Biocatalytic asymmetric amination of carbonyl functional groups - a synthetic biology approach to organic chemistry

Identification of a novel thermostable transaminase and its application in L-phosphinothricin biosynthesis

Transaminase (TA) is a crucial biocatalyst for enantioselective production of the herbicide L-phosphinothricin (L-PPT). The use of enzymatic cascades has been shown to effectively overcome the unfavorable thermodynamic equilibrium of TA-catalyzed transamination reaction, also increasing demand for TA stability. In this work, a novel thermostable transaminase (PtTA) from Pseudomonas thermotolerans was mined and characterized. The PtTA showed a high specific activity (28.63 U/mg) towards 2-oxo-4-[(hydroxy)(methyl)phosphinoyl]butyric acid (PPO), with excellent thermostability and substrate tolerance. Two cascade systems driven by PtTA were developed for L-PPT biosynthesis, including asymmetric synthesis of L-PPT from PPO and deracemization of D, L-PPT. For the asymmetric synthesis of L-PPT from PPO, a three-enzyme cascade was constructed as a recombinant Escherichia coli (E. coli G), by co-expressing PtTA, glutamate dehydrogenase (GluDH) and D-glucose dehydrogenase (GDH). Complete conversion of 400 mM PPO was achieved using only 40 mM amino donor L-glutamate. Furthermore, by coupling D-amino acid aminotransferase (Ym DAAT) from Bacillus sp. YM-1 and PtTA, a two-transaminase cascade was developed for the one-pot deracemization of D, L-PPT. Under the highest reported substrate concentration (800 mM D, L-PPT), a 90.43% L-PPT yield was realized. The superior catalytic performance of the PtTA-driven cascade demonstrated that the thermodynamic limitation was overcome, highlighting its application prospect for L-PPT biosynthesis. KEY POINTS: • A novel thermostable transaminase was mined for L-phosphinothricin biosynthesis. • The asymmetric synthesis of L-phosphinothricin was achieved via a three-enzyme cascade. • Development of a two-transaminase cascade for D, L-phosphinothricin deracemization.

Enantioselective Amination of 3-Substituted-2-benzofuranones via Non-covalent N-Heterocyclic Carbene Catalysis

Herein, an efficient method for asymmetric α-amination of 2-benzofuranones with N-heterocyclic carbene (NHC) catalysis is reported. The process is based on non-covalent interaction of NHC with substrate, facilitating the formation of a chiral ion-pair that encompasses enolate and azolium salt. The activated enolate adds to an electrophilic amine source with sufficient facial control to furnish an enantioenriched product having an amine substituted quaternary stereocenter. The process displays a broad substrate scope. A preparative scale synthesis has been achieved. Preliminary mechanistic investigations based on experimental and DFT studies suggest a reaction pathway that involves non-covalent substrate/NHC interactions and essentially implicate the role of π-π interaction in diastereomeric transition states for stereo-chemical discrimination.

Development of an aminotransferase-driven biocatalytic cascade for deracemization of d,l-phosphinothricin

2-oxo-4-[(hydroxy)(methyl)phosphinoyl]butyric acid (PPO) is the essential precursor keto acid for the asymmetric biosynthesis of herbicide l-phosphinothricin (l-PPT). Developing a biocatalytic cascade for PPO production with high efficiency and low cost is highly desired. Herein, a d-amino acid aminotransferase from Bacillus sp. YM-1 (Ym DAAT) with high activity (48.95 U/mg) and affinity (Km  = 27.49 mM) toward d-PPT was evaluated. To circumvent the inhibition of by-product d-glutamate (d-Glu), an amino acceptor (α-ketoglutarate) regeneration cascade was constructed as a recombinant Escherichia coli (E. coli D), by coupling Ym d-AAT, d-aspartate oxidase from Thermomyces dupontii (TdDDO) and catalase from Geobacillus sp. CHB1. Moreover, the regulation of the ribosome binding site was employed to overcome the limiting step of expression toxic protein TdDDO in E. coli BL21(DE3). The aminotransferase-driven whole-cell biocatalytic cascade (E. coli D) showed superior catalytic efficiency for the synthesis of PPO from d,l-phosphinothricin (d,l-PPT). It revealed the production of PPO exhibited high space-time yield (2.59 g L-1  h-1 ) with complete conversion of d-PPT to PPO at high substrate concentration (600 mM d,l-PPT) in 1.5 L reaction system. This study first provides the synthesis of PPO from d,l-PPT employing an aminotransferase-driven biocatalytic cascade.

