Asymmetric Amination of Secondary Alcohols by using a Redox-Neutral Two-Enzyme Cascade

Biocatalytic Hydrogen-Borrowing Cascade in Organic Synthesis

Biocatalytic hydrogen borrowing represents an environmentally friendly and highly efficient synthetic method. This innovative approach involves converting various substrates into high-value-added products, typically via a one-pot, two/three-step sequence encompassing dehydrogenation (intermediate transformation) and hydrogenation processes employing the hydride shuffling between NAD(P)+ and NAD(P)H. Represented key transformations in hydrogen borrowing include stereoisomer conversion within alcohols, conversion between alcohols and amines, conversion of allylic alcohols to saturated carbonyl counterparts, and α,β-unsaturated aldehydes to saturated carboxylic acids, etc. The direct transformation methodology and environmentally benign characteristics of hydrogen borrowing have contributed to its advancements in fine chemical synthesis or drug developments. Over the past decades, the hydrogen borrowing strategy in biocatalysis has led to the creation of diverse catalytic systems, demonstrating substantial potential for straightforward synthesis as well as asymmetric transformations. This perspective serves as a detailed exposition of the recent advancements in biocatalytic reactions employing the hydrogen borrowing strategy. It provides insights into the potential of this approach for future development, shedding light on its promising prospects in the field of biocatalysis.

Biocatalytic reductive aminations with NAD(P)H-dependent enzymes: enzyme discovery, engineering and synthetic applications

Chiral amines are pivotal building blocks for the pharmaceutical industry. Asymmetric reductive amination is one of the most efficient and atom economic methodologies for the synthesis of optically active amines. Among the various strategies available, NAD(P)H-dependent amine dehydrogenases (AmDHs) and imine reductases (IREDs) are robust enzymes that are available from various sources and capable of utilizing a broad range of substrates with high activities and stereoselectivities. AmDHs and IREDs operate via similar mechanisms, both involving a carbinolamine intermediate followed by hydride transfer from the co-factor. In addition, both groups catalyze the formation of primary and secondary amines utilizing both organic and inorganic amine donors. In this review, we discuss advances in developing AmDHs and IREDs as biocatalysts and focus on evolutionary history, substrate scope and applications of the enzymes to provide an outlook on emerging industrial biotechnologies of chiral amine production.

Engineering of formate dehydrogenase for improving conversion potential of carbon dioxide to formate

Formate dehydrogenase (FDH) is a D-2-hydroxy acid dehydrogenase, which can reversibly reduce CO2 to formate and thus act as non-photosynthetic CO2 reductase. In order to increase catalytic efficiency of formate dehydrogenase for CO2 reduction, two mutants V328I/F285W and V354G/F285W were obtained of which reduction activity was about two times more than the parent CbFDHM2, and the formate production from CO2 catalyzed by mutants were 2.9 and 2.7-fold higher than that of the parent CbFDHM2. The mutants had greater potential in CO2 reduction. The optimal temperature for V328I/F285W and V354G/F285W was 55 °C, and they showed increasement of relative activity under 45 °C to 55 °C compared with parent. The optimal pH for the mutants was 9.0, and they showed excellent stability in pH 4.0-11.5. The kcat/Km values of mutants were 1.75 times higher than that of the parent. Then the molecular basis for its improvement of biochemical characteristics were preliminarily elucidated by computer-aided methods. All of these results further established a solid foundation for molecular modification of formate dehydrogenase and CO2 reduction.

Taming secondary benzylic cations in catalytic asymmetric SN1 reactions

Benzylic stereogenic centers are ubiquitous in natural products and pharmaceuticals. A potentially general, though challenging, approach toward their selective creation would be asymmetric unimolecular nucleophilic substitution (SN1) reactions that proceed through highly reactive benzylic cations. We now report a broadly applicable solution to this problem by identifying chiral counteranions that pair with secondary benzylic cations to engage in catalytic asymmetric C-C, C-O, and C-N bond-forming reactions with excellent enantioselectivity. The critical cationic intermediate can be accessed from different precursors via Lewis- or Brønsted acid catalysis. Key to our strategy is the use of only weakly basic, confined counteranions that are posited to prolong the lifetime of the carbocation, thereby avoiding nonproductive deprotonation pathways to the corresponding styrene.

