A recombinant ketoreductase tool-box. Assessing the substrate selectivity and stereoselectivity toward the reduction of β-ketoesters

Biocatalytic dynamic kinetic reductive resolution with ketoreductase from Klebsiella pneumoniae: the asymmetric synthesis of functionalized tetrahydropyrans

Ketoreductase from growing cells of Klebsiella pneumoniae (NBRC 3319) acts as an efficient reagent for converting racemic α-benzyl/cinnamyl substituted-β-ketoesters to the corresponding β-hydroxy esters with excellent yields and stereoselectivities (ee and de >99 %). The reactions described herein followed a biocatalytic dynamic kinetic reductive resolution (DKRR) pathway, which is reported for the first time with such substrates. It was found that the enzyme system can accept substituted mono-aryl rings with different electronic natures. In addition, it also accepts a substituted naphthyl ring and heteroaryl ring in the α-position of the parent β-ketoester. The synthesized enantiopure β-hydroxy esters were then synthetically manipulated to valuable tetrahydropyran building blocks.

Efficient chiral synthesis by Saccharomyces cerevisiae spore encapsulation of Candida parapsilosis Glu228Ser/(S)-carbonyl reductase II and Bacillus sp. YX-1 glucose dehydrogenase in organic solvents

BACKGROUND

Saccharomyces cerevisiae AN120 osw2∆ spores were used as a host with good resistance to unfavorable environment. This work was undertaken to develop a new yeast spore-encapsulation of Candida parapsilosis Glu228Ser/(S)-carbonyl reductase II and Bacillus sp. YX-1 glucose dehydrogenase for efficient chiral synthesis in organic solvents.

RESULTS

The spore microencapsulation of E228S/SCR II and GDH in S. cerevisiae AN120 osw2∆ catalyzed (R)-phenylethanol in a good yield with an excellent enantioselectivity (up to 99%) within 4 h. It presented good resistance and catalytic functions under extreme temperature and pH conditions. The encapsulation produced several chiral products with over 70% yield and over 99% enantioselectivity in ethyl acetate after being recycled for 4-6 times. It increased substrate concentration over threefold and reduced the reaction time two to threefolds compared to the recombinant Escherichia coli containing E228S and glucose dehydrogenase.

CONCLUSIONS

This work first described sustainable enantioselective synthesis without exogenous cofactors in organic solvents using yeast spore-microencapsulation of coupled alcohol dehydrogenases.

Synthesis of the GPR40 Partial Agonist MK-8666 through a Kinetically Controlled Dynamic Enzymatic Ketone Reduction

A scalable and efficient synthesis of the GPR40 agonist MK-8666 was developed from a simple pyridine building block. The key step to set the stereochemistry at two centers relied on an enzymatic dynamic kinetic reduction of an unactivated ketone. Directed evolution was leveraged to generate an optimized ketoreductase that provided the desired trans alcohol in >30:1 dr and >99% ee. Further, it was demonstrated that all four diastereomers of this hydroxy-ester could be prepared in high yield and selectivity. Subsequently, a challenging intramolecular displacement was carried out to form the cyclopropane ring system with perfect control of endo/exo selectivity. The endgame coupling strategy relied on a Pd-catalyzed C-O coupling to join the headpiece chloropyridine with the benzylic alcohol tailpiece.

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

The application of biocatalysis to the synthesis of chiral molecules is one of the greenest technologies for the replacement of chemical routes due to its environmentally benign reaction conditions and unparalleled chemo-, regio- and stereoselectivities. We have been interested in searching for carbonyl reductase enzymes and assessing their substrate specificity and stereoselectivity. We now report a gene cluster identified in Candida parapsilosis that consists of four open reading frames including three putative stereospecific carbonyl reductases (scr1, scr2, and scr3) and an alcohol dehydrogenase (cpadh). These newly identified three stereospecific carbonyl reductases (SCRs) showed high catalytic activities for producing (S)-1-phenyl-1,2-ethanediol from 2-hydroxyacetophenone with NADPH as the coenzyme. Together with CPADH, all four enzymes from this cluster are carbonyl reductases with novel anti-Prelog stereoselectivity. SCR1 and SCR3 exhibited distinct specificities to acetophenone derivatives and chloro-substituted 2-hydroxyacetophenones, and especially very high activities towards ethyl 4-chloro-3-oxobutyrate, a β-ketoester with important pharmaceutical potential. Our study also showed that genomic mining is a powerful tool for the discovery of new enzymes.

