Fluorescence-based high-throughput screening system for R-ω-transaminase engineering and its substrate scope extension

Rich analytic toolbox for the exploration, characterization, screening, and application studies of ω-transaminases

Omega-transaminases (ωTAs) constitute an important class of biocatalysts in the pharmaceutical, agrochemical, and fine chemical industries, because of their generally good performance in the efficient, enantiospecific, and environment-friendly synthesis of chiral amines that possess diverse chemical structures and biological activities. However, their practical applications are often hindered by unfavorable reaction equilibria, product inhibition, limited robustness, and relatively small accommodation for substrates. Many efforts, including the exploration of novel enzymes from various environments and the targeted engineering of identified enzymes, have been made to develop more specific and efficient ωTA catalysts. A simple, rapid, and accurate evaluation of enzyme activity is important. In addition to the classic chromatography-based methods, to date, at least 18 analytic methods, which are based on cell growth or colorimetry/spectrophotometry, pH, fluorescence and conductivity changes, have been developed and applied in both qualitative and quantitative analyses of ωTAs. These methods differ in terms of their principles, accuracy, throughput, simplicity, and cost-effectiveness. Here, we present a detailed examination of the advantages and drawbacks of these methods. Guidance for method selection from the perspective of practical applications is proposed to assist investigators in choosing appropriate methods according to different research purposes and existing conditions.

Synthesis of C-N bonds by nicotinamide-dependent oxidoreductase: an overview

Compounds containing chiral C-N bonds play a vital role in the composition of biologically active natural products and small pharmaceutical molecules. Therefore, the development of efficient and convenient methods for synthesizing compounds containing chiral C-N bonds is a crucial area of research. Nicotinamide-dependent oxidoreductases (NDOs) emerge as promising biocatalysts for asymmetric synthesis of chiral C-N bonds due to their mild reaction conditions, exceptional stereoselectivity, high atom economy, and environmentally friendly nature. This review aims to present the structural characteristics and catalytic mechanisms of various NDOs, including imine reductases/ketimine reductases, reductive aminases, EneIRED, and amino acid dehydrogenases. Additionally, the review highlights protein engineering strategies employed to modify the stereoselectivity, substrate specificity, and cofactor preference of NDOs. Furthermore, the applications of NDOs in synthesizing essential medicinal chemicals, such as noncanonical amino acids and chiral amine compounds, are extensively examined. Finally, the review outlines future perspectives by addressing challenges and discussing the potential of utilizing NDOs to establish efficient biosynthesis platforms for C-N bond synthesis. In conclusion, NDOs provide an economical, efficient, and environmentally friendly toolbox for asymmetric synthesis of C-N bonds, thus contributing significantly to the field of pharmaceutical chemical development.

A Self-Assembled Amino Acid Hydrogel for Immobilization and Protection of Enzymes

Enzymes are essential biological catalysts, which have merits such as specificity, high efficiency, and mild-acting conditions. Due to the characteristics of enzymes, problems such as poor operational stability and difficulty in reuse limit the practical application of enzymes. These problems can often be solved by immobilization of enzymes. Commonly used enzyme immobilization materials include biochar, chitosan, polymer, and metal-organic frameworks, which often do not match the nature of the enzyme. This study utilizes the self-assembled amino acid hydrogel Fmoc-Y-OMe as the immobilizing material. The hydrogelator Fomc-Y-OMe has advantages like simple synthesis, easy immobilization, environmental friendliness, and good compatibility with proteins. It is able to protect enzyme activity at high temperatures and under a wide range of acid-base conditions and has excellent versatility. In particular, immobilized polyethylene terephthalate degrading enzyme (PETase) can significantly degrade polyethylene terephthalate (PET) film at 70 °C, while free PETase completely loses its catalytic capacity at such high temperatures. The excellent performance of self-assembled hydrogels to protect the catalytic activity of enzymes at high temperatures is highlighted.