Improving the Thermostability and Activity of Transaminase From Aspergillus terreus by Charge-Charge Interaction

Transaminases that promote the amination of ketones into amines are an emerging class of biocatalysts for preparing a series of drugs and their intermediates. One of the main limitations of (R)-selective amine transaminase from Aspergillus terreus (At-ATA) is its weak thermostability, with a half-life (t _1/2) of only 6.9 min at 40°C. To improve its thermostability, four important residue sites (E133, D224, E253, and E262) located on the surface of At-ATA were identified using the enzyme thermal stability system (ETSS). Subsequently, 13 mutants (E133A, E133H, E133K, E133R, E133Q, D224A, D224H, D224K, D224R, E253A, E253H, E253K, and E262A) were constructed by site-directed mutagenesis according to the principle of turning the residues into opposite charged ones. Among them, three substitutions, E133Q, D224K, and E253A, displayed higher thermal stability than the wild-type enzyme. Molecular dynamics simulations indicated that these three mutations limited the random vibration amplitude in the two α-helix regions of 130-135 and 148-158, thereby increasing the rigidity of the protein. Compared to the wild-type, the best mutant, D224K, showed improved thermostability with a 4.23-fold increase in t _1/2 at 40°C, and 6.08°C increase in T 50 10 . Exploring the three-dimensional structure of D224K at the atomic level, three strong hydrogen bonds were added to form a special "claw structure" of the α-helix 8, and the residues located at 151-156 also stabilized the α-helix 9 by interacting with each other alternately.

Multi-enzyme pyruvate removal system to enhance (R)-selective reductive amination of ketones

Biocatalytic transamination is widely used in industrial production of chiral chemicals. Here, we constructed a novel multi-enzyme system to promote the conversion of the amination reaction. Firstly, we constructed the ArR-ωTA/TdcE/FDH/LDH multi-enzyme system, by combination of (R)-selective ω-transaminase derived from Arthrobacter sp. (ArR-ωTA), formate dehydrogenase (FDH) derived from Candida boidinii, formate acetyltransferase (TdcE) and lactate dehydrogenase (LDH) derived from E. coli MG1655. This multi-enzyme system was used to efficiently remove the by-product pyruvate by TdcE and LDH to facilitate the transamination reaction. The TdcE/FDH pathway was found to dominate the by-product pyruvate removal in the transamination reaction. Secondly, we optimized the reaction conditions, including d-alanine, DMSO, and pyridoxal phosphate (PLP) with different concentration of 2-pentanone (as a model substrate). Thirdly, by using the ArR-ωTA/TdcE/FDH/LDH system, the conversions of 2-pentanone, 4-phenyl-2-butanone and cyclohexanone were 84.5%, 98.2% and 79.3%, respectively.

The ArR-ωTA/TdcE/FDH/LDH system is an efficient system for increasing the conversion in the transamination reaction.

Tuning amino acid dehydrogenases with featured sequences for L-phosphinothricin synthesis by reductive amination

Biosynthesizing unnatural chiral amino acids is challenging due to the limited reductive amination activity of amino acid dehydrogenase (AADH). Here, for the asymmetric synthesis of l-phosphinothricin from 2-oxo-4-[(hydroxy)(methyl)phosphinoyl]butyric acid (PPO), a glutamate dehydrogenase gene (named GluDH3) from Pseudomonas monteilii was selected, cloned and expressed in Escherichia coli (E. coli). To boost its activity, a "two-step"-based computational approach was developed and applied to select the potential beneficial amino acid positions on GluDH3. l-phosphinothricin was synthesized by GluDH-catalyzed asymmetric amination using the d-glucose dehydrogenase from Exiguobacterium sibiricum (EsGDH) for NADPH regeneration. Using lyophilized E. coli cells that co-expressed GluDH3_V375S and EsGDH, up to 89.04 g L^-1 PPO loading was completely converted to l-phosphinothricin within 30 min at 35 °C with a space-time yield of up to 4.752 kg·L^-1·d^-1. The beneficial substitution V375S with increased polar interactions between K90, T193, and substrate PPO exhibited 168.2-fold improved catalytic efficiency (k_cat/K_M) and 344.8-fold enhanced specific activity. After the introduction of serine residues into other GluDHs at specific positions, forty engineered GluDHs exhibited the catalytic functions of "glufosinate dehydrogenase" towards PPO.

Transaminase-mediated synthesis of enantiopure drug-like 1-(3′,4′-disubstituted phenyl)propan-2-amines

Transaminases (TAs) offer an environmentally and economically attractive method for the direct synthesis of pharmaceutically relevant disubstituted 1-phenylpropan-2-amine derivatives starting from prochiral ketones. In this work, we report the application of immobilised whole-cell biocatalysts with (R)-transaminase activity for the synthesis of novel disubstituted 1-phenylpropan-2-amines. After optimisation of the asymmetric synthesis, the (R)-enantiomers could be produced with 88–89% conversion and >99% ee, while the (S)-enantiomers could be selectively obtained as the unreacted fraction of the corresponding racemic amines in kinetic resolution with >48% conversion and >95% ee.