Enantioconvergent transformations of secondary alcohols through borrowing hydrogen catalysis

Direct substitution of readily available alcohols is recognized as a key research area in green chemical synthesis. Starting from simple racemic secondary alcohols, the achievement of catalytic enantioconvergent transformations of the substrates will be highly desirable for efficient access to valuable enantiopure compounds. To accomplish such attractive yet challenging transformations, the strategy of the enantioconvergent borrowing hydrogen methodology has proven to be uniquely effective and versatile. This review aims to provide an overview of the impressive progress made on this topic of research that has only thrived in the past decade. In particular, the conversion of racemic secondary alcohols to enantioenriched chiral amines, N-heterocycles, higher-order alcohols and ketones will be discussed in detail.

Photobiocatalytic Strategies for Organic Synthesis

Biocatalysis has revolutionized chemical synthesis, providing sustainable methods for preparing various organic molecules. In enzyme-mediated organic synthesis, most reactions involve molecules operating from their ground states. Over the past 25 years, there has been an increased interest in enzymatic processes that utilize electronically excited states accessed through photoexcitation. These photobiocatalytic processes involve a diverse array of reaction mechanisms that are complementary to one another. This comprehensive review will describe the state-of-the-art strategies in photobiocatalysis for organic synthesis until December 2022. Apart from reviewing the relevant literature, a central goal of this review is to delineate the mechanistic differences between the general strategies employed in the field. We will organize this review based on the relationship between the photochemical step and the enzymatic transformations. The review will include mechanistic studies, substrate scopes, and protein optimization strategies. By clearly defining mechanistically-distinct strategies in photobiocatalytic chemistry, we hope to illuminate future synthetic opportunities in the area.

Discovery of an Imine Reductase for Reductive Amination of Carbonyl Compounds with Sterically Challenging Amines

The synthesis of structurally diverse amines is of fundamental significance in the pharmaceutical industry due to the ubiquitous presence of amine motifs in biologically active molecules. Biocatalytic reductive amination for amine production has attracted great interest owing to its synthetic advantages. Herein, we report the direct synthesis of a wide range of sterically demanding secondary amines, including several important active pharmaceutical ingredients and pharmaceutical intermediates, via reductive amination of carbonyl substrates and bulky amine nucleophiles employing imine reductases. Key to success for this route is the identification of an imine reductase from Penicillium camemberti with unusual substrate specificity and its further engineering, which empowered the accommodation of a broad range of sterically demanding amine nucleophiles encompassing linear alkyl and (hetero)aromatic (oxy)alkyl substituents and the formation of final amine products with up to >99% conversion. The practical utility of the biocatalytic route has been demonstrated by its application in the preparative synthesis of the anti-hyperparathyroidism drug cinacalcet.

Substrate-Specific Engineering of Amino Acid Dehydrogenase Superfamily for Synthesis of a Variety of Chiral Amines and Amino Acids

Amino acid dehydrogenases (AADHs) are a group of enzymes that catalyze the reversible reductive amination of keto acids with ammonia to produce chiral amino acids using either nicotinamide adenine dinucleotide (NAD+) or nicotinamide adenine dinucleotide phosphate (NADP+) as cofactors. Among them, glutamate dehydrogenase, valine dehydrogenase, leucine dehydrogenase, phenylalanine dehydrogenase, and tryptophan dehydrogenase have been classified as a superfamily of amino acid dehydrogenases (s-AADHs) by previous researchers because of their conserved structures and catalytic mechanisms. Owing to their excellent stereoselectivity, high atom economy, and low environmental impact of the reaction pathway, these enzymes have been extensively engineered to break strict substrate specificities for the synthesis of high value-added chiral compounds (chiral amino acids, chiral amines, and chiral amino alcohols). Substrate specificity engineering of s-AADHs mainly focuses on recognition engineering of the substrate side chain R group and substrate backbone carboxyl group. This review summarizes the reported studies on substrate specificity engineering of s-AADHs and reports that this superfamily of enzymes shares substrate specificity engineering hotspots (the inside of the pocket, substrate backbone carboxyl anchor sites, substrate entrance tunnel, and hinge region), which sheds light on the substrate-specific tailoring of these enzymes.