Advances in the Molecular Modification of Microbial ω-Transaminases for Asymmetric Synthesis of Bulky Chiral Amines

ω-Transaminases are biocatalysts capable of asymmetrically synthesizing high-value chiral amines through the reductive amination of carbonyl compounds, and they are ubiquitously distributed across diverse microorganisms. Despite their broad natural occurrence, the industrial utility of naturally occurring ω-transaminases remains constrained by their limited catalytic efficiency toward sterically bulky substrates. Over recent decades, the use of structure-guided molecular modifications, leveraging three-dimensional structures, catalytic mechanisms, and machine learning-driven predictions, has emerged as a transformative strategy to address this limitation. Notably, these advancements have unlocked unprecedented progress in the asymmetric synthesis of bulky chiral amines, which is exemplified by the industrial-scale production of sitagliptin using engineered ω-transaminases. This review systematically explores the structural and mechanistic foundations of ω-transaminase engineering. We first delineate the substrate binding regions of these enzymes, focusing on their defining features such as substrate tunnels and dual pockets. These structural elements serve as critical targets for rational design to enhance substrate promiscuity. Next, we dissect the catalytic and substrate recognition mechanisms of (S)- and (R)-ω-transaminases. Drawing on these insights, we consolidate recent advances in engineering ω-transaminases to highlight their performance in synthesizing bulky chiral amines and aim to guide future research and the industrial implementation of tailored ω-transaminases.

Advances in biosynthesis of chiral amide herbicides and the key enzymes: dimethenamid-P and S-metolachlor as case studies

Chiral amide herbicides represent a significant class of agrochemicals, widely used for effective weed control. Prominent examples include S-metolachlor and dimethenamid-P, both of which share the intermediate (S)-1-methoxy-2-propylamine, a key structural component in their synthesis. Developing green and sustainable methods for producing this intermediate is crucial for enhancing the environmental and economic feasibility of herbicide manufacturing. Biosynthesis, with its advantages in sustainability and efficiency, has emerged as a pivotal approach in pesticide production. This review explores the classification and current development status of chiral amide herbicides, including their varieties and applications in the agricultural market. It outlines the synthesis pathways for S-metolachlor and dimethenamid-P, covering both chemical and biosynthetic routes. The review also highlights the functional properties of the key enzymes involved in the biosynthesis of (S)-1-methoxy-2-propylamine, focusing on the potential for enzyme engineering and creation to optimize these pathways. The challenges and future development directions for amide herbicides are discussed, with an emphasis on overcoming synthetic and ecological barriers.

Challenges and good practices on transaminase-catalysed synthesis of optically pure amines

Transaminase-catalysed synthesis of chiral amines has been highlighted as an important strategy to get access to optically pure primary amines with high selectivity. However, their application is still hindered from several factors. The co-factor instability leads to instability of the protein itself, while the thermodynamics typically do not favor the desired amination reaction. Thus, several strategies have been suggested to tackle the thermodynamic issue, while parameters that initially may seem trivial, such as selection of buffer salt, pH and temperature, recently were studied more thoroughly. In this chapter we provide a review on the suggested strategies with specific commentaries on their application, as well as protocol for the synthesis of optically pure amines with the two most commonly used amine donors, namely alanine and isopropylamine.

Directed evolution and metabolic engineering generate an Escherichia coli cell factory for de novo production of 4-hydroxymandelate

4-hydroxymandelate is a high-value aromatic compound used in the medicine, cosmetics, food, and chemical industry. However, existing natural extraction and chemical synthesis methods are costly and lead to environmental pollution. This study employed metabolic engineering and directed evolution strategies for de novo 4-hydroxymandelate biosynthesis. Two key challenges were addressed: insufficient precursor supply and limited activity of crucial enzymes. Through gene overexpression and multi-level gene interference using CRISPRi, An Escherichia coli chassis capable of producing the key precursor 4-hydroxyphenylpyruvate and the titer reached 5.05 mM (0.91 g/L). A mutant clone was obtained, HmaSV152G, which showed a 5.13-fold improvement in the catalytic rate. During fermentation, a high production of 194.87 mM (32.768 g/L) 4-hydroxymandelate was achieved in 76 h with a batch supply of glucose in a 5-L bioreactor. This study demonstrated the great potential of biosensors in protein engineering and provides a reference for large-scale production of other high-value aromatic compounds.

