Rapid asymmetric reduction of ethyl 4-chloro-3-oxobutanoate using a thermostabilized mutant of ketoreductase ChKRED20

Preparation of chiral aryl alcohols: a controllable enzymatic strategy via light-driven NAD(P)H regeneration

Controllable and mild photoenzymatic production of chiral alcohols was realized by coupling a photochemical NAD(P)H regeneration system with (R)- or (S)-selective ketoreductases.

Biocatalytic approaches for the synthesis of optically pure vic-halohydrins

Enantiopure vicinal halohydrins (vic-halohydrins) are highly valuable building blocks for the synthesis of many different natural products and pharmaceuticals, and biocatalytic methods for their synthesis have received considerable interest. This review emphasizes the application of biocatalytic approaches as an efficient alternative or complement to conventional chemical reactions, with a special focus on the asymmetric reductions catalyzed by ketoreductases, kinetic resolution catalyzed using lipases or esterases, stereoselective biotransformation catalyzed by halohydrin dehalogenases, asymmetric hydroxylation catalyzed by cytochrome P450 monooxygenases, asymmetric dehalogenation catalyzed by haloalkane dehalogenases, and aldehyde condensation catalyzed by aldolases. Although many chiral vic-halohydrins have been successfully synthesized using wild-type biocatalysts, their enantioselectivity is often too low for enantiopure synthesis. To overcome these limitations, catalytic properties of wild-type enzymes have been improved by rational and semi-rational protein design or directed evolution. This review briefly introduces the research status in this field, highlighting aspects of basic academic research in the biocatalytic synthesis of optically active vic-halohydrins by employing such unconventional approaches. KEY POINTS: • Outlines the enzymatic strategies for the production of enantiopure vic-halohydrins • Highlights recent advances in biocatalytic production of enantiopure vic-halohydrins • Provide guidance for efficient preparation of enantiopure vic-halohydrins.

Biocatalytic Reduction Reactions from a Chemist's Perspective

Reductions play a key role in organic synthesis, producing chiral products with new functionalities. Enzymes can catalyse such reactions with exquisite stereo-, regio- and chemoselectivity, leading the way to alternative shorter classical synthetic routes towards not only high-added-value compounds but also bulk chemicals. In this review we describe the synthetic state-of-the-art and potential of enzymes that catalyse reductions, ranging from carbonyl, enone and aromatic reductions to reductive aminations.

Functional characterization of an (R)-selective styrene monooxygenase from streptomyces sp. NRRL S-31

Styrene monooxygenases (SMOs) are two-component enzymes known to catalyze the epoxidation of styrene to (S)-styrene oxide. In this work, we identified a new oxygenase component, named StStyA, from the genome of Streptomyces sp. NRRL S-31. StStyA displayed complementary stereoselectivity to all of the known SMOs when coupled with a known reductase component (PsStyB), which made it the first natural SMO that produces (R)-styrene oxide. Accordingly, a plasmid co-expressing StStyA and PsStyB was constructed, which led to an artificial two-component SMO, named StStyA/B. When applied in the bio-epoxidation of nine aromatic alkenes, the enzyme showed activity toward five alkenes, and consistently displayed (R)-selectivity. Excellent stereoselectivity was achieved for all five substrates with enantiomeric excesses ranging from 91% to >99%ee.

Structure-guided engineering of ChKRED20 from Chryseobacterium sp. CA49 for asymmetric reduction of aryl ketoesters

ChKRED20 is a robust NADH-dependent ketoreductase identified from the genome of Chryseobacterium sp. CA49 that can use 2-propanol as the ultimate reducing agent. The wild-type can reduce over 100 g/l ketones for some pharmaceutical relevant substrates, exhibiting a remarkable potential for industrial application. In this work, to overcome the limitation of ChKRED20 to aryl ketoesters, we first refined the X-ray crystal structure of ChKRED20/NAD⁺ complex at a resolution of 1.6 Å, and then performed three rounds of iterative saturation mutagenesis at critical amino acid sites to reshape the active cavity of the enzyme. For methyl 2-oxo-2-phenylacetate and ethyl 3-oxo-3-phenylpropanoate, several gain-of-activity mutants were achieved, and for ethyl 2-oxo-4-phenylbutanoate, improved mutants were achieved with k_cat/K_m increasing to 196-fold of the wild-type. All three substrates were completely reduced at 100 g/l loading catalyzed with selected ChKRED20 mutants, and deliver the corresponding chiral alcohols with >90% isolated yield and 97 - >99%ee.

Single mutations of ketoreductase ChKRED20 enhance the bioreductive production of (1S)-2-chloro-1-(3, 4-difluorophenyl) ethanol

(1S)-2-chloro-1-(3, 4-difluorophenyl) ethanol ((S)-CFPL) is an intermediate for the drug ticagrelor, and is manufactured via chemical approaches. To develop a biocatalytic solution to (S)-CFPL, an inventory of ketoreductases from Chryseobacterium sp. CA49 were rescreened, and ChKRED20 was found to catalyze the reduction of the ketone precursor with excellent stereoselectivity (>99 % ee). After screening an error-prone PCR library of the wild-type ChKRED20, two mutants, each bearing a single amino acid substitution of H145L or L205M, were identified with significantly increased activity. Then, the two critical positions were each randomized by constructing saturation mutagenesis libraries, which delivered several mutants with further enhanced activity. Among them, the mutant L205A was the best performer with a specific activity of 178 μmol/min/mg, ten times of that of the wild-type. Its k _cat/K _m increased by 15 times and half-life at 50 °C increased by 70 %. The mutant catalyzed the complete conversion of 150 and 200 g/l substrate within 6 and 20 h, respectively, to yield enantiopure (S)-CFPL with an isolated yield of 95 %.