High coenzyme affinity chimeric amine dehydrogenase based on domain engineering

NADH-dependent phenylalanine amine dehydrogenase (F-AmDH) engineered from phenylalanine dehydrogenase (PheDH) catalyzes the synthesis of aromatic chiral amines from prochiral ketone substrates. However, its low coenzyme affinity and catalytic efficiency limit its industrial application. Here, we developed a chimeric amine dehydrogenase, cFLF-AmDH, based on the relative independence of the structure at the domain level, combined with a substrate-binding domain from F-AmDH and a high-affinity cofactor-binding domain from leucine amine dehydrogenase (L-AmDH). The kinetic parameters indicated that cFLF-AmDH showed a twofold improvement in affinity for NADH and a 4.4-fold increase in catalytic efficiency (kcat/Km) compared with the parent F-AmDH. Meanwhile, cFLF-AmDH also showed higher thermal stability, with the half-life increased by 60% at 55 °C and a broader substrate spectrum, than the parent F-AmDH. Molecular dynamics simulations suggested that the constructed cFLF-AmDH had a more stable structure than the parent F-AmDH, thereby improving the affinity of the coenzyme. The reaction rate increased by 150% in the reductive amination reaction catalyzed by cFLF-AmDH. When the NAD+ concentration was 0.05 mM, the conversion rate was increased by 150%. These results suggest that the chimeric protein by domain shuffling from different domain donors not only increased the cofactor affinity and catalytic efficiency, but also changed the specificity and thermal stability. Our study highlights that domain engineering is another effective method for creating biodiversity with different catalytic properties.

Stereo-divergent enzyme cascades to convert racemic 4-phenyl-2-butanol into either (S)- or (R)- corresponding chiral amine

The synthesis of enantiopure chiral amines from racemic alcohols is a key transformation in the chemical industry, e.g., in the production of active pharmaceutical ingredients (APIs). However, this reaction remains challenging. In this work, we propose a one-pot enzymatic cascade for the direct conversion of a racemic alcohol into either (S) - or (R) -enantiomers of the corresponding amine, with in-situ cofactor recycling. This enzymatic cascade consists of two enantio-complementary alcohol dehydrogenases, both NADH and NADPH oxidase for in-situ recycling of NAD(P) + cofactors, and either (S) - or (R) -enantioselective transaminase. This cell-free biocatalytic system has been successfully applied to the conversion of racemic 4-phenyl-2-butanol into the high value (S) - or (R) -enantiomers of the amine reaching good (73% (S) ) and excellent (>99% (R) ) enantioselectivities.

Biosynthesis of Chiral Amino Alcohols via an Engineered Amine Dehydrogenase in E. coli

Chiral amino alcohols are prevalent synthons in pharmaceuticals and synthetic bioactive compounds. The efficient synthesis of chiral amino alcohols using ammonia as the sole amino donor under mild conditions is highly desired and challenging in organic chemistry and biotechnology. Our previous work explored a panel of engineered amine dehydrogenases (AmDHs) derived from amino acid dehydrogenase (AADH), enabling the one-step synthesis of chiral amino alcohols via the asymmetric reductive amination of α-hydroxy ketones. Although the AmDH-directed asymmetric reduction is in a high stereoselective manner, the activity is yet fully excavated. Herein, an engineered AmDH derived from a leucine dehydrogenase from Sporosarcina psychrophila (SpAmDH) was recruited as the starting enzyme, and the combinatorial active-site saturation test/iterative saturation mutagenesis (CAST/ISM) strategy was applied to improve the activity. After three rounds of mutagenesis in an iterative fashion, the best variant wh84 was obtained and proved to be effective in the asymmetric reductive amination of 1-hydroxy-2-butanone with 4-fold improvements in k_cat/K_m and total turnover number (TTN) values compared to those of the starting enzyme, while maintaining high enantioselectivity (ee >99%) and thermostability (T₅₀¹⁵ >53°C). In preparative-scale reaction, the conversion of 100 and 200 mM 1-hydroxy-2-butanone catalyzed by wh84 was up to 91–99%. Insights into the source of an enhanced activity were gained by the computational analysis. Our work expands the catalytic repertoire and toolbox of AmDHs.

