Structure-Guided Directed Evolution of a Carbonyl Reductase Enables the Stereoselective Synthesis of (2S,3S)-2,2-Disubstituted-3-hydroxycyclopentanones via Desymmetric Reduction

Catalytic Asymmetric Desymmetrization of Cyclic 1,3-Diketones Using Chiral Boro-phosphates

Herein, we report a chiral boro-phosphate-catalyzed reductive amination for the desymmetrization of 2,2-disubstituted 1,3-cyclopentadiones with pinacolborane as the reducing agent, delivering chiral β-amino ketones with an all-carbon quaternary stereocenter in good yields (≤94%), high enantioselectivities (≤97% ee), and excellent diastereoselectivities (>20:1 dr). This reaction has a broad substrate scope and high functional group tolerance. The importance of the chiral products was also demonstrated through the preparation of multifunctional building blocks and heterocycles.

Effective engineering of a ketoreductase for the biocatalytic synthesis of an ipatasertib precursor

Semi-rational enzyme engineering is a powerful method to develop industrial biocatalysts. Profiting from advances in molecular biology and bioinformatics, semi-rational approaches can effectively accelerate enzyme engineering campaigns. Here, we present the optimization of a ketoreductase from Sporidiobolus salmonicolor for the chemo-enzymatic synthesis of ipatasertib, a potent protein kinase B inhibitor. Harnessing the power of mutational scanning and structure-guided rational design, we created a 10-amino acid substituted variant exhibiting a 64-fold higher apparent kcat and improved robustness under process conditions compared to the wild-type enzyme. In addition, the benefit of algorithm-aided enzyme engineering was studied to derive correlations in protein sequence-function data, and it was found that the applied Gaussian processes allowed us to reduce enzyme library size. The final scalable and high performing biocatalytic process yielded the alcohol intermediate with ≥ 98% conversion and a diastereomeric excess of 99.7% (R,R-trans) from 100 g L-1 ketone after 30 h. Modelling and kinetic studies shed light on the mechanistic factors governing the improved reaction outcome, with mutations T134V, A238K, M242W and Q245S exerting the most beneficial effect on reduction activity towards the target ketone.

Semirational Engineering of a Thermostable Carbonyl Reductase for the Precision Synthesis of (2R,3R)-2-Methyl-2-benzyl-3-hydroxycyclopentanone and Its Analogues

2,2-Disubstituted-3-hydroxycyclopentanones are important chiral intermediates for natural products and pharmaceuticals. Through semirational engineering of a thermostable carbonyl reductase CBCR from Cupriavidus sp. BIS7, a mutant L91C/F93I was obtained. Mutant L91C/F93I showed 4- to 36-fold enhanced activities toward 2-methyl-2-benzyl-1,3-cyclopentanedione and its analogues, affording the (2R,3R)-stereoisomers with >99% ee and >99% de. Enzyme-substrate docking studies were performed to reveal the molecular basis for the activity and stereoselectivity improvements.

Structural analysis of an anthrol reductase inspires enantioselective synthesis of enantiopure hydroxycycloketones and β-halohydrins

Asymmetric reduction of prochiral ketones, particularly, reductive desymmetrization of 2,2-disubstituted prochiral 1,3-cyclodiketones to produce enantiopure chiral alcohols is challenging. Herein, an anthrol reductase CbAR with the ability to accommodate diverse bulky substrates, like emodin, for asymmetric reduction is identified. We firstly solve crystal structures of CbAR and CbAR-Emodin complex. It reveals that Tyr210 is critical for emodin recognition and binding, as it forms a hydrogen-bond interaction with His162 and π-π stacking interactions with emodin. This ensures the correct orientation for the stereoselectivity. Then, through structure-guided engineering, variant CbAR-H162F can convert various 2,2-disubstituted 1,3-cyclodiketones and α-haloacetophenones to optically pure (2S, 3S)-ketols and (R)-β-halohydrins, respectively. More importantly, their stereoselectivity mechanisms are also well explained by the respective crystal structures of CbAR-H162F-substrate complex. Therefore, this study demonstrates that an in-depth understanding of catalytic mechanism is valuable for exploiting the promiscuity of anthrol reductases to prepare diverse enantiopure chiral alcohols.

