Biocatalytic Asymmetric Reduction of γ‐Keto Esters to Access Optically Active γ‐Aryl‐γ‐butyrolactones

Simultaneously Enhanced Catalytic Activity and Thermostability of a Baeyer-Villiger Monooxygenase from Oceanicola granulosus by Reshaping the Binding Pocket

Enzymatic oxygenation of various cyclic ketones into lactones via Baeyer-Villiger monooxygenases (BVMOs) could provide a promising route for synthesizing fragrances and pharmaceutical ingredients. However, unsatisfactory catalytic activity and thermostability restricted their applications in the pharmaceutical and food industries. In this study, we successfully improved the catalytic activity and thermostability of a Baeyer-Villiger monooxygenase (OgBVMO) from Oceanicola granulosus by reshaping the binding pocket. As a result, mutant OgBVMO-Re displayed a 1.0- to 6.4-fold increase in the activity toward branched cyclic ketones tested, accompanied by a 3 °C higher melting point, and a 2-fold longer half-life time (t1/2 (45 °C)). Molecular dynamics simulations revealed that reshaping the binding pocket achieved strengthened motion correlation between amino acid residues, appropriate size of the substrate-binding pocket, beneficial surface characteristics, lower energy barriers, and shorter nucleophilic distance. This study well demonstrated the trade-off between the enzyme activity and thermostability by reshaping the substrate-binding pocket, paving the way for further engineering other enzymes.

A pH-dependent shift of redox cofactor specificity in a benzyl alcohol dehydrogenase of aromatoleum aromaticum EbN1

We characterise a reversible bacterial zinc-containing benzyl alcohol dehydrogenase (BaDH) accepting either NAD+ or NADP+ as a redox cofactor. Remarkably, its redox cofactor specificity is pH-dependent with the phosphorylated cofactors favored at lower and the dephospho-forms at higher pH. BaDH also shows different steady-state kinetic behavior with the two cofactor forms. From a structural model, the pH-dependent shift may affect the charge of a histidine in the 2'-phosphate-binding pocket of the redox cofactor binding site. The enzyme is phylogenetically affiliated to a new subbranch of the Zn-containing alcohol dehydrogenases, which share this conserved residue. BaDH appears to have some specificity for its substrate, but also turns over many substituted benzyl alcohol and benzaldehyde variants, as well as compounds containing a conjugated C=C double bond with the aldehyde carbonyl group. However, compounds with an sp3-hybridised C next to the alcohol/aldehyde group are not or only weakly turned over. The enzyme appears to contain a Zn in its catalytic site and a mixture of Zn and Fe in its structural metal-binding site. Moreover, we demonstrate the use of BaDH in an enzyme cascade reaction with an acid-reducing tungsten enzyme to reduce benzoate to benzyl alcohol. KEY POINTS: •Zn-containing BaDH has activity with either NAD + or NADP+ at different pH optima. •BaDH converts a broad range of substrates. •BaDH is used in a cascade reaction for the reduction of benzoate to benzyl alcohol.

Catalytic Stability of S-1-(4-Hydroxyphenyl)-Ethanol Dehydrogenase from Aromatoleum aromaticum

Derived from the denitrifying bacterium Aromatoleum aromaticum EbN1 (Azoarcus sp.), the enzyme S-1-(4-hydroxyphenyl)-ethanol dehydrogenase (S-HPED) belongs to the short-chain dehydrogenase/reductase family. Using research techniques like UV-Vis spectroscopy, dynamic light scattering, thermal-shift assay and HPLC, we investigated the catalytic and structural stability of S-HPED over a wide temperature range and within the pH range of 5.5 to 9.0 under storage and reaction conditions. The relationship between aggregation and inactivation of the enzyme in various pH environments was also examined and interpreted. At pH 9.0, where the enzyme exhibited no aggregation, we characterized thermally induced enzyme inactivation. Through isothermal and multitemperature analysis of inactivation data, we identified and confirmed the first-order inactivation mechanism under these pH conditions and determined the kinetic parameters of the inactivation process. Additionally, we report the positive impact of glucose as an enzyme stabilizer, which slows down the dynamics of S-HPED inactivation over a wide range of pH and temperature and limits enzyme aggregation. Besides characterizing the stability of S-HPED, the enzyme's catalytic activity and high stereospecificity for 10 prochiral carbonyl compounds were positively verified, thus expanding the spectrum of substrates reduced by S-HPED. Our research contributes to advancing knowledge about the biocatalytic potential of this catalyst.

Classification and functional origins of stereocomplementary alcohol dehydrogenases for asymmetric synthesis of chiral secondary alcohols: A review

Alcohol dehydrogenases (ADHs) mediated biocatalytic asymmetric reduction of ketones have been widely applied in the synthesis of optically active secondary alcohols with highly reactive hydroxyl groups ligated to the stereogenic carbon and divided into (R)- and (S)-configurations. Stereocomplementary ADHs could be applied in the synthesis of both enantiomers and are increasingly accepted as the "first of choice" in green chemistry due to the high atomic economy, low environmental factor, 100 % theoretical yield, and high environmentally friendliness. Due to the equal importance of complementary alcohols, development of stereocomplementary ADHs draws increasing attention. This review is committed to summarize recent advance in discovery of naturally evolved and tailor-made stereocomplementary ADHs, unveil the molecular mechanism of stereoselective catalysis in views of classification and functional basis, and provide guidance for further engineering the stereoselectivity of ADHs for the industrial biosynthesis of chiral secondary alcohol of industrial relevance.

