Purification, characterization, and molecular cloning of a novel amine:pyruvate transaminase from Vibrio fluvialis JS17

Biotransamination of Furan-Based Aldehydes with Isopropylamine: Enzyme Screening and pH Influence

Furan-based amines are highly valuable compounds which can be directly obtained via reductive amination from easily accessible furfural, 5-(hydroxymethyl)furfural (HMF) and 2,5-diformylfuran (DFF). Herein the biocatalytic amination of these carbonyl derivatives is disclosed using amine transaminases (ATAs) and isopropylamine (IPA) as amine donors. Among the different biocatalysts tested, the ones from Chromobacterium violaceum (Cv-TA), Arthrobacter citreus (ArS-TA), and variants from Arthrobacter sp. (ArRmut11-TA) and Vibrio fluvialis (Vf-mut-TA), afforded high levels of product formation (>80 %) at 100-200 mM aldehyde concentration. The transformations were studied in terms of enzyme and IPA loading. The pH influence was found as a key factor and attributed to the imine/aldehyde equilibrium that can arise from the high reactivity of the carbonyl substrates with a nucleophilic amine such as IPA.

Co-Immobilization of a Multi-Enzyme Cascade: (S)-Selective Amine Transaminases, l-Amino Acid Oxidase and Catalase

An enzyme cascade was established previously consisting of a recycling system with an l-amino acid oxidase (hcLAAO4) and a catalase (hCAT) for different α-keto acid co-substrates of (S)-selective amine transaminases (ATAs) in kinetic resolutions of racemic amines. Only 1 mol % of the co-substrate was required and l-amino acids instead of α-keto acids could be applied. However, soluble enzymes cannot be reused easily. Immobilization of hcLAAO4, hCAT and the (S)-selective ATA from Vibrio fluvialis (ATA-Vfl) was addressed here. Immobilization of the enzymes together rather than on separate beads showed higher reaction rates most likely due to fast co-substrate channeling between ATA-Vfl and hcLAAO4 due to their close proximity. Co-immobilization allowed further reduction of the co-substrate amount to 0.1 mol % most likely due to a more efficient H2 O2 -removal caused by the stabilized hCAT and its proximity to hcLAAO4. Finally, the co-immobilized enzyme cascade was reused in 3 cycles of preparative kinetic resolutions to produce (R)-1-PEA with high enantiomeric purity (97.3 %ee). Further recycling was inefficient due to the instability of ATA-Vfl, while hcLAAO4 and hCAT revealed high stability. An engineered ATA-Vfl-8M was used in the co-immobilized enzyme cascade to produce (R)-1-(3-ethoxy-4-methoxyphenyl)-2-(methylsulfonyl)ethanamine, an apremilast-intermediate, with a 1,000 fold lower input of the co-substrate.

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.

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.

Recombinant L-Amino Acid Oxidase with broad substrate spectrum for Co-Substrate Recycling in (S)-Selective Transaminase-Catalyzed Kinetic Resolutions

Chiral and enantiopure amines can be produced by enantioselective transaminases via kinetic resolution of amine racemates. This transamination reaction requires stoichiometric amounts of co-substrate. A dual-enzyme recycling system overcomes this limitation: L-amino acid oxidases (LAAO) recycle the accumulating co-product of ( S )-selective transaminases in the kinetic resolution of racemic amines to produce pure ( R )-amines. However, availability of suitable LAAOs is limited. Here we use the heterologously produced, highly active fungal hcLAAO4 with broad substrate spectrum. H 2 O 2 as by-product of hcLAAO4 is detoxified by a catalase. The final system allows using sub-stoichiometric amounts of 1 mol% of the transaminase co-substrate as well as the initial application of L-amino acids instead of α-keto acids. With an optimized protocol, synthetic potential of this kinetic resolution cascade was proven at the preparative scale (>90 mg) by the synthesis of highly enantiomerically pure ( R )-methylbenzylamine (>99 %ee) at complete conversion (50 %).