Evolving ω-amine transaminase AtATA guided by substrate-enzyme binding free energy for enhancing activity and stability against non-natural substrates

In the field of chiral amine synthesis, ω-amine transaminase (ω-ATA) is one of the most established enzymes capable of asymmetric amination under optimal conditions. However, the applicability of ω-ATA toward more non-natural complex molecules remains limited due to its low transamination activity, thermostability, and narrow substrate scope. Here, by employing a combined approach of computational virtual screening strategy and combinatorial active-site saturation test/iterative saturation mutagenesis strategy, we have constructed the best variant M14C3-V5 (M14C3-V62A-V116S-E117I-L118I-V147F) with improved ω-ATA from Aspergillus terreus (AtATA) activity and thermostability toward non-natural substrate 1-acetylnaphthalene, which is the ketone precursor for producing the intermediate (R)-(+)-1-(1-naphthyl)ethylamine [(R)-NEA] of cinacalcet hydrochloride, showing activity enhancement of up to 3.4-fold compared to parent enzyme M14C3 (AtATA-F115L-M150C-H210N-M280C-V149A-L182F-L187F). The computational tools YASARA, Discovery Studio, Amber, and FoldX were applied for predicting mutation hotspots based on substrate-enzyme binding free energies and to show the possible mechanism with features related to AtATA structure, catalytic activity, and stability in silico analyses. M14C3-V5 achieved 71.8% conversion toward 50 mM 1-acetylnaphthalene in a 50 mL preparative-scale reaction for preparing (R)-NEA. Moreover, M14C3-V5 expanded the substrate scope toward aromatic ketone compounds. The generated virtual screening strategy based on the changes in binding free energies has successfully predicted the AtATA activity toward 1-acetylnaphthalene and related substrates. Together with experimental data, these approaches can serve as a gateway to explore desirable performances, expand enzyme-substrate scope, and accelerate biocatalysis.IMPORTANCEChiral amine is a crucial compound with many valuable applications. Their asymmetric synthesis employing ω-amine transaminases (ω-ATAs) is considered an attractive method. However, most ω-ATAs exhibit low activity and stability toward various non-natural substrates, which limits their industrial application. In this work, protein engineering strategy and computer-aided design are performed to evolve the activity and stability of ω-ATA from Aspergillus terreus toward non-natural substrates. After five rounds of mutations, the best variant, M14C3-V5, is obtained, showing better catalytic efficiency toward 1-acetylnaphthalene and higher thermostability than the original enzyme, M14C3. The robust combinational variant acquired displayed significant application value for pushing the asymmetric synthesis of aromatic chiral amines to a higher level.

Visual monitoring of biocatalytic processes using small molecular fluorescent probes: strategies-mechanisms-applications

Real-time monitoring of biocatalytic-based processes is significantly improved and simplified when they can be visualized. Visual monitoring can be achieved by integrating a fluorescent unit with the biocatalyst. Herein, we outline the design strategies of fluorescent probes for monitoring biocatalysis: (1) probes for monitoring biocatalytic transfer: γ-glutamine is linked to the fluorophore as both a recognition group and for intramolecular charge transfer (ICT) inhibition; the probe is initially in an off state and is activated via the transfer of the γ-glutamine group and the release of the free amino group, which results in restoration of the "Donor-π-Acceptor" (D-π-A) system and fluorescence recovery. (2) Probes for monitoring biocatalytic oxidation: a propylamine is connected to the fluorophore as a recognition group, which cages the hydroxyl group, leading to the inhibition of ICT; propylamine is oxidized and subsequently β-elimination occurs, resulting in exposure of the hydroxyl group and fluorescence recovery. (3) Probes for monitoring biocatalytic reduction: a nitro group attached to a fluorophore as a fluorescence quenching group, this is converted to an amino group by catalytic reduction, resulting in fluorescence recovery. (4) Probes for monitoring biocatalytic hydrolysis: β-D-galactopyranoside or phosphate acts as a recognition group attached to hydroxyl groups of the fluorophore; the subsequent biocatalytic hydrolysis reaction releases the hydroxyl group resulting in fluorescence recovery. Following these 4 mechanisms, fluorophores including cyanine, coumarin, rhodamine, and Nile-red, have been used to develop systems for monitoring biocatalytic reactions. We anticipate that these strategies will result in systems able to rapidly diagnose and facilitate the treatment of serious diseases.