Immobilised whole-cell (R)-transaminases (TAs) enabled synthesis of either (R)- or (S)-enantiomers of drug-like amines from prochiral ketones or from racemic amines, respectively, in >95% ee.

Enantioselective Carbonyl Catalysis Enabled by Chiral Aldehydes

Organocatalytic methods have achieved spectacular advancements for the preparation of chiral molecules in highly enantioenriched forms. The fast development of this field can mainly be attributed to the evolution of general and reliable activation modes. The discovery and identification of new activation modes are therefore highly desirable to push the boundaries of asymmetric reactions. In this Minireview, recent advances in enantioselective carbonyl catalysis, one useful subbranch of organocatalysis for the efficient activation of simple amines, will be summarized. With elegantly designed chiral aldehyde catalysts, highly enantioselective and efficient asymmetric reactions can be developed. Continued development of enantioselective carbonyl catalysis is expected in the future.

Crystal structure of the aromatic-amino-acid aminotransferase fromStreptococcus mutans

Streptococcus mutans, a facultatively aerobic and Gram-positive bacterium, is the primary causative agent of dental caries and contributes to the multispecies biofilm known as dental plaque. In this study, the aromatic-amino-acid aminotransferase fromStreptococcus mutans(SmAroAT) was recombinantly expressed inEscherichia coli. An effective purification protocol was established. The recombinant protein was crystallized using the hanging-drop vapor-diffusion method with PEG 3350 as the primary precipitant. The crystal structure ofSmAroAT was solved at 2.2 Å resolution by the molecular-replacement method. Structural analysis indicated that the proteins of the aromatic-amino-acid aminotransferase family have conserved structural elements that might play a role in substrate binding. These results may help in obtaining a better understanding of the catabolism and biosynthesis of aromatic amino acids.

Enzymatic asymmetric synthesis of chiral amino acids

This review summarizes the progress achieved in the enzymatic asymmetric synthesis of chiral amino acids from prochiral substrates.

Enzymatic synthesis of chiral amino-alcohols by coupling transketolase and transaminase-catalyzed reactions in a cascading continuous-flow microreactor system

Rapid biocatalytic process development and intensification continues to be challenging with currently available methods. Chiral amino‐alcohols are of particular interest as they represent key industrial synthons for the production of complex molecules and optically pure pharmaceuticals. (2S,3R)‐2‐amino‐1,3,4‐butanetriol (ABT), a building block for the synthesis of protease inhibitors and detoxifying agents, can be synthesized from simple, non‐chiral starting materials, by coupling a transketolase‐ and a transaminase‐catalyzed reaction. However, until today, full conversion has not been shown and, typically, long reaction times are reported, making process modifications and improvement challenging. In this contribution, we present a novel microreactor‐based approach based on free enzymes, and we report for the first time full conversion of ABT in a coupled enzyme cascade for both batch and continuous‐flow systems. Using the compartmentalization of the reactions afforded by the microreactor cascade, we overcame inhibitory effects, increased the activity per unit volume, and optimized individual reaction conditions. The transketolase‐catalyzed reaction was completed in under 10 min with a volumetric activity of 3.25 U ml⁻¹. Following optimization of the transaminase‐catalyzed reaction, a volumetric activity of 10.8 U ml⁻¹ was attained which led to full conversion of the coupled reaction in 2 hr. The presented approach illustrates how continuous‐flow microreactors can be applied for the design and optimization of biocatalytic processes.

Direct Catalytic Asymmetric Reductive Amination of Aliphatic Ketones Utilizing Diphenylmethanamine as Coupling Partner

The highly efficient direct catalytic reductive amination of ketones with diphenylmethanamine catalyzed by iridium-phosphoramidite complexes is described. As an effective coupling partner, diphenylmethanamine is suitable for a wide range of ketones to provide chiral amines in high yields and enantioselectivity. The chiral monodentate phosphoramidite ligands are tunable and competent to accommodate substrates with different structures.

Iridium-catalyzed direct asymmetric reductive amination of aromatic ketones

Diphenylmethanamine offers excellent stereocontrol and easy deprotection to provide primary amines in the studied asymmetric reductive amination.

Catalytic Promiscuity of Transaminases: Preparation of Enantioenriched β-Fluoroamines by Formal Tandem Hydrodefluorination/Deamination

Transaminases are valuable enzymes for industrial biocatalysis and enable the preparation of optically pure amines. For these transformations they require either an amine donor (amination of ketones) or an amine acceptor (deamination of racemic amines). Herein transaminases are shown to react with aromatic β-fluoroamines, thus leading to simultaneous enantioselective dehalogenation and deamination to form the corresponding acetophenone derivatives in the absence of an amine acceptor. A series of racemic β-fluoroamines was resolved in a kinetic resolution by tandem hydrodefluorination/deamination, thus giving the corresponding amines with up to greater than 99 % ee. This protocol is the first example of exploiting the catalytic promiscuity of transaminases as a tool for novel transformations.