Cascade chiral amine synthesis catalyzed by site-specifically co-immobilized alcohol and amine dehydrogenases

Sustainable and efficient production of chiral amines was realized with an oriented co-immobilized dual-enzyme system via SiBP-tag.

Engineering the activity of amine dehydrogenase in the asymmetric reductive amination of hydroxyl ketones

An engineered AmDH derived from a leucine dehydrogenase was used as the starting enzyme to improve its activity in the synthesis of (R)-3-amino-1-butanol. Preparative-scale synthesis of the (R)-product (90% yield, >99%) was performed on a gram-scale.

High Regio- and Stereoselective Multi-enzymatic Synthesis of All Phenylpropanolamine Stereoisomers from β-Methylstyrene

We present a one‐pot cascade for the synthesis of phenylpropanolamines (PPAs) in high optical purities (er and dr up to >99.5 %) and analytical yields (up to 95 %) by using 1‐phenylpropane‐1,2‐diols as key intermediates. This bioamination entails the combination of an alcohol dehydrogenase (ADH), an ω‐transaminase (ωTA) and an alanine dehydrogenase to create a redox‐neutral network, which harnesses the exquisite and complementary regio‐ and stereo‐selectivities of the selected ADHs and ωTAs. The requisite 1‐phenylpropane‐1,2‐diol intermediates were obtained from trans‐ or cis‐β‐methylstyrene by combining a styrene monooxygenase with epoxide hydrolases. Furthermore, in selected cases, the envisioned cascade enabled to obtain the structural isomer (1S,2R)‐1‐amino‐1‐phenylpropan‐2‐ol in high optical purity (er and dr >99.5 %). This is the first report on an enzymatic method that enables to obtain all of the four possible PPA stereoisomers in great enantio‐ and diastereo‐selectivity.

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.

PQQ-dependent Dehydrogenase Enables One-pot Bi-enzymatic Enantio-convergent Biocatalytic Amination of Racemic sec-Allylic Alcohols

The asymmetric amination of secondary racemic allylic alcohols bears several challenges like the reactivity of the bi-functional substrate/product as well as of the α,β-unsaturated ketone intermediate in an oxidation-reductive amination sequence. Heading for a biocatalytic amination cascade with a minimal number of enzymes, an oxidation step was implemented relying on a single PQQ-dependent dehydrogenase with low enantioselectivity. This enzyme allowed the oxidation of both enantiomers at the expense of iron(III) as oxidant. The stereoselective amination of the α,β-unsaturated ketone intermediate was achieved with transaminases using 1-phenylethylamine as formal reducing agent as well as nitrogen source. Choosing an appropriate transaminase, either the (R)- or (S)-enantiomer was obtained in optically pure form (>98 % ee). The enantio-convergent amination of the racemic allylic alcohols to one single allylic amine enantiomer was achieved in one pot in a sequential cascade.

Recent advances in artificial enzyme cascades for the production of value-added chemicals

Enzyme cascades are efficient tools to perform multi-step synthesis in one-pot in a green and sustainable manner, enabling non-natural synthesis of valuable chemicals from easily available substrates by artificially combining two or more enzymes. Bioproduction of many high-value chemicals such as chiral and highly functionalised molecules have been achieved by developing new enzyme cascades. This review summarizes recent advances on engineering and application of enzyme cascades to produce high-value chemicals (alcohols, aldehydes, ketones, amines, carboxylic acids, etc) from simple starting materials. While 2-step enzyme cascades are developed for versatile enantioselective synthesis, multi-step enzyme cascades are engineered to functionalise basic chemicals, such as styrenes, cyclic alkanes, and aromatic compounds. New cascade reactions have also been developed for producing valuable chemicals from bio-based substrates, such as ʟ-phenylalanine, and renewable feedstocks such as glucose and glycerol. The challenges in current process and future outlooks in the development of enzyme cascades are also addressed.