Stereoselective synthesis of (R)-(+)-1-(1-naphthyl)ethylamine by ω-amine transaminase immobilized on amino modified multi-walled carbon nanotubes and biocatalyst recycling

Immobilized enzymes exhibit favorable advantages in biocatalysis, such as high operation stability, feasible reusability, and improved organic solvents tolerance. Herein, an immobilized ω-amine transaminase AtATA@MWCNTs-NH2 is successfully prepared using amino modified multi-walled carbon nanotubes as carrier and glutaraldehyde as crosslinker. Under the optimum immobilization conditions, the activity recovery is 78.7%. Compared with purified enzyme AtATA, AtATA@MWCNTs-NH2 possesses superior stability, even in harsh conditions (e.g., high temperature, acidic or alkali environment, and different kind of organic solvents). To simplify the separation and extraction of products, we choose methanol (10%, v/v) as the cosolvent, replacing DMSO (20%, v/v) in our previous work, for the catalytic reaction of AtATA@MWCNTs-NH2. AtATA@MWCNTs-NH2 can be used for stereoselective synthesis (R)-(+)- 1(1-naphthyl)ethylamine ((R)-NEA) for 15 cycles, with the e.e.p (enantiomeric excess) > 99.5%. The catalytic process of AtATA@MWCNTs-NH2 achieves cycle production of (R)-NEA using methanol as cosolvent.

The rapid high-throughput screening of ω-transaminases via a colorimetric method using aliphatic α-diketones as amino acceptors

ω-Transaminases (ω-TAs) are widely available for the production of chiral amines and unnatural amino acids. Herein, a rapid spectrophotometric method was developed for screening ω-TAs based on the colored products that can be generated from transamination reactions between aliphatic α-diketones and amino donors catalyzed by ω-TAs. The possible mechanism of the formation of the colored product was investigated according to LC-Q-TOF-MS analysis. Among seven diketones, 2,3-butanedione was selected as the most suitable amino acceptor for colorimetric screening of ω-TAs with high efficiency, high sensitivity, and low background interference. Meanwhile, the absorbance of the colored product generated by 2,3-butanedione catalyzed by ω-TAs in this method was linearly correlated with the results by HPLC analysis. This method was also confirmed to effectively screen ω-TA mutants with high activity towards isopropylamine.

Efficient Synthesis of Key Chiral Intermediate in Painkillers (R)-1-[3,5-Bis(trifluoromethyl)phenyl]ethanamine by Bienzyme Cascade System with R-ω-Transaminase and Alcohol Dehydrogenase Functions

(R)-1-[3,5-bis(trifluoromethyl)phenyl]ethanamine, a key chiral intermediate of selective tetrodotoxin-sensitive blockers, was efficiently synthesized by a bienzyme cascade system formed by with R-ω-transaminase (ATA117) and an alcohol dehydrogenase (ADH) co-expression system. Herein, we report that the use of ATA117 as the biocatalyst for the amination of 3,5-bistrifluoromethylacetophenone led to the highest efficiency in product performance (enantiomeric excess > 99.9%). Moreover, to further improve the product yield, ADH was introduced into the reaction system to promote an equilibrium shift. Additionally, bienzyme cascade system was constructed by five different expression systems, including two tandem expression recombinant plasmids (pETDuet-ATA117-ADH and pACYCDuet-ATA117-ADH) and three co-expressed dual-plasmids (pETDuet-ATA117/pET28a-ADH, pACYCDuet-ATA117/pET28a-ADH, and pACYCDuet-ATA117/pETDuet-ADH), utilizing recombinant engineered bacteria. Subsequent studies revealed that as compared with ATA117 single enzyme, the substrate handling capacity of BL21(DE3)/pETDuet-ATA117-ADH (0.25 g wet weight) developed for bienzyme cascade system was increased by 1.50 folds under the condition of 40 °C, 180 rpm, 0.1 M pH9 Tris-HCl for 24 h. To the best of our knowledge, ours is the first report demonstrating the production of (R)-1-[3,5-bis(trifluoromethyl)phenyl]ethanamine using a bienzyme cascade system, thus providing valuable insights into the biosynthesis of chiral amines.

