Three in One: Temperature, Solvent and Catalytic Stability by Engineering the Cofactor-Binding Element of Amine Transaminase

Immobilization of His6-tagged amine transaminases in microreactors using functionalized nonwoven nanofiber membranes

Process intensification is crucial for industrial implementation of biocatalysis and can be achieved by continuous process operation in miniaturized reactors with efficiently immobilized biocatalysts, enabling their long-term use. Due to their extremely large surface-to-volume ratio, nanomaterials are promising supports for enzyme immobilization. In this work, different functionalized nanofibrous nonwoven membranes were embedded in a two-plate microreactor to enable immobilization of hexahistidine (His6)-tagged amine transaminases (ATAs) in flow. A membrane coated with Cu2+ ions gave the best results regarding His6-tagged ATAs immobilization among the membranes tested yielding an immobilization yield of up to 95.3 % for the purified N-His6-ATA-wt enzyme. Moreover, an efficient one-step enzyme immobilization process from overproduced enzyme in Escherichia coli cell lysate was developed and yielded enzyme loads up to 1088 U mL-1. High enzyme loads resulted in up to 80 % yields of acetophenone produced from 40 mM (S)-α-methylbenzylamine in less than 4 min using a continuously operated microreactor. Up to 81 % of the initial activity was maintained in a 5-day continuous microreactor operation with immobilized His6-tagged ATA constructs. The highest turnover number within the indicated time was 7.23·106, which indicates that this immobilization approach using advanced material and reactor system is highly relevant for industrial implementation.

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.

Turning thermostability of Aspergillus terreus (R)-selective transaminase At-ATA by synthetic shuffling

As versatile and green biocatalysts for the asymmetric amination of ketones, the insufficient thermostability of transaminases always limits its broad application in the pharmaceutical and fine chemical industries. Here, synthetic shuffling technology was used to enhance stability of (R)-selective transaminase from Aspergillus terreus. The results showed that 30 out of 5000 mutants had improved thermostability by color-based screening method, among which mutants with residual enzyme activity higher than 50% at 45 °C for 10 min were selected for further analysis. Especially, the half-inactivation temperature (T₅₀¹⁰), half-life (t_1/2), and melting temperature (T_m) of the best mutant M14 (M280C-H210N-M150C-F115L) were 13.7 °C, 165.8 min, and 13.9 °C higher than that of the wild type (WT), respectively. M14 also exhibited a significant biocatalytic efficiency toward acetophenone and 1-acetylnaphthalene, the yield of which were 265.6% and 117.5% higher than WT, respectively. Based on molecular dynamics simulation, improved catalytic efficiency of M14 could be attributed to its increased hydrogen bonds interaction around the mutation sites. Additionally, the introduction of disulfide bond combined with above mutations has a synergistic effect on the improved protein thermostability.

Comparison of Four Immobilization Methods for Different Transaminases

Biocatalytic syntheses often require unfavorable conditions, which can adversely affect enzyme stability. Consequently, improving the stability of biocatalysts is needed, and this is often achieved by immobilization. In this study, we aimed to compare the stability of soluble and immobilized transaminases from different species. A cysteine in a consensus sequence was converted to a single aldehyde by the formylglycine-generating enzyme for directed single-point attachment to amine beads. This immobilization was compared to cross-linked enzyme aggregates (CLEAs) and multipoint attachments to glutaraldehyde-functionalized amine- and epoxy-beads. Subsequently, the reactivity and stability (i.e., thermal, storage, and solvent stability) of all soluble and immobilized transaminases were analyzed and compared under different conditions. The effect of immobilization was highly dependent on the type of enzyme, the immobilization strategy, and the application itself, with no superior immobilization technique identified. Immobilization of HAGA-beads often resulted in the highest activities of up to 62 U/g beads, and amine beads were best for the hexameric transaminase from Luminiphilus syltensis. Furthermore, the immobilization of transaminases enabled its reusability for at least 10 cycles, while maintaining full or high activity. Upscaled kinetic resolutions (partially performed in a SpinChemTM reactor) resulted in a high conversion, maintained enantioselectivity, and high product yields, demonstrating their applicability.

