselectivity of perakine reductase (PR) was engineered through rational design. We identified Arg127 as a control site of chemoselectivity. Mutation of Arg127 switched the chemoselectivity of PR between CO and CC or led to non-selectivity towards α,β-unsaturated ketones, leading to the production of allylic alcohols, saturated ketones, or a mixture of both. This study provides an example for developing novel reductases for α,β-unsaturated ketones.

Chemoselective reduction of α,β-unsaturated ketones to allylic alcohols under catalyst-free conditions

A strategy of chemoselective reduction of α, β-unsaturated ketones was developed in our group. H3N·BH3 would prefer to coordinate with CO bond, forming six-membered ring, and ketones were hydrogenated via concerted double-hydrogen-transfer process.

Gene mining, codon optimization and analysis of binding mechanism of an aldo-keto reductase with high activity, better substrate specificity and excellent solvent tolerance

The biosynthesis of chiral alcohols has important value and high attention. Aldo–keto reductases (AKRs) mediated reduction of prochiral carbonyl compounds is an interesting way of synthesizing single enantiomers of chiral alcohols due to the high enantio-, chemo- and regioselectivity of the enzymes. However, relatively little research has been done on characterization and apply of AKRs to asymmetric synthesis of chiral alcohols. In this study, the AKR from Candida tropicalis MYA-3404 (C. tropicalis MYA-3404), was mined and characterized. The AKR shown wider optimum temperature and pH. The AKR exhibited varying degrees of catalytic activity for different substrates, suggesting that the AKR can catalyze a variety of substrates. It is worth mentioning that the AKR could catalytic reduction of keto compounds with benzene rings, such as cetophenone and phenoxyacetone. The AKR exhibited activity on N,N-dimethyl-3-keto-3-(2-thienyl)-1-propanamine (DKTP), a key intermediate for biosynthesis of the antidepressant drug duloxetine. Besides, the AKR still has high activity whether in a reaction system containing 10%-30% V/V organic solvent. What’s more, the AKR showed the strongest stability in six common organic solvents, DMSO, acetonitrile, ethyl acetate, isopropanol, ethanol, and methanol. And, it retains more that 70% enzyme activity after 6 hours, suggesting that the AKR has strong solvent tolerance. Furthermore, the protein sequences of the AKR and its homology were compared, and a 3D model of the AKR docking with coenzyme NADPH were constructed. And the important catalytic and binding sites were identified to explore the binding mechanism of the enzyme and its coenzyme. These properties, predominant organic solvents resistance and extensive substrate spectrum, of the AKR making it has potential applications in the pharmaceutical field.

Regioselective asymmetric bioreduction of trans-4-phenylbut-3-en-2-one by whole-cell of Weissella cibaria N9 biocatalyst

There is a considerable interest in the asymmetric production of chiral allylic alcohols, the main building blocks of many functional molecules. The asymmetric reduction of α,β-unsaturated ketones is difficult with traditional chemical protocols in a regioselective and stereoselective manner. In this study, the reductive capacity of whole cell of Leuconostoc mesenteroides N6, Weissella paramesenteroides N7, Weissella cibaria N9, and Leuconostoc pseudomesenteroides N13 was investigated as whole-cell biocatalysts in the enantioselective reduction of (E)-4-phenylbut-3-en-2-one (1). The biocatalytic reduction of 1 to (S,E)-4-phenylbut-3-en-2-ol ((S,E)-2) using the whole cell of W. cibaria N9 isolated from Turkish sourdough was developed in a regioselective fashion, occurring with excellent conversion and recovering the product in good yield. In biocatalytic reduction reactions, the conversion of the substrate and the enantiomeric excess (ee) of the product are significantly affected by optimization parameters such as temperature, agitation rate, pH, and incubation time. Effects of these parameters on ee and conversion were investigated comprehensively. In addition, to our knowledge, this is the first report on production of (S,E)-2 using whole-cell biocatalyst in excellent yield, conversion with enantiopure form and at gram scale. These findings pave the way for the use of whole cell of W. cibaria N9 for challenging higher substrate concentrations of different α,β-unsaturated ketones for regioselective reduction at industrial scale.

Single-Point Mutant Inverts the Stereoselectivity of a Carbonyl Reductase toward β-Ketoesters with Enhanced Activity

Enzyme stereoselectivity control is still a major challenge. To gain insight into the molecular basis of enzyme stereo-recognition and expand the source of antiPrelog carbonyl reductase toward β-ketoesters, rational enzyme design aiming at stereoselectivity inversion was performed. The designed variant Q139G switched the enzyme stereoselectivity toward β-ketoesters from Prelog to antiPrelog, providing corresponding alcohols in high enantiomeric purity (89.1-99.1 % ee). More importantly, the well-known trade-off between stereoselectivity and activity was not found. Q139G exhibited higher catalytic activity than the wildtype enzyme, the enhancement of the catalytic efficiency (k_cat /K_m ) varied from 1.1- to 27.1-fold. Interestingly, the mutant Q139G did not lead to reversed stereoselectivity toward aromatic ketones. Analysis of enzyme-substrate complexes showed that the structural flexibility of β-ketoesters and a newly formed cave together facilitated the formation of the antiPrelog-preferred conformation. In contrast, the relatively large and rigid structure of the aromatic ketones prevents them from forming the antiPrelog-preferred conformation.

Fine-tuning of the substrate binding mode to enhance the catalytic efficiency of an ortho-haloacetophenone-specific carbonyl reductase

Fine-tuning of the substrate binding mode was successfully applied for enhancing the catalytic efficiency of an ortho-haloacetophenone-specific carbonyl reductase.