Control of Absolute Stereochemistry in Transition-Metal-Catalysed Hydrogen-Borrowing Reactions

Hydrogen-borrowing catalysis represents a powerful method for the alkylation of amine or enolate nucleophiles with non-activated alcohols. This approach relies upon a catalyst that can mediate a strategic series of redox events, enabling the formation of C-C and C-N bonds and producing water as the sole by-product. In the majority of cases these reactions have been employed to target achiral or racemic products. In contrast, the focus of this Minireview is upon hydrogen-borrowing-catalysed reactions in which the absolute stereochemical outcome of the process can be controlled. Asymmetric hydrogen-borrowing catalysis is rapidly emerging as a powerful approach for the synthesis of enantioenriched amine and carbonyl containing products and examples involving both C-N and C-C bond formation are presented. A variety of different approaches are discussed including use of chiral auxiliaries, asymmetric catalysis and enantiospecific processes.

One-Pot Synthesis of Chiral N-Arylamines by Combining Biocatalytic Aminations with Buchwald-Hartwig N-Arylation

The combination of biocatalysis and chemo-catalysis increasingly offers chemists access to more diverse chemical architectures. Here, we describe the combination of a toolbox of chiral-amine-producing biocatalysts with a Buchwald-Hartwig cross-coupling reaction, affording a variety of α-chiral aniline derivatives. The use of a surfactant allowed reactions to be performed sequentially in the same flask, preventing the palladium catalyst from being inhibited by the high concentrations of ammonia, salts, or buffers present in the aqueous media in most cases. The methodology was further extended by combining with a dual-enzyme biocatalytic hydrogen-borrowing cascade in one pot to allow for the conversion of a racemic alcohol to a chiral aniline.

A Simple Biosystem for the High-Yielding Cascade Conversion of Racemic Alcohols to Enantiopure Amines

The amination of racemic alcohols to produce enantiopure amines is an important green chemistry reaction for pharmaceutical manufacturing, requiring simple and efficient solutions. Herein, we report the development of a cascade biotransformation to aminate racemic alcohols. This cascade utilizes an ambidextrous alcohol dehydrogenase (ADH) to oxidize a racemic alcohol, an enantioselective transaminase (TA) to convert the ketone intermediate to chiral amine, and isopropylamine to recycle PMP and NAD⁺ cofactors via the reversed cascade reactions. The concept was proven by using an ambidextrous CpSADH-W286A engineered from (S)-enantioselective CpSADH as the first example of evolving ambidextrous ADHs, an enantioselective BmTA, and isopropylamine. A biosystem containing isopropylamine and E. coli (CpSADH-W286A/BmTA) expressing the two enzymes was developed for the amination of racemic alcohols to produce eight useful and high-value (S)-amines in 72-99 % yield and 98-99 % ee, providing with a simple and practical solution to this type of reaction.

Impact and relevance of alcohol dehydrogenase enantioselectivities on biotechnological applications

Alcohol dehydrogenases (ADHs) catalyze the reversible reduction of a carbonyl group to its corresponding alcohol. ADHs are widely employed for organic synthesis due to their lack of harm to the environment, broad substrate acceptance, and high enantioselectivity. This review focuses on the impact and relevance of ADH enantioselectivities on their biotechnological application. Stereoselective ADHs are beneficial to reduce challenging ketones such as ketones owning two bulky substituents or similar-sized substituents to the carbonyl carbon. Meanwhile, in cascade reactions, non-stereoselective ADHs can be utilized for the quantitative oxidation of racemic alcohol to ketone and dynamic kinetic resolution.