Fusion of Formate Dehydrogenase and Alanine Dehydrogenase as an Amino Donor Regenerating System Coupled to Transaminases

Fusion enzymes are attractive tools for facilitating the assembly of biocatalytic cascades for chemical synthesis. This approach can offer great advantages for cooperative redox cascades that need the constant supply of a donor molecule. In this work, we have developed a self-sufficient bifunctional enzyme that can be coupled to transaminase-catalyzed reactions for the efficient recycling of the amino donor (L-alanine). By genetic fusion of an alanine dehydrogenase (AlaDH) and a formate dehydrogenase (FDH), a redox-complementary system was applied to recycle the amino donor and the cofactor (NADH), respectively. AlaDH and FDH were assembled in both combinations (FDH-AlaDH and AlaDH-FDH), with a 2.5-fold higher enzymatic activity of the latter system. Then, AlaDH-FDH was coupled to two different S-selective transaminases for the synthesis of vanillyl amine (10 mM) reaching up to 99 % conversion in 24 h in both cases. Finally, the multienzyme system was reused for at least 3 consecutive cycles when implemented in dialysis-assisted biotransformations.

Characterization of proteins from the 3N5M family reveals an operationally stable amine transaminase

Abstract

Amine transaminases (ATA) convert ketones into optically active amines and are used to prepare active pharmaceutical ingredients and building blocks. Novel ATA can be identified in protein databases due to the extensive knowledge of sequence-function relationships. However, predicting thermo- and operational stability from the amino acid sequence is a persisting challenge and a vital step towards identifying efficient ATA biocatalysts for industrial applications. In this study, we performed a database mining and characterized selected putative enzymes of the β-alanine:pyruvate transaminase cluster (3N5M) — a subfamily with so far only a few described members, whose tetrameric structure was suggested to positively affect operational stability. Four putative transaminases (TA-1: Bilophilia wadsworthia, TA-5: Halomonas elongata, TA-9: Burkholderia cepacia, and TA-10: Burkholderia multivorans) were obtained in a soluble form as tetramers in E. coli. During comparison of these tetrameric with known dimeric transaminases we found that indeed novel ATA with high operational stabilities can be identified in this protein subfamily, but we also found exceptions to the hypothesized correlation that a tetrameric assembly leads to increased stability. The discovered ATA from Burkholderia multivorans features a broad substrate specificity, including isopropylamine acceptance, is highly active (6 U/mg) in the conversion of 1-phenylethylamine with pyruvate and shows a thermostability of up to 70 °C under both, storage and operating conditions. In addition, 50% (v/v) of isopropanol or DMSO can be employed as co-solvents without a destabilizing effect on the enzyme during an incubation time of 16 h at 30 °C.

Key points

• Database mining identified a thermostable amine transaminase in the β-alanine:pyruvate transaminase subfamily.

• The tetrameric transaminase tolerates 50% DMSO and isopropanol under operating conditions at 30 °C.

• A tetrameric structure is not necessarily associated with a higher operational stability

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-12071-1.

The identification and application of a robust ω-transaminase with high tolerance of substrate and isopropylamine from a directed soil metagenome

The ω-transaminase-mediated asymmetric amination of a ketone substrate has gained significant attention for its immense potential to synthesize chiral amine pharmaceuticals and precursors. However, few of these have been authentically...

Biophysical characterization of the inactivation of E. coli transketolase by aqueous co-solvents