Non-Canonical Amino Acid-Based Engineering of (R)-Amine Transaminase

Non-canonical amino acids (ncAAs) have been utilized as an invaluable tool for modulating the active site of the enzymes, probing the complex enzyme mechanisms, improving catalytic activity, and designing new to nature enzymes. Here, we report site-specific incorporation of p-benzoyl phenylalanine (pBpA) to engineer (R)-amine transaminase previously created from d-amino acid aminotransferase scaffold. Replacement of the single Phe88 residue at the active site with pBpA exhibits a significant 15-fold and 8-fold enhancement in activity for 1-phenylpropan-1-amine and benzaldehyde, respectively. Reshaping of the enzyme's active site afforded an another variant F86A/F88pBpA, with 30% higher thermostability at 55°C without affecting parent enzyme activity. Moreover, various racemic amines were successfully resolved by transaminase variants into (S)-amines with excellent conversions (∼50%) and enantiomeric excess (>99%) using pyruvate as an amino acceptor. Additionally, kinetic resolution of the 1-phenylpropan-1-amine was performed using benzaldehyde as an amino acceptor, which is cheaper than pyruvate. Our results highlight the utility of ncAAs for designing enzymes with enhanced functionality beyond the limit of 20 canonical amino acids.

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.

Active-site engineering of ω-transaminase from Ochrobactrum anthropi for preparation of L-2-aminobutyric acid

BACKGROUND

The unnatural amino acid, L-2-aminobutyric acid (L-ABA) is an essential chiral building block for various pharmaceutical drugs, such as the antiepileptic drug levetiracetam and the antituberculosis drug ethambutol. The present study aims at obtaining variants of ω-transaminase from Ochrobactrum anthropi (OATA) with high catalytic activity to α-ketobutyric acid through protein engineering.

RESULTS

Based on the docking model using α-ketobutyric acid as the ligand, 6 amino acid residues, consisting of Y20, L57, W58, G229, A230 and M419, were chosen for saturation mutagenesis. The results indicated that L57C, M419I, and A230S substitutions demonstrated the highest elevation of enzymatic activity among 114 variants. Subsequently, double substitutions combining L57C and M419I caused a further increase of the catalytic efficiency to 3.2-fold. This variant was applied for threonine deaminase/OATA coupled reaction in a 50-mL reaction system with 300 mM L-threonine as the substrate. The reaction was finished in 12 h and the conversion efficiency of L-threonine into L-ABA was 94%. The purity of L-ABA is 75%, > 99% ee. The yield of L-ABA was 1.15 g.

CONCLUSION

This study provides a basis for further engineering of ω-transaminase for producing chiral amines from keto acids substrates.

Identification, Characterization, and Site-Specific Mutagenesis of a Thermostable ω-Transaminase from Chloroflexi bacterium