Enzymatic Assembly of Diverse Lactone Structures: An Intramolecular C-H Functionalization Strategy

Lactones are cyclic esters with extensive applications in materials science, medicinal chemistry, and the food and perfume industries. Nature's strategy for the synthesis of many lactones found in natural products always relies on a single type of retrosynthetic strategy, a C-O bond disconnection. Here, we describe a set of laboratory-engineered enzymes that use a new-to-nature C-C bond-forming strategy to assemble diverse lactone structures. These engineered "carbene transferases" catalyze intramolecular carbene insertions into benzylic or allylic C-H bonds, which allow for the synthesis of lactones with different ring sizes and ring scaffolds from simple starting materials. Starting from a serine-ligated cytochrome P450 variant previously engineered for other carbene-transfer activities, directed evolution generated a variant P411-LAS-5247, which exhibits a high activity for constructing a five-membered ε-lactone, lactam, and cyclic ketone products (up to 5600 total turnovers (TTN) and >99% enantiomeric excess (ee)). Further engineering led to variants P411-LAS-5249 and P411-LAS-5264, which deliver six-membered δ-lactones and seven-membered ε-lactones, respectively, overcoming the thermodynamically unfavorable ring strain associated with these products compared to the γ-lactones. This new carbene-transfer activity was further extended to the synthesis of complex lactone scaffolds based on fused, bridged, and spiro rings. The enzymatic platform developed here complements natural biosynthetic strategies for lactone assembly and expands the structural diversity of lactones accessible through C-H functionalization.

Biocatalytic characterization of an alcohol dehydrogenase variant deduced from Lactobacillus kefir in asymmetric hydrogen transfer

Hydrogen transfer biocatalysts to prepare optically pure alcohols are in need, especially when it comes to sterically demanding ketones, whereof the bioreduced products are either essential precursors of pharmaceutically relevant compounds or constitute APIs themselves. In this study, we report on the biocatalytic potential of an anti-Prelog (R)-specific Lactobacillus kefir ADH variant (Lk-ADH-E145F-F147L-Y190C, named Lk-ADH Prince) employed as E. coli/ADH whole-cell biocatalyst and its characterization for stereoselective reduction of prochiral carbonyl substrates. Key enzymatic reaction parameters, including the reaction medium, evaluation of cofactor-dependency, organic co-solvent tolerance, and substrate loading, were determined employing the drug pentoxifylline as a model prochiral ketone. Furthermore, to tap the substrate scope of Lk-ADH Prince in hydrogen transfer reactions, a broad range of 34 carbonylic derivatives was screened. Our data demonstrate that E. coli/Lk-ADH Prince exhibits activity toward a variety of structurally different ketones, furnishing optically active alcohol products at the high conversion of 65-99.9% and in moderate-to-high isolated yields (38-91%) with excellent anti-Prelog (R)-stereoselectivity (up to >99% ee) at substrate concentrations up to 100 mM.

Chemoenzymatic approach towards the synthesis of the antitumor and antileishmanial marine metabolite (+)-Harzialactone A via the stereoselective, biocatalyzed reduction of a prochiral ketone

As a rich source of biological active compounds, marine natural products have been increasingly screened as candidates for developing new drugs. Among the several marine products and metabolites, (+)-Harzialactone A has drawn considerable attention for its antitumor and antileishmanial activity. In this work a chemoenzymatic approach has been implemented for the preparation of the marine metabolite (+)-Harzialactone A. The synthesis involved a stereoselective, biocatalyzed reduction of the prochiral ketone 4-oxo-5-phenylpentanoic acid or the corresponding esters, all generated by chemical reactions. A collection of different promiscuous oxidoreductases (both wild-type and engineered) and diverse microorganism strains were investigated to mediate the bioconversions. After co-solvent and co-substrate investigation in order to enhance the bioreduction performance, T. molischiana in presence of NADES (choline hydrochloride-glucose) and ADH442 were identified as the most promising biocatalysts, allowing the obtainment of the (S)-enantiomer with excellent ee (97% to > 99% respectively) and good to excellent conversion (88% to 80% respectively). The successful attempt in this study provides a new chemoenzymatic approach for the synthesis of (+)-Harzialactone A.