Transketolase (TK) has been previously engineered, using semi-rational directed evolution and substrate walking, to accept increasingly aliphatic, cyclic, and then aromatic substrates. This has ultimately led to the poor water solubility of new substrates, as a potential bottleneck to further exploitation of this enzyme in biocatalysis. Here we used a range of biophysical studies to characterise the response of both E. coli apo- and holo-TK activity and structure to a range of polar organic co-solvents: acetonitrile (AcCN), n-butanol (nBuOH), ethyl acetate (EtOAc), isopropanol (iPrOH), and tetrahydrofuran (THF). The mechanism of enzyme deactivation was found to be predominantly via solvent-induced local unfolding. Holo-TK is thermodynamically more stable than apo-TK and yet for four of the five co-solvents it retained less activity than apo-TK after exposure to organic solvents, indicating that solvent tolerance was not simply correlated to global conformational stability. The co-solvent concentrations required for complete enzyme inactivation was inversely proportional to co-solvent log(P), while the unfolding rate was directly proportional, indicating that the solvents interact with and partially unfold the enzyme through hydrophobic contacts. Small amounts of aggregate formed in some cases, but this was not sufficient to explain the enzyme inactivation. TK was found to be tolerant to 15% (v/v) iPrOH, 10% (v/v) AcCN, or 6% (v/v) nBuOH over 3 h. This work indicates that future attempts to engineer the enzyme to better tolerate co-solvents should focus on increasing the stability of the protein to local unfolding, particularly in and around the cofactor-binding loops.

Hydrogel-Based Enzyme and Cofactor Co-Immobilization for Efficient Continuous Transamination in a Microbioreactor

A microbioreactor was developed in which selected amine transaminase was immobilized together with the cofactor pyridoxal phosphate (PLP) to allow efficient continuous transamination. The enzyme and cofactor were retained in a porous copolymeric hydrogel matrix formed in a two-plate microreactor with an immobilization efficiency of over 97%. After 10 days of continuous operation, 92% of the initial productivity was retained and no leaching of PLP or enzyme from the hydrogel was observed. The microbioreactor with co-immobilized cofactor showed similar performance with and without the addition of exogenous PLP, suggesting that the addition of PLP is not required during the process. The space-time yield of the microbioreactor was 19.91 g L⁻¹ h⁻¹, while the highest achieved biocatalyst productivity was 5.4 mg mg_enzyme⁻¹ h⁻¹. The immobilized enzyme also showed better stability over a wider pH and temperature range than the free enzyme. Considering the time and cost efficiency of the immobilization process and the possibility of capacity expansion, such a system is of great potential for industrial application.

Computational Redesign of an ω-Transaminase from Pseudomonas jessenii for Asymmetric Synthesis of Enantiopure Bulky Amines

ω-Transaminases (ω-TA) are attractive biocatalysts for the production of chiral amines from prochiral ketones via asymmetric synthesis. However, the substrate scope of ω-TAs is usually limited due to steric hindrance at the active site pockets. We explored a protein engineering strategy using computational design to expand the substrate scope of an (S)-selective ω-TA from Pseudomonas jessenii (PjTA-R6) toward the production of bulky amines. PjTA-R6 is attractive for use in applied biocatalysis due to its thermostability, tolerance to organic solvents, and acceptance of high concentrations of isopropylamine as amino donor. PjTA-R6 showed no detectable activity for the synthesis of six bicyclic or bulky amines targeted in this study. Six small libraries composed of 7–18 variants each were separately designed via computational methods and tested in the laboratory for ketone to amine conversion. In each library, the vast majority of the variants displayed the desired activity, and of the 40 different designs, 38 produced the target amine in good yield with >99% enantiomeric excess. This shows that the substrate scope and enantioselectivity of PjTA mutants could be predicted in silico with high accuracy. The single mutant W58G showed the best performance in the synthesis of five structurally similar bulky amines containing the indan and tetralin moieties. The best variant for the other bulky amine, 1-phenylbutylamine, was the triple mutant W58M + F86L + R417L, indicating that Trp58 is a key residue in the large binding pocket for PjTA-R6 redesign. Crystal structures of the two best variants confirmed the computationally predicted structures. The results show that computational design can be an efficient approach to rapidly expand the substrate scope of ω-TAs to produce enantiopure bulky amines.

Efficient Amino Donor Recycling in Amination Reactions: Development of a New Alanine Dehydrogenase in Continuous Flow and Dialysis Membrane Reactors

Transaminases have arisen as one of the main biocatalysts for amine production but despite their many advantages, their stability is still a concern for widespread application. One of the reasons for their instability is the need to use an excess of the amino donor when trying to synthesise amines with unfavourable equilibria. To circumvent this, recycling systems for the amino donor, such as amino acid dehydrogenases or aldolases, have proved useful to push the equilibria while avoiding high amino donor concentrations. In this work, we report the use of a new alanine dehydrogenase from the halotolerant bacteria Halomonas elongata which exhibits excellent stability to different cosolvents, combined with the well characterised CbFDH as a recycling system of L-alanine for the amination of three model substrates with unfavourable equilibria. In a step forward, the amino donor recycling system has been co-immobilised and used in flow with success as well as re-used as a dialysis enclosed system for the amination of an aromatic aldehyde.