In the present study, we have identified an ω-transaminase (ω-TA) from Chloroflexi bacterium from the genome database by using two ω-TA sequences (ATA117 Arrmut11 from Arthrobacter sp. KNK168 and amine transaminase from Aspergillus terreus NIH2624) as templates in a BLASTP search and motif sequence alignment. The protein sequence of the ω-TA from C. bacterium (CbTA) shows 38% sequence identity to that of ATA117 Arrmut11. The gene sequence of CbTA was inserted into pRSF-Duet1 and functionally expressed in Escherichia coli BL21(DE3). The results showed that the recombinant CbTA has a specific activity of 1.19 U/mg for (R)-α-methylbenzylamine [(R)-MBA] at pH 8.5 and 45 °C. The substrate acceptability test showed that CbTA has significant reactivity to aromatic amino donors and amino receptors. More importantly, CbTA also exhibited good affinity toward some cyclic substrates. The homology model of CbTA was built by Discovery Studio, and docking was performed to describe the relative activity toward some substrates. CbTA evolved by site-specific mutagenesis and found that the Q192G mutant increased the activity to (R)-MBA by around 9.8-fold. The Q192G mutant was then used to convert two cyclic ketones, N-Boc-3-pyrrolidinone and N-Boc-3-piperidone, and both the conversions were obviously improved compared to that of the parental CbTA.

Fluorescence-based screening for engineered aldo-keto reductase KmAKR with improved catalytic performance and extended substrate scope

BACKGROUND

Aldo-keto reductases-catalyzed transformations of ketones to chiral alcohols have become an established biocatalytic process step in the pharmaceutical industry. Previously, we have discovered an aldo-keto reductase (AKR) from Kluyveromyces marxianus that is active to the aliphatic tert-butyl 6-substituted (5R/S)-hydroxy-3-oxohexanoates, but it is inactive to aromatic ketones. In order to acquire an excellent KmAKRmutant for ensuring the simultaneous improvement of activity-thermostability toward tert-butyl 6-cyano-(5R)-hydroxy-3-oxohexanoate ((5R)-1) and broadening the universal application prospects toward more substrates covering both aliphatic and aromatic ketones, a fluorescence-based high-throughput (HT) screening technique was established.

MAIN METHODS AND MAJOR RESULTS

The directed evolution of KmAKR variant M5 (KmAKR-W297H/Y296W/K29H/Y28A/T63M) produced the "best" variant M5-Q213A/T23V. It exhibited enhanced activity-thermostability toward (5R)-1, improved activity toward all 18 test substrates and strict R-stereoselectivity toward 10 substrates in comparison to M5. The enhancement of enzymatic activity and the extension of substrate scope covering aromatic ketones are proposed to be largely attributed to pushing the binding pocket of M5-Q213A/T23V to the enzyme surface, decreasing the steric hindrance at the entrance and enhancing the flexibility of loops surrounding the active center. In addition, combined with 0.94 g dry cell weight (DCW) L^-1 glucose dehydrogenase from Exiguobacterium sibiricum (EsGDH) for NADPH regeneration, 2.81 g DCW L^-1 M5-Q213A/T23V completely converted (5R)-1 of up to 450 g L^-1 at 120 g g^-1 substrates/catalysts (S/C), yielding the corresponding optically pure tert-butyl 6-cyano-(3R,5R)-dihydroxyhexanoate ((3R,5R)-2, > 99.5% d.e._p ) with a space-time yield (STY) of 1.08 kg L^-1 day^-1 .

CONCLUSIONS

A fluorescence-based HT screening system was developed to tailor KmAKR's activity, thermostability and substrate scope. The "best" variant M5-Q213A/T23V holds great potential application for the synthesis of aliphatic/aromatic R-configuration alcohols.

Kinetics of enzyme-catalysed desymmetrisation of prochiral substrates: product enantiomeric excess is not always constant

The kinetics of enzymatic desymmetrisation were analysed for the most common kinetic mechanisms: ternary complex ordered (prochiral ketone reduction); ping-pong second (ketone amination, diol esterification, desymmetrisation in the second half reaction); ping-pong first (diol ester hydrolysis) and ping-pong both (prochiral diacids). For plausible values of enzyme kinetic parameters, the product enantiomeric excess (ee) can decline substantially as the reaction proceeds to high conversion. For example, an ee of 0.95 at the start of the reaction can decline to less than 0.5 at 95% of equilibrium conversion, but for different enzyme properties it will remain almost unchanged. For most mechanisms a single function of multiple enzyme rate constants (which can be termed ee decline parameter, eeDP) accounts for the major effect on the tendency for the ee to decline. For some mechanisms, the concentrations or ratios of the starting materials have an important influence on the fall in ee. For the application of enzymatic desymmetrisation it is important to study if and how the product ee declines at high conversion.