Chemoenzymatic Synthesis of Tenofovir

We report on novel chemoenzymatic routes toward tenofovir using low-cost starting materials and commercial or homemade enzyme preparations as biocatalysts. The biocatalytic key step was accomplished either via stereoselective reduction using an alcohol dehydrogenase or via kinetic resolution using a lipase. By employing a suspension of immobilized lipase from Burkholderia cepacia (Amano PS-IM) in a mixture of vinyl acetate and toluene, the desired (R)-ester (99% ee) was obtained on a 500 mg scale (60 mM) in 47% yield. Alternatively, stereoselective reduction of 1-(6-chloro-9H-purin-9-yl) propan-2-one (84 mg, 100 mM) catalyzed by lyophilized E. coli cells harboring recombinant alcohol dehydrogenase (ADH) from Lactobacillus kefir (E. coli/Lk-ADH Prince) allowed one to reach quantitative conversion, 86% yield and excellent optical purity (>99% ee) of the corresponding (R)-alcohol. The key (R)-intermediate was transformed into tenofovir through "one-pot" aminolysis-hydrolysis of (R)-acetate in NH3-saturated methanol, alkylation of the resulting (R)-alcohol with tosylated diethyl(hydroxymethyl) phosphonate, and bromotrimethylsilane (TMSBr)-mediated cleavage of the formed phosphonate ester into the free phosphonic acid. The elaborated enzymatic strategy could be applicable in the asymmetric synthesis of tenofovir prodrug derivatives, including 5'-disoproxil fumarate (TDF, Viread) and 5'-alafenamide (TAF, Vemlidy). The molecular basis of the stereoselectivity of the employed ADHs was revealed by molecular docking studies.

Inactivation and aggregation of R-specific 1-(4-hydroxyphenyl)-ethanol dehydrogenase from Aromatoleum aromaticum

R-specific 1-(4-hydroxyphenyl)-ethanol dehydrogenase (R-HPED) is a promising biotool for stereoselective synthesis of chiral aromatic alcohols. This work focused on the evaluation of its stability under storage and in-process conditions in the pH range from 5.5 to 8.5. The relationship between the dynamics of aggregation and activity loss under various pH conditions and in the presence of glucose, serving as a stabilizer, was analysed using spectrophotometric techniques and dynamic light scattering. pH 8.5 was indicated as a representative environment in which the enzyme, despite relatively low activity, shows high stability and the highest total product yield. Based on a series of inactivation experiments, the mechanism of thermal inactivation at pH 8.5 was modelled. The irreversible first-order mechanism of R-HPED inactivation in the temperature range of 47.5-60 °C was verified by isothermal and multi-temperature evaluation of data, confirming that in the alkaline pH 8.5, R-HPED aggregation is the secondary process occurring at already inactivated protein molecules. The rate constants were from 0.029 min^-1 to 0.380 min^-1 for a buffer solution but they decreased to 0.011 min^-1 and 0.161 min^-1, respectively, when 1.5 M glucose was added as a stabilizer. The activation energy was however about 200 kJ mol^-1 in both cases.

Tungsten Enzyme Using Hydrogen as an Electron Donor to Reduce Carboxylic Acids and NAD. ACS Catal 2022;12:8707-8717. [PMID: 35874620 PMCID: PMC9295118 DOI: 10.1021/acscatal.2c02147]

Tungsten-dependent aldehyde oxidoreductases (AORs) catalyze the oxidation of aldehydes to acids and are the only known enzymes reducing non-activated acids using electron donors with low redox potentials. We report here that AOR from Aromatoleum aromaticum (AOR_Aa) catalyzes the reduction of organic acids not only with low-potential Eu(II) or Ti(III) complexes but also with H₂ as an electron donor. Additionally, AOR_Aa catalyzes the H₂-dependent reduction of NAD⁺ or benzyl viologen. The rate of H₂-dependent NAD⁺ reduction equals to 10% of that of aldehyde oxidation, representing the highest H₂ turnover rate observed among the Mo/W enzymes. As AOR_Aa simultaneously catalyzes the reduction of acids and NAD⁺, we designed a cascade reaction utilizing a NAD(P)H-dependent alcohol dehydrogenase to reduce organic acids to the corresponding alcohols with H₂ as the only reductant. The newly discovered W-hydrogenase side activity of AOR_Aa may find applications in either NADH recycling or conversion of carboxylic acids to more useful biochemicals.

Unraveling the mechanism of enantio-controlling switches of an alcohol dehydrogenase toward sterically small ketone

Efficient synthesis of chiral compounds under mild conditions is highly desirable in the chemical and pharmaceutical communities, but it often faces difficulties. Although various enzymes have been harnessed as biocatalysts...

Chemoenzymatic deracemization of lisofylline catalyzed by a (laccase/TEMPO)-alcohol dehydrogenase system

This article reports on a novel biocatalytic method for the synthesis of both enantiomers of lisofylline based on Trametes versicolor laccase, TEMPO as a redox mediator and stereocomplementary recombinant alcohol dehydrogenases as biocatalysts.

Reducing Challenges in Organic Synthesis with Stereoselective Hydrogenation and Tandem Catalysis

Tandem catalysis enables the rapid construction of complex architectures from simple building blocks. This Perspective shares our interest in combining stereoselective hydrogenation with transformations such as isomerization, oxidation, and epimerization to solve diverse challenges. We highlight the use of tandem hydrogenation for preparing complex natural products from simple prochiral building blocks and present tandem catalysis involving transfer hydrogenation and dynamic kinetic resolution. Finally, we underline recent breakthroughs and opportunities for asymmetric hydrogenation.