Stabilization of ω-transaminase from Pseudomonas fluorescens by immobilization techniques

Transaminases are a class of enzymes with promising applications for the preparation and resolution of a vast diversity of valued amines. Their poor operational stability has fueled many investigations on its stabilization due to their biotechnological relevance. In this work, we screened the stabilization of the tetrameric ω-transaminase from Pseudomonas fluorescens (PfωTA) through both carrier-bound and carrier-free immobilization techniques. The best heterogeneous biocatalyst was the PfωTA immobilized as cross-linked enzyme aggregates (PfωTA-CLEA) which resulted after studying different parameters as the precipitant, additives and glutaraldehyde concentrations. The best conditions for maximum recovered activity (29 %) and maximum thermostability at 60 ºC and 70 ºC (100 % and 71 % residual activity after 1 h, respectively) were achieved by enzyme precipitation with 90% acetone or ethanol, in presence of BSA (100 mg/mL) and employing glutaraldehyde (100 mM) as cross-linker. Studies on different conditions for PfωTA-CLEA preparation yielded a biocatalyst that exhibited 31 and 4.6 times enhanced thermal stability at 60 °C and 70 °C, respectively, compared to its soluble counterpart. The PfωTA-CLEA was successfully used in the bioamination of 4-hydroxybenzaldehyde to 4-hydroxybenzylamine. To the best of our knowledge, this is the first report describing a transaminase cross-linked enzyme aggregates as immobilization strategy to generate a biocatalyst with outstanding thermostability.

Robust ω-Transaminases by Computational Stabilization of the Subunit Interface

Transaminases are attractive catalysts for the production of enantiopure amines. However, the poor stability of these enzymes often limits their application in biocatalysis. Here, we used a framework for enzyme stability engineering by computational library design (FRESCO) to stabilize the homodimeric PLP fold type I ω-transaminase from Pseudomonas jessenii. A large number of surface-located point mutations and mutations predicted to stabilize the subunit interface were examined. Experimental screening revealed that 10 surface mutations out of 172 tested were indeed stabilizing (6% success), whereas testing 34 interface mutations gave 19 hits (56% success). Both the extent of stabilization and the spatial distribution of stabilizing mutations showed that the subunit interface was critical for stability. After mutations were combined, 2 very stable variants with 4 and 6 mutations were obtained, which in comparison to wild type (T _m ^app = 62 °C) displayed T _m ^app values of 80 and 85 °C, respectively. These two variants were also 5-fold more active at their optimum temperatures and tolerated high concentrations of isopropylamine and cosolvents. This allowed conversion of 100 mM acetophenone to (S)-1-phenylethylamine (>99% enantiomeric excess) with high yield (92%, in comparison to 24% with the wild-type transaminase). Crystal structures mostly confirmed the expected structural changes and revealed that the most stabilizing mutation, I154V, featured a rarely described stabilization mechanism: namely, removal of steric strain. The results show that computational interface redesign can be a rapid and powerful strategy for transaminase stabilization.

Tuning Enzyme Activity for Nonaqueous Solvents: Engineering an Enantioselective "Michaelase" for Catalysis in High Concentrations of Ethanol