High level and enantioselective production of L-phenylglycine from racemic mandelic acid by engineered Escherichia coli using response surface methodology

L-Phenylglycine (L-PHG) is a member of unnatural amino acids, and becoming more and more important as intermediate for pharmaceuticals, food additives and agrochemicals. However, the existing synthetic methods for L-PHG mainly rely on toxic cyanide chemistry and multistep processes. To provide green, safe and high enantioselective alternatives, we envisaged cascade biocatalysis for the one-pot synthesis of L-PHG from racemic mandelic acid. A engineered E. coli strain was established to co-express mandelate racemase, D-mandelate dehydrogenase and L-leucine dehydrogenase and catalyze a 3-step reaction in one pot, enantioselectively transforming racemic mandelic acid to give L-PHG (e.e. >99 %). After the conditions for biosynthesis of L-PHG optimized by response surface methodology, the yield and space-time yield of L-PHG can reach 87.89 % and 79.70 g·L^-1·d^-1, which was obviously improved. The high-yielding and enantioselective synthetic methods use cheap and green reagents, and E. coli whole-cell catalysts, thus providing green and useful alternative methods for manufacturing L-PHG.

Development of an engineered thermostable amine dehydrogenase for the synthesis of structurally diverse chiral amines

Structurally diverse chiral amines and amino alcohols were synthesized using an engineered thermostable amine dehydrogenase, demonstrating its extensive synthesis potential.

Regio- and stereoselective multi-enzymatic aminohydroxylation of β-methylstyrene using dioxygen, ammonia and formate

We report an enzymatic route for the formal regio- and stereoselective aminohydroxylation of β-methylstyrene that consumes only dioxygen, ammonia and formate; carbonate is the by-product. The biocascade entails highly selective epoxidation, hydrolysis and hydrogen-borrowing alcohol amination. Thus, β-methylstyrene was converted into 1R,2R and 1S,2R-phenylpropanolamine in 59-63% isolated yields, and up to >99.5: <0.5 dr and er.

Sequential Two-Step Stereoselective Amination of Allylic Alcohols through the Combination of Laccases and Amine Transaminases

A sequential two-step chemoenzymatic methodology for the stereoselective synthesis of (3E)-4-(het)arylbut-3-en-2-amines in a highly selective manner and under mild reaction conditions is described. The approach consists of oxidation of the corresponding racemic alcohol precursors by the use of a catalytic system made up of the laccase from Trametes versicolor and the oxy-radical TEMPO, followed by the asymmetric reductive bio-transamination of the corresponding ketone intermediates. Optimisation of the oxidation reaction, exhaustive amine transaminase screening for the bio-transaminations and the compatibility of the two enzymatic reactions were studied in depth in search of a design of a compatible sequential cascade. This synthetic strategy was successful and the combinations of enzymes displayed a broad substrate scope, with 16 chiral amines being obtained in moderate to good isolated yields (29-75 %) and with excellent enantiomeric excess values (94 to >99 %). Interestingly, both amine enantiomers can be achieved, depending on the selectivity of the amine transaminase employed in the system.

Enzymatic self-sufficient hydride transfer processes

A number of self-sufficient hydride transfer processes have been reported in biocatalysis, with a common feature being the dependence on nicotinamide as a cofactor. This cofactor is provided in catalytic amounts and serves as a hydride shuttle to connect two or more enzymatic redox events, usually ensuring overall redox neutrality. Creative systems were designed to produce synthetic sequences characterized by high hydride economy, typically going in hand with excellent atom economy. Several redox enzymes have been successfully combined in one-pot one-step to allow functionalization of a large variety of molecules while preventing by-product formation. This review analyzes and classifies the various strategies, with a strong focus on efficiency, which is evaluated here in terms of the hydride economy and measured by the turnover number of the nicotinamide cofactor(s). The review ends with a critical evaluation of the reported systems and highlights areas where further improvements might be desirable.