Simultaneous directed evolution of coupled enzymes for efficient asymmetric synthesis of l-phosphinothricin

The traditional strategy to improve the efficiency of an entire coupled enzyme system relies on separate direction of the evolution of enzymes involved in their respective enzymatic reactions. This strategy can lead to enhanced single-enzyme catalytic efficiency but may also lead to loss of coordination among enzymes. This study aimed to overcome such shortcomings by executing a directed evolution strategy on multiple enzymes in one combined group that catalyzes the asymmetric biosynthesis of l-phosphinothricin. The genes of a glutamate dehydrogenase from Pseudomonas moorei (PmGluDH) and a glucose dehydrogenase from Exiguobacterium sibiricum (EsGDH), along with other gene parts (promoters, ribosomal binding sites (RBSs), and terminators) were simultaneously evolved. The catalytic efficiency of PmGluDH was boosted by introducing the beneficial mutation A164G (from 1.29 s^-1mM^-1 to 183.52 s^-1mM^-1), and the EsGDH expression level was improved by optimizing the linker length between the RBS and the start codon of gdh. The total turnover numbers of the bioreaction increased from 115 (GluDH WT^NADPH) to 5846 (A164G^NADPH coupled with low expression of EsGDH), and to 33950 (A164G^NADPH coupled with high expression of EsGDH). The coupling efficiency was increased from ∼30% (GluDH_WT with low expression of GDH) to 83.3% (GluDH_A164G with high expression of GDH). In the batch production of l-phosphinothricin utilizing whole-cell catalysis, the strongest biocatalytic reaction exhibited a high space-time yield (6410 g·L^-1·d^-1) with strict stereoselectivity (>99% enantiomeric excess).Importance: The traditional strategy to improve multienzyme-catalyzed reaction efficiency may lead to enhanced single-enzyme catalytic efficiency but may also result in loss of coordination among enzymes. We describe a directed evolution strategy of an entire coupled enzyme system to simultaneously enhance enzyme coordination and catalytic efficiency. The simultaneous evolution strategy was applied to a multienzyme-catalyzed reaction for the asymmetric synthesis of l-phosphinothricin, which not only enhanced the catalytic efficiency of GluDH but also improved the coordination between GluDH and GDH. Since this strategy is enzyme-independent, it may be applicable to other coupled enzyme systems for chiral chemical synthesis.

Creation of a robust and R-selective ω-amine transaminase for the asymmetric synthesis of sitagliptin intermediate on a kilogram scale

The creation of an R-selective ω-amine transaminase (ω-ATA) as biocatalyst is crucial for the asymmetric amination of prochiral ketones to produce sitagliptin intermediates because rare ω-ATAs are R-selective in nature and most of them suffer from poor stability and low activity toward bulky prochiral ketones. Here, the gene of an R-selective ω-ATA was cloned from Arthrobacter cumminsii ZJUT212 (AcATA) and expressed in Escherichia coli. The best variants (M1 + M122H and M1+T134 G) were obtained using a semi-rational protein design after screening. These variants not only exhibited improved activity and substrate affinity but also enhanced stability in aqueous phase containing 20 % dimethyl sulfoxide. The conversion of asymmetric amination on 50 g/L pro-sitagliptin ketone PTfpB (1-[1-piperidinyl]-4-[2,4,5-trifluorophenyl]-1,3-butanedione) achieved 92 %, with an extremely high e.e. of >99 %, using 2 g_DCW/L E. coli cells harboring M1 + M122H as biocatalyst. In the kilogram-scale experiment, approximately 40 kg of (R)-APTfpB (e.e. >99 %) was produced within 30 h when 50 kg PTfpB was used as the substrate. Furthermore, the space-time yield reached ≈32 g/(L·d).

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.