Enzymes have evolved to function under aqueous conditions and may not exhibit features essential for biocatalytic application, such as the ability to function in high concentrations of an organic solvent. Consequently, protein engineering is often required to tune an enzyme for catalysis in non‐aqueous solvents. In this study, we have used a collection of nearly all single mutants of 4‐oxalocrotonate tautomerase, which promiscuously catalyzes synthetically useful Michael‐type additions of acetaldehyde to various nitroolefins, to investigate the effect of each mutation on the ability of this enzyme to retain its “Michaelase” activity in elevated concentrations of ethanol. Examination of this mutability landscape allowed the identification of two hotspot positions, Ser30 and Ala33, at which mutations are beneficial for catalysis in high ethanol concentrations. The “hotspot” position Ala33 was then randomized in a highly enantioselective, but ethanol‐sensitive 4‐OT variant (L8F/M45Y/F50A) to generate an improved enzyme variant (L8F/A33I/M45Y/F50A) that showed great ethanol stability and efficiently catalyzes the enantioselective addition of acetaldehyde to nitrostyrene in 40 % ethanol (permitting high substrate loading) to give the desired γ‐nitroaldehyde product in excellent isolated yield (89 %) and enantiopurity (ee=98 %). The presented work demonstrates the power of mutability‐landscape‐guided enzyme engineering for efficient biocatalysis in non‐aqueous solvents.

The biotechnological potential of marine bacteria in the novel lineage of Pseudomonas pertucinogena

Marine habitats represent a prolific source for molecules of biotechnological interest. In particular, marine bacteria have attracted attention and were successfully exploited for industrial applications. Recently, a group of Pseudomonas species isolated from extreme habitats or living in association with algae or sponges were clustered in the newly established Pseudomonas pertucinogena lineage. Remarkably for the predominantly terrestrial genus Pseudomonas, more than half (9) of currently 16 species within this lineage were isolated from marine or saline habitats. Unlike other Pseudomonas species, they seem to have in common a highly specialized metabolism. Furthermore, the marine members apparently possess the capacity to produce biomolecules of biotechnological interest (e.g. dehalogenases, polyester hydrolases, transaminases). Here, we summarize the knowledge regarding the enzymatic endowment of the marine Pseudomonas pertucinogena bacteria and report on a genomic analysis focusing on the presence of genes encoding esterases, dehalogenases, transaminases and secondary metabolites including carbon storage compounds.

Insight into the dimer dissociation process of the Chromobacterium violaceum (S)-selective amine transaminase

One of the main factors hampering the implementation in industry of transaminase-based processes for the synthesis of enantiopure amines is their often low storage and operational stability. Our still limited understanding of the inactivation processes undermining the stability of wild-type transaminases represents an obstacle to improving their stability through enzyme engineering. In this paper we present a model describing the inactivation process of the well-characterized (S)-selective amine transaminase from Chromobacterium violaceum. The cornerstone of the model, supported by structural, computational, mutagenesis and biophysical data, is the central role of the catalytic lysine as a conformational switch. Upon breakage of the lysine-PLP Schiff base, the strain associated with the catalytically active lysine conformation is dissipated in a slow relaxation process capable of triggering the known structural rearrangements occurring in the holo-to-apo transition and ultimately promoting dimer dissociation. Due to the occurrence in the literature of similar PLP-dependent inactivation models valid for other non-transaminase enzymes belonging to the same fold-class, the role of the catalytic lysine as conformational switch might extend beyond the transaminase enzyme group and offer new insight to drive future non-trivial engineering strategies.

Enhancing PLP-Binding Capacity of Class-III ω-Transaminase by Single Residue Substitution

Transaminases are pyridoxal-5′-phosphate (PLP) binding enzymes, broadly studied for their potential industrial application. Their affinity for PLP has been related to their performance and operational stability and while significant differences in PLP requirements have been reported, the environment of the PLP-binding pocket is highly conserved. In this study, thorough analysis of the residue interaction network of three homologous transaminases Halomonas elongata (HeTA), Chromobacterium violaceum (CvTA), and Pseudomonas fluorescens (PfTA) revealed a single residue difference in their PLP binding pocket: an asparagine at position 120 in HeTA. N120 is suitably positioned to interact with an aspartic acid known to protonate the PLP pyridinium nitrogen, while the equivalent position is occupied by a valine in the other two enzymes. Three different mutants were constructed (HeTA-N120V, CvTA-V124N, and PfTA-V129N) and functionally analyzed. Notably, in HeTA and CvTA, the asparagine variants, consistently exhibited a higher thermal stability and a significant decrease in the dissociation constant (K_d) for PLP, confirming the important role of N120 in PLP binding. Moreover, the reaction intermediate pyridoxamine-5′-phosphate (PMP) was released more slowly into the bulk, indicating that the mutation also enhances their PMP binding capacity. The crystal structure of PfTA, elucidated in this work, revealed a tetrameric arrangement with the PLP binding sites near the subunit interface. In this case, the V129N mutation had a negligible effect on PLP-binding, but it reduced its temperature stability possibly destabilizing the quaternary structure.