Titanium-Catalyzed Cyano-Borrowing Reaction for the Direct Amination of Cyanohydrins with Ammonia

α-Aminonitrile was an important building block in natural products and key intermedia in organic chemistry. Herein, the direct amination of cyanohydrins with the partner of ammonia to synthesis N-unprotected α-aminonitriles is developed. The reaction proceeds via titanium-catalyzed cyano-borrowing reaction, which features high atom economy and simple operation. A broad range of ketone or aldehyde cyanohydrins was tolerated with ammonia, and the N-unprotected α-aminonitriles were synthesis with moderate to high yields under mild reaction conditions.

Synthesis of enantiomerically pure alcohols and amines via biocatalytic deracemisation methods

Deracemisation via chemo-enzymatic or multi-enzymatic approaches is the optimum substitute for kinetic resolution, which suffers from the limitation of a theoretical maximum 50% yield albeit high enantiomeric excess is attainable. This review covers the recent progress in various deracemisation approaches applied to the synthesis of enantiomerically pure alcohols and amines, such as (1) dynamic kinetic resolution, (2) cyclic deracemisation, (3) linear deracemisation (including stereoinversion) and (4) enantioconvergent methods.

Generation of amine dehydrogenases with increased catalytic performance and substrate scope from ε-deaminating L-Lysine dehydrogenase

Amine dehydrogenases (AmDHs) catalyse the conversion of ketones into enantiomerically pure amines at the sole expense of ammonia and hydride source. Guided by structural information from computational models, we create AmDHs that can convert pharmaceutically relevant aromatic ketones with conversions up to quantitative and perfect chemical and optical purities. These AmDHs are created from an unconventional enzyme scaffold that apparently does not operate any asymmetric transformation in its natural reaction. Additionally, the best variant (LE-AmDH-v1) displays a unique substrate-dependent switch of enantioselectivity, affording S- or R-configured amine products with up to >99.9% enantiomeric excess. These findings are explained by in silico studies. LE-AmDH-v1 is highly thermostable (Tm of 69 °C), retains almost entirely its catalytic activity upon incubation up to 50 °C for several days, and operates preferentially at 50 °C and pH 9.0. This study also demonstrates that product inhibition can be a critical factor in AmDH-catalysed reductive amination.

Efficient synthesis of enantiopure amines from alcohols using resting E. coli cells and ammonia

α-Chiral amines are pivotal building blocks for chemical manufacturing. Stereoselective amination of alcohols is receiving increased interest due to its higher atom-efficiency and overall improved environmental footprint compared with other chemocatalytic and biocatalytic methods. We previously developed a hydrogen-borrowing amination by combining an alcohol dehydrogenase (ADH) with an amine dehydrogenase (AmDH) in vitro. Herein, we implemented the ADH-AmDH bioamination in resting Escherichia coli cells for the first time. Different genetic constructs were created and tested in order to obtain balanced expression levels of the dehydrogenase enzymes in E. coli. Using the optimized constructs, the influence of several parameters towards the productivity of the system were investigated such as the intracellular NAD+/NADH redox balance, the cell loading, the survival rate of recombinant E. coli cells, the possible toxicity of the components of the reaction at different concentrations and the influence of different substrates and cosolvents. In particular, the cofactor redox-balance for the bioamination was maintained by the addition of moderate and precise amounts of glucose. Higher concentrations of certain amine products resulted in toxicity and cell death, which could be alleviated by the addition of a co-solvent. Notably, amine formation was consistent using several independently grown E. coli batches. The optimized E. coli/ADH-AmDH strains produced enantiopure amines from the alcohols with up to 80% conversion and a molar productivity up to 15 mM. Practical applicability was demonstrated in a gram-scale biotransformation. In summary, the present E. coli-ADH-AmDH system represents an important advancement towards the development of 'green', efficient and selective biocatalytic processes for the amination of alcohols.