Asymmetric synthesis of both antipodes of beta-hydroxy nitriles and beta-hydroxy carboxylic acids via enzymatic reduction or sequential reduction/hydrolysis

Use of isolated carbonyl reductases in the reduction of aromatic beta-ketonitriles have completely eliminated the competing alpha-ethylation, which is often observed with whole cell biocatalysts. By choosing suitable recombinant carbonyl reductase, the reduction of beta-ketonitriles afforded (R)- or (S)-beta-hydroxy nitriles with excellent optical purity and yield. Subsequently, nitrilase-catalyzed hydrolysis of the obtained optically pure beta-hydroxy nitriles led to the corresponding beta-hydroxy carboxylic acids in high yields. More importantly, the sequential enzymatic reduction and hydrolysis could be carried out in "two-step-one-pot" fashion without the isolation of intermediates beta-hydroxy nitriles, lowering the cost and minimizing the environmental impact. This allows ready access to both antipodes of chiral beta-hydroxy nitriles and beta-hydroxy carboxylic acids of pharmaceutical importance with excellent optical purity.

Structure-based dissociation of a type I polyketide synthase module

Individual modules of modular polyketide synthases (PKSs) such as 6-deoxyerythronolide B synthase (DEBS) consist of conserved, covalently linked domains separated by unconserved intervening linker sequences. To better understand the protein-protein and enzyme-substrate interactions in modular catalysis, we have exploited recent structural insights to prepare stand-alone domains of selected DEBS modules. When combined in vitro, ketosynthase (KS), acyl transferase (AT), and acyl carrier protein (ACP) domains of DEBS module 3 catalyzed methylmalonyl transfer and diketide substrate elongation. When added to a minimal PKS, ketoreductase domains from DEBS modules 1, 2, and 6 showed specificity for the beta-ketoacylthioester substrate, but not for either the ACP domain carrying the polyketide substrate or the KS domain that synthesized the substrate. With insights into catalytic efficiency and specificity of PKS modules, our results provide guidelines for constructing optimal hybrid PKS systems.

A molecular modeling study on the enantioselectivity of aryl alkyl ketone reductions by a NADPH-dependent carbonyl reductase

Automated structural analysis of Sporobolomyces salmonicolor carbonyl reductase (SSCR) indicates that the two largest potential receptor sites are in the vicinity of the nicotinamide reductant. The largest receptor site is a scalene triangle with sides of approximately 8 A by 9 A by 13 A, which is narrow in width; one corner is surrounded by hydrophilic residues that can favorably bond with the ketone oxygen. Docking aryl alkyl ketones shows a distinct preference for binding to the largest receptor site, and for conformations that place the carbonyl oxygen of the substrate in the hydrophilic corner of the largest receptor site. Favorable docking conformations for aryl alkyl ketones fall into two low-energy ensembles. These conformational ensembles are distinguished by the positions of the substituents, presenting either the Si- or Re-face of the ketone to the nicotinamide reductant. For the ketones investigated here, there is a correspondence between the major enantiomer of the alcohol obtained from the reduction of the ketone and the conformer found to have the most stable interaction energy with the receptor site in all cases. The receptor site modeling, docking simulations, molecular dynamics, and enzyme-substrate geometry optimizations lead to a model for understanding the enantioselectivity of this NADPH-dependent carbonyl reductase.

Stereoselective ketone reduction by a carbonyl reductase from Sporobolomyces salmonicolor. Substrate specificity, enantioselectivity and enzyme-substrate docking studies

In our effort to search for effective carbonyl reductases, the activity and enantioselectivity of a carbonyl reductase from Sporobolomyces salmonicolor have been evaluated toward the reduction of a variety of ketones. This carbonyl reductase (SSCR) reduces a broad spectrum of ketones including aliphatic and aromatic ketones, as well as alpha- and beta-ketoesters. Among these substrates, SSCR shows highest activity for the reduction of alpha-ketoesters. Aromatic alpha-ketoesters are reduced to (S)-alpha-hydroxy esters, while (R)-enantiomers are obtained from the reduction of aliphatic counterparts. This interesting observation is consistent with enzyme-substrate docking studies, which show that hydride transfer occurs at the different faces of carbonyl group for aromatic and aliphatic alpha-ketoesters. It is worthy to note that sterically bulky ketone substrates, such as 2'-methoxyacetophenone, 1-adamantyl methyl ketone, ethyl 4,4-dimethyl-3-oxopentanoate and ethyl 3,3-dimethyl-2-oxobutanoate, are reduced to the corresponding alcohols with excellent optical purity. Thus, SSCR possesses an unusually broad substrate specificity and is especially useful for the reduction of ketones with sterically bulky substituents.