The identification and use of robust transaminases from a domestic drain metagenome

Transaminases remain one of the most promising biocatalysts for use in chiral amine synthesis, however their industrial implementation has been hampered by their general instability towards, for example, high amine donor concentrations and organic solvent content. Herein we describe the identification, cloning and screening of 29 novel transaminases from a household drain metagenome. The most promising enzymes were fully characterised and the effects of pH, temperature, amine donor concentration and co-solvent determined. Several enzymes demonstrated good substrate tolerance as well as an unprecedented robustness for a wild-type transaminase. One enzyme in particular readily accepted IPA as an amine donor giving the same conversion with 2-50 equivalents, as well as being tolerant to a number of co-solvents, and operational in up to 50% DMSO - a characteristic as yet unobserved in a wild-type transaminase. This work highlights the value of using metagenomics for biocatalyst discovery from niche environments, and here has led to the identification of one of the most robust native transaminases described to date, with respect to IPA and DMSO tolerance.

Chemo-Enzymatic Synthesis of Pyrazines and Pyrroles

Herein we report the biocatalytic synthesis of substituted pyrazines and pyrroles using a transaminase (ATA) to mediate the key amination step of the ketone precursors. Treatment of α-diketones with ATA-113 in the presence of a suitable amine donor yielded the corresponding α-amino ketones which underwent oxidative dimerization to the pyrazines. Selective amination of α-diketones in the presence of β-keto esters afforded substituted pyrroles in a biocatalytic equivalent of the classical Knorr pyrrole synthesis. Finally we have shown that pyrroles can be prepared by internal amine transfer catalyzed by a transaminase in which no external amine donor is required.

Investigating Pervaporation for In Situ Acetone Removal as Process Intensification Tool in ω-Transaminase Catalyzed Chiral Amine Synthesis

Hydrophobic pervaporation (PV), allowing for the separation of an organic component from an aqueous stream, was investigated for in situ acetone removal from a transamination reaction. A poly(dimethylsiloxane) membrane was applied in a coupled enzymatic process at 5 L scale. Among the four components, there was no loss of donor and product amines through PV which was highly desirable. However, in addition to removal of acetone, there was also an unwanted loss of acetophenone (substrate ketone) because of PV. The coupled enzyme-PV process resulted in 13% more product formation compared to the control process (where no PV was applied) after 9 h. Results from a qualitative simulation study (based on partial vapor pressures and a vapor-liquid equilibrium of the feed solution) indicated that PV might have an advantage over direct distillation strategy for selective removal of acetone from the reaction medium. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2731, 2019.

Structural dynamics of the transaminase active site revealed by the crystal structure of a co-factor free omega-transaminase from Vibrio fluvialis JS17

Omega (ω)-transaminase catalyzes the transfer of an amino group from a non-α position amino acid, or an amine compound with no carboxylic group, to an amino acceptor, and has been studied intensively because of its high potential utility in industry and pharmatheutics. The ω-transaminase from Vibrio fluvialis JS17 (Vfat) is an amine:pyruvate transaminase capable of the stereo-selective transamination of arylic chiral amines. This enzyme exhibits extraordinary enantio-selectivity, and has a rapid reaction rate for chiral amine substrates. In this study, we report the crystal structure of the apo form of Vfat. The overall structure of Vfat was typical of other class III aminotransferase exhibiting an N-terminal helical domain, a small domain, and a large domain. Interestingly, the two subunits of apo Vfat in the asymmetric unit had different structures. A comparison of the overall structure to other transaminases, revealed that the structures of the N-terminal helical domain and the large domain can be affected by cofactor occupancy, but the structural rearrangement in these regions can occur independently.