Kinetic Resolution of Racemic Amines to Enantiopure (S)-amines by a Biocatalytic Cascade Employing Amine Dehydrogenase and Alanine Dehydrogenase

Amine dehydrogenases (AmDHs) efficiently catalyze the NAD(P)H-dependent asymmetric reductive amination of prochiral carbonyl substrates with high enantioselectivity. AmDH-catalyzed oxidative deamination can also be used for the kinetic resolution of racemic amines to obtain enantiopure amines. In the present study, kinetic resolution was carried out using a coupled-enzyme cascade consisting of AmDH and alanine dehydrogenase (AlaDH). AlaDH efficiently catalyzed the conversion of pyruvate to alanine, thus recycling the nicotinamide cofactors and driving the reaction forward. The ee values obtained for the kinetic resolution of 25 and 50 mM rac-α-methylbenzylamine using the purified enzymatic systems were only 54 and 43%, respectively. The use of whole-cells apparently reduced the substrate/product inhibition, and the use of only 30 and 40 mgDCW/mL of whole-cells co-expressing AmDH and AlaDH efficiently resolved 100 mM of rac-2-aminoheptane and rac-α-methylbenzylamine into the corresponding enantiopure (S)-amines. Furthermore, the applicability of the reaction protocol demonstrated herein was also successfully tested for the efficient kinetic resolution of wide range of racemic amines.

Mechanistic Insight into the Catalytic Promiscuity of Amine Dehydrogenases: Asymmetric Synthesis of Secondary and Primary Amines

Biocatalytic asymmetric amination of ketones, by using amine dehydrogenases (AmDHs) or transaminases, is an efficient method for the synthesis of α-chiral primary amines. A major challenge is to extend amination to the synthesis of secondary and tertiary amines. Herein, for the first time, it is shown that AmDHs are capable of accepting other amine donors, thus giving access to enantioenriched secondary amines with conversions up to 43 %. Surprisingly, in several cases, the promiscuous formation of enantiopure primary amines, along with the expected secondary amines, was observed. By conducting practical laboratory experiments and computational experiments, it is proposed that the promiscuous formation of primary amines along with secondary amines is due to an unprecedented nicotinamide (NAD)-dependent formal transamination catalysed by AmDHs. In nature, this type of mechanism is commonly performed by pyridoxal 5'-phosphate aminotransferase and not by dehydrogenases. Finally, a catalytic pathway that rationalises the promiscuous NAD-dependent formal transamination activity and explains the formation of the observed mixture of products is proposed. This work increases the understanding of the catalytic mechanism of NAD-dependent aminating enzymes, such as AmDHs, and will aid further research into the rational engineering of oxidoreductases for the synthesis of α-chiral secondary and tertiary amines.

Enhancing cofactor recycling in the bioconversion of racemic alcohols to chiral amines with alcohol dehydrogenase and amine dehydrogenase by coupling cells and cell-free system

Alcohol dehydrogenase (ADH) and amine dehydrogenase (AmDH)-catalyzed one-pot cascade conversion of an alcohol to an amine provides a simple preparation of chiral amines. To enhance the cofactor recycling in this reaction, we report a new concept of coupling whole-cells with the cell-free system to enable separated intracellular and extracellular cofactor regeneration and recycling. This was demonstrated by the respective biotransformation of racemic 4-phenyl-2-butanol 1a and 1-phenyl-2-propanol 1b to (R)-4-phenylbutan-2-amine 3a and (R)-1-phenylpropan-2-amine 3b. Escherichia coli cells expressing S-enantioselective CpsADH, R-enantioselective PfODH, and NADH oxidase (NOX) was developed to oxidize racemic alcohols 1a-b to ketones 2a-b with full conversion via intracellular NAD⁺ recycling. AmDH and glucose dehydrogenase (GDH) were used to convert ketones 2a-b to amines (R)-3a-b with 89-94% conversion and 891-943 times recycling of NADH. Combining the cells and enzymes for the cascade transformation of racemic alcohols 1a-b gave 70% and 48% conversion to the amines (R)-3a and (R)-3b in 99% ee, with a total turnover number (TTN) of 350 and 240 for NADH recycling, respectively. Improved results were obtained by using the E. coli cells with immobilized AmDH and GDH: (R)-3a was produced in 99% ee with 71-84% conversion and a TTN of 1410-1260 for NADH recycling, the highest value so far for the ADH-AmDH-catalyzed cascade conversion of alcohols to amines. The concept might be generally applicable to this type of reactions.