Ketoreductase Catalyzed (Dynamic) Kinetic Resolution for Biomanufacturing of Chiral Chemicals

Biocatalyzed asymmetric reduction of ketones is an environmentally friendly approach and one of the most cost-effective routes for producing chiral alcohols. In comparison with the well-studied reduction of prochiral ketones to generate chiral alcohols with one chiral center, resolution of racemates by ketoreductases (KREDs) to produce chiral compounds with at least two chiral centers is also an important strategy in asymmetric synthesis. The development of protein engineering and the combination with chemo-catalysts further enhanced the application of KREDs in the efficient production of chiral alcohols with high stereoselectivity. This review discusses the advances in the research area of KRED catalyzed asymmetric synthesis for biomanufacturing of chiral chemicals with at least two chiral centers through the kinetic resolution (KR) approach and the dynamic kinetic resolution (DKR) approach.

Biotransformation Enables Innovations Toward Green Synthesis of Steroidal Pharmaceuticals

Steroids have been widely used in birth-control, prevention, and treatment of various diseases, representing the largest sector after antibiotics in the global pharmaceutical market. The steroidal active pharmaceutical ingredients (APIs) have been produced via partial synthetic processes first mainly from sapogenins, which was converted into 16-dehydropregnenolone by the famous "Marker Degradation". Traditional mutation and screening, and process engineering have resulted in the industrial production of 4-androstene-3,17-dione (AD), androst-1,4-diene-3,17-dione (ADD), 9α-hydroxy-androsta-4-ene-3,17-dione (9α-OH-AD), and so on, which serve as the key intermediates for the synthesis of steroidal APIs. Recently, genetic and metabolic engineering have generated highly efficient microbial strains for the production of these precursors, leading to the replacement of sapogenins with phytosterols as the starting materials. Further advances in synthetic biology hold promise in the design and construction of microbial cell factories for the industrial production of steroidal intermediates and/or APIs from simple carbon sources such as glucose. Integration of biotransformation into the synthesis of steroidal APIs can greatly reduce the number of reaction steps, achieve lower waste discharge and higher production efficiency, thus enabling a greener steroidal pharmaceutical industry.

Directed evolution of an alcohol dehydrogenase for the desymmetric reduction of 2,2-disubstituted cyclopenta-1,3-diones by enzymatic hydrogen transfer

Directed evolution of carbonyl reductase TbADH created mutant Tb2 with balanced activity toward ethyl secodione and isopropanol, enabling the desymmetric reduction of ethyl secodione to give (13R,17S)-ethyl secol with 94% yield, >99% ee and >99% de.

Engineering a carbonyl reductase for the scalable preparation of (S)-3-Cyclopentyl-3-hydroxypropanenitrile, the key building block of ruxolitinib

( S )- 3-Cyclopentyl-3-hydroxypropanenitrile is the key precursor for the synthesis of ruxolitinib. The bioreduction of 3-cyclopentyl-3-ketopropanenitrile ( 1a ) offers an attractive method to access this important compound. A carbonyl reductase (PhADH) from Paraburkholderia hospita catalyzed the reduction of 1a giving the ( S )-alcohol ( 1b ) with 85% ee. Rational engineering of PhADH resulted in a double mutant H93C/A139L, which enhanced the enantioselectivity from 85% to >98%, as well as a 6.3-fold improvement in the specific activity. The bioreduction of 1a was performed at 200 g/L (1.5 M) substrate concentration, leading to isolation of ( S )- 1b in 91% yield. Similarly, using this mutant enzyme 3-cyclohexyl-3-ketopropanenitrile ( 2a) and 3-phenyl-3-ketopropanenitrile ( 3a ) were reduced at high concentration affording the corresponding alcohols in >99% ee, and 90% and 92% yield, respectively. The results showed that the variant H93C/A139L was a powerful biocatalyst for reduction of β-substituted-β-ketonitriles.