ω-Transaminases for the amination of functionalised cyclic ketones

The potential of a number of enantiocomplementary ω-transaminases (ω-TAms) in the amination of cyclic ketones has been investigated. After a preliminary screening of several compounds with increasing complexity, different approaches to shift the equilibrium of the reaction to the amine products were studied, and reaction conditions (temperature and pH) optimised. Interestingly, 2-propylamine as an amine donor was tolerated by all five selected ω-TAms, and therefore used in further experiments. Due to the higher conversions observed and interest in chiral amines studies then focused on the amination of α-tetralone and 2-methylcyclohexanone. Both ketones were aminated to give the corresponding amine with at least one of the employed enzymes. Moreover, the amination of 2-methylcyclohexanone was investigated in more detail due to the different stereoselectivities observed with TAms used. The highest yields and stereoselectivities were obtained using the ω-TAm from Chromobacterium violaceum (CV-TAm), producing 2-methylcyclohexylamine with complete stereoselectivity at the (1S)-amine position and up to 24 : 1 selectivity for the cis : trans [(1S,2R) : (1S,2S)] isomer.

Organocatalytic synthesis of optically active β-branched α-amino esters via asymmetric biomimetic transamination

This paper describes an efficient asymmetric biomimetic transamination of α-keto esters with a quinine-derived chiral base as the catalyst, giving a variety of β-branched α-amino esters in 50-96% yield and 87-95% ee.

Progress in asymmetric biomimetic transamination of carbonyl compounds

Transamination of α-keto acids is an important process to form α-amino acids in biological systems. Various biomimetic transamination systems have been developed for carbonyl compounds with chiral vitamin B₆ analogues, artificial transaminase mimics, chiral nitrogen sources, and chiral catalysts. This review provides a brief summary in this area.

Synthesis of (R)- or (S)-valinol using ω-transaminases in aqueous and organic media

Valinol is part of numerous pharmaceuticals and has various other important applications. Optically pure valinol (ee >99%) was prepared employing different ω-transaminases from the corresponding prochiral hydroxy ketone. By the choice of the enzyme the (R)- as well as the (S)-enantiomer were accessible. Reductive amination was performed in organic solvent (MTBE) using 2-propyl amine as amine donor whereas alanine was applied in or in aqueous medium. Transformations in phosphate buffer were successfully performed even at 200 mM substrate concentration (20.4 g/L) leading to 99% (R) and 94% (S) conversion with perfect optical purity (>99% ee).

New generation of biocatalysts for organic synthesis

The use of enzymes as catalysts for the preparation of novel compounds has received steadily increasing attention over the past few years. High demands are placed on the identification of new biocatalysts for organic synthesis. The catalysis of more ambitious reactions reflects the high expectations of this field of research. Enzymes play an increasingly important role as biocatalysts in the synthesis of key intermediates for the pharmaceutical and chemical industry, and new enzymatic technologies and processes have been established. Enzymes are an important part of the spectrum of catalysts available for synthetic chemistry. The advantages and applications of the most recent and attractive biocatalysts--reductases, transaminases, ammonia lyases, epoxide hydrolases, and dehalogenases--will be discussed herein and exemplified by the syntheses of interesting compounds.

Engineering the meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum by site saturation mutagenesis for D-phenylalanine synthesis

In order to enlarge the substrate binding pocket of the meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum to accommodate larger 2-keto acids, four amino acid residues (Phe146, Thr171, Arg181, and His227) were targeted for site saturation mutagenesis. Among all mutants, the single mutant H227V had a specific activity of 2.39 ± 0.06 U · mg(-1), which was 35.1-fold enhancement over the wild-type enzyme.

Asymmetric Preparation of prim-, sec-, and tert-Amines Employing Selected Biocatalysts

This account focuses on the application of ω-transaminases, lyases, and oxidases for the preparation of amines considering mainly work from our own lab. Examples are given to access α-chiral primary amines from the corresponding ketones as well as terminal amines from primary alcohols via a two-step biocascade. 2,6-Disubstituted piperidines, as examples for secondary amines, are prepared by biocatalytical regioselective asymmetric monoamination of designated diketones followed by spontaneous ring closure and a subsequent diastereoselective reduction step. Optically pure tert-amines such as berbines and N-methyl benzylisoquinolines are obtained by kinetic resolution via an enantioselective aerobic oxidative C-C bond formation.