Characterization of the stability of Vibrio fluvialis JS17 amine transaminase

The amine transaminase from Vibrio fluvialis (Vf-ATA) is an attractive enzyme with applications within Biocatalysis for the preparation of chiral amines. Various catalytic properties of Vf-ATA have been investigated, but a biophysical characterization of its stability has been lacking. Today, the industrial application of Vf-ATA is limited by its low operational stability. In order to enhance the knowledge regarding the structural stability of ATAs, general characterizations of different ATAs are required. In this work, the stability of Vf-ATA was explored. First, the affinity between enzyme and pyridoxal-5'-phosphate (PLP) (K_D value of 7.9 μM) was determined. Addition of PLP to enzyme preparations significantly improved the enzyme thermal stability by preventing enzyme unfolding. With the aim to understand if this was due to the PLP phosphate group coordination into the phosphate group binding cup, the effect of phosphate buffer on the enzyme stability was compared to HEPES buffer. Low concentrations of phosphate buffer showed a positive effect on the enzyme initial activity, while higher phosphate buffer concentrations prevented cofactor dissociation. Additionally, the effects of various amine or ketone substrates on the enzyme stability were explored. All tested amines caused a concentration dependent enzyme inactivation, while the corresponding ketones showed no or stabilizing effects. The enzyme inactivation due to the presence of amine can be connected to the formation of PMP, which forms in the presence of amines in the absence of ketone. Since PMP is not covalently bound to the enzyme, it could readily leave the enzyme upon formation. Exploring the different stability effects of cofactor, substrates, additives and buffer system on ATAs seems to be important in order to understand and improve the general performance of ATAs.

Development of microreactors with surface-immobilized biocatalysts for continuous transamination

The industrial importance of optically pure compounds has thrown a spotlight on ω-transaminases that have shown a high potential for the synthesis of bioactive compounds with a chiral amine moiety. The implementation of biocatalysts in industrial processes relies strongly on fast and cost effective process development, including selection of a biocatalyst form and the strategy for its immobilization. Here, microscale reactors with selected surface-immobilized amine-transaminase (ATA) in various forms are described as platforms for high-throughput process development. Wild type ATA (ATA-wt) from a crude cell extract, as well as Escherichia coli cells intracellularly overexpressing the enzyme, were immobilized on the surfaces of meander microchannels of disposable plastics by means of reactor surface silanization and glutaraldehyde bonding. In addition, a silicon/glass microchannel reactor was used for immobilization of an ATA-wt, genetically engineered to contain a silica-binding module (SBM) at the N-terminus (N-SBM-ATA-wt), leading to immobilization on the non-modified inner microchannel surface. Microreactors with surface-immobilized biocatalysts were coupled with a quenching system and at-line HPLC analytics and evaluated based on continuous biotransformation, yielding acetophenone and l-alanine. E. coli cells and N-SBM-ATA-wt were efficiently immobilized and yielded a volumetric productivity of up to 14.42 g L^-1 h^-1, while ATA-wt small load resulted in two orders of magnitude lower productivity. The miniaturized reactors further enabled in-operando characterization of biocatalyst stability, crucial for successful transfer to a production scale.

Amine transaminases in chiral amines synthesis: recent advances and challenges

Transaminases, which catalyze the stereoselective transfer of an amino group between an amino donor and a prochiral ketone substrate, are interesting biocatalytic tools for the generation of optically pure chiral amines. In particular, amine transaminases (ATAs) are of industrial interest because they are capable of performing reductive amination reactions using a broad range of amine donors and acceptors. The most remarkable example of ATAs industrial application is in the production process of the anti-hyperglycaemic drug sitagliptin (Januvia^®/Janumet^®), which generated around 6 billion U.S. dollars of revenue to Merck in 2016. In this review, an update about the availability of microbial ATAs, discovered by both screening and database-mining approaches, or obtained by protein engineering of wild-type enzymes, will be provided. Current challenges in ATAs application and possible solutions will be also discussed. In particular, innovative biocatalytic process strategies aimed at the improvement of ATAs performances in chiral amines synthesis, e.g., using in situ product removal process strategies or flow reactors, will be presented. The progress in the industrial exploitation of these enzymes will be highlighted by selected examples of large-scale ATA-catalyzed processes.