Tuning enzymatic properties by protein engineering toward catalytic tetrad of carbonyl reductase

Enzyme engineering toward catalytic-tetrad residues usually results in activity loss. Unexpectedly, we found that a directed evolution campaign yielded a beneficial residue A100 in KmCR (a carbonyl reductase from Kluyveromyces marxianus ZJB14056), which is a residue of catalytic tetrad and conserved according to multiple sequence alignment. Inspired by this finding, we performed saturation mutagenesis on all the four residues of catalytic tetrad of KmCR. A number of variants with improved enzymatic activities were obtained. Among them, the variant KmCR_A100S exhibited increased catalytic efficiency (k_cat /K_M  = 47.3 s^-1 ·mM^-1 ), improved stereoselectivity (from moderate selectivity (de_P  = 66.7%) to strict (S)-selectivity (de_P > 99.5%)), and extended substrate scope, compared to those of KmCR_WT. In silico analysis showed that a relay system was rebuilt in KmCR via the beneficial residue S100. Furthermore, comparison of 11 protein engineering campaigns indicated that the beneficial position is easily overlooked due to the long distance (>10 Å) from ketone substrates. Since CRs share similar catalytic mechanism, the knowledge gained from this study has universal significance to CR engineering.

Modulating the active site lid of an alcohol dehydrogenase from Ralstonia sp. enabled efficient stereospecific synthesis of 17β-hydroxysteroids

Enzymatic stereospecific reduction of 17-oxosteroids offers an attractive approach to access 17β-hydroxysteroids of pharmaceutical importance. In this study, by adjusting the flexibility of α6-helix at the substrate entrance of the alcohol dehydrogenase from Ralstonia sp. (RasADH), the catalytic activity toward the stereospecific 17β-reduction of androstenedione was improved without sacrifice of the enantioselectivity. Among the mutants, F205I and F205A exhibited up to 623- and 523-fold improvement in catalytic efficiency, respectively, towards a range of different 17-oxosteroids compared to the wild-type enzyme. The corresponding 17β-hydroxysteroids were prepared in optically pure form with high space-time productivity and isolated yields using F205I as the biocatalyst, indicating that these mutants are promising biocatalysts for this useful transformation. These results suggest that modulating the flexibility of the active site lid offers an effective approach to engineer alcohol dehydrogenase for accommodating bulky steroidal substrates.

Biosynthesis of chiral cyclic and heterocyclic alcohols via CO/C–H/C–O asymmetric reactions

This review covers the recent progress in various biological approaches applied to the synthesis of enantiomerically pure cyclic and heterocyclic alcohols through CO/C–H/C–O asymmetric reactions.

Highly Diastereoselective Synthesis of 2,2-Disubstituted Cyclopentane-1,3-diols via Stepwise Ketone Reduction Enabling Concise Chirality Construction

A series of 2,2-disubstituted trans,cis-cyclopentane-1,3-diols were synthesized in >99% dr through enzymatic reduction of enantiopure 2,2-disubstituted 3-hydroxycyclopentane-1-ones, which were prepared by highly stereoselective enzymatic reduction of the corresponding cyclodiketones. For 2-benzyl-2-methyl-3-oxocyclopentyl acetate, acetylation of the hydroxyl group significantly affected the reduction stereoselectivity, giving trans,cis-, trans,trans-, and cis,cis-2-benzyl-2-methyl-cyclopentane-1,3-diols in stereomerically pure form. This efficient and environmentally friendly method provides a practical approach to the synthesis of these chiral building blocks in single stereoisomeric form, demonstrating the power of biocatalysis in the concise chirality construction of complex chiral molecules.