Engineering Novel (R)-Selective Transaminase for Efficient Symmetric Synthesis of d-Alanine

d-Alanine belongs to nonessential amino acids that have diverse applications in the fields of food and health care. (R)-transaminase [(R)-TA]-catalyzed asymmetric amination of pyruvate is a feasible alternative for the synthesis of d-alanine, but low catalytic efficiency and thermostability limit enzymatic utilization. In this work, several potential (R)-TAs were discovered using NCBI database mining synchronously with enzymatic structure-function analysis, among which Capronia epimyces TA (CeTA) showed the highest activity for amination of pyruvate using (R)-α-methylbenzylamine as the donor. Furthermore, enzymatic residues surrounding a large catalysis pocket were subjected to saturation and combinatorial mutagenesis, and positive mutant F113T showed dramatic improvement in activity and thermostability. Molecular modeling indicated that the substitution of phenylalanine with threonine afforded alleviation of steric hindrance in the pocket and induced formation of additional hydrogen bonds with neighboring residues. Finally, using recombinant cells containing F113T as a biocatalyst, the conversion yield of amination of 100 mM pyruvate to d-alanine achieved up to 95.2%, which seemed to be the highest level in the literature regarding synthesis of d-alanine using TAs. The inherent characteristics rendered CeTA F113T a promising platform for efficient preparation of d-alanine operating with high productivity. IMPORTANCE d-Alanine is an important compound with many valuable applications. Its asymmetric synthesis employing (R)-ω-TA is considered an attractive choice. According to the stereoselectivity, ω-TAs have either (R)- or (S)-enantiopreference. There has been a variety of literature regarding screening, characterizing, and molecular modification of (S)-ω-TAs; in contrast, the research about (R)-ω-TA has lagged behind. In this work, we identify several (R)-ω-TAs and succeeded in creating mutant F113T, which showed not only better efficiency toward pyruvate but also higher thermostability compared with the original enzyme. The obtained original enzymes and positive mutants displayed important application value for pushing symmetric synthesis of d-alanine to a higher level.

High-Yield Synthesis of Enantiopure 1,2-Amino Alcohols from l-Phenylalanine via Linear and Divergent Enzymatic Cascades

Enantiomerically pure 1,2-amino alcohols are important compounds due to their biological activities and wide applications in chemical synthesis. In this work, we present two multienzyme pathways for the conversion of l-phenylalanine into either 2-phenylglycinol or phenylethanolamine in the enantiomerically pure form. Both pathways start with the two-pot sequential four-step conversion of l-phenylalanine into styrene via subsequent deamination, decarboxylation, enantioselective epoxidation, and enantioselective hydrolysis. For instance, after optimization, the multienzyme process could convert 507 mg of l-phenylalanine into (R)-1-phenyl-1,2-diol in an overall isolated yield of 75% and >99% ee. The opposite enantiomer, (S)-1-phenyl-1,2-diol, was also obtained in a 70% yield and 98–99% ee following the same approach. At this stage, two divergent routes were developed to convert the chiral diols into either 2-phenylglycinol or phenylethanolamine. The former route consisted of a one-pot concurrent interconnected two-step cascade in which the diol intermediate was oxidized to 2-hydroxy-acetophenone by an alcohol dehydrogenase and then aminated by a transaminase to give enantiomerically pure 2-phenylglycinol. Notably, the addition of an alanine dehydrogenase enabled the connection of the two steps and made the overall process redox-self-sufficient. Thus, (S)-phenylglycinol was isolated in an 81% yield and >99.4% ee starting from ca. 100 mg of the diol intermediate. The second route consisted of a one-pot concurrent two-step cascade in which the oxidative and reductive steps were not interconnected. In this case, the diol intermediate was oxidized to either (S)- or (R)-2-hydroxy-2-phenylacetaldehyde by an alcohol oxidase and then aminated by an amine dehydrogenase to give the enantiomerically pure phenylethanolamine. The addition of a formate dehydrogenase and sodium formate was required to provide the reducing equivalents for the reductive amination step. Thus, (R)-phenylethanolamine was isolated in a 92% yield and >99.9% ee starting from ca. 100 mg of the diol intermediate. In summary, l-phenylalanine was converted into enantiomerically pure 2-phenylglycinol and phenylethanolamine in overall yields of 61% and 69%, respectively. This work exemplifies how linear and divergent enzyme cascades can enable the synthesis of high-value chiral molecules such as amino alcohols from a renewable material such as l-phenylalanine with high atom economy and improved sustainability.

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...

Four-Step Pathway from Phenylpyruvate to Benzylamine, an Intermediate to the High-Energy Propellant CL-20

Benzylamine is a commodity chemical used in the synthesis of motion-sickness treatments and anticonvulsants, in dyeing textiles, and as a precursor to the high-energy propellant CL-20. Because chemical production generates toxic waste streams, biosynthetic alternatives have been explored, recently resulting in a functional nine-step pathway from central metabolism (phenylalanine) in E. coli. We report a novel four-step pathway for benzylamine production, which generates the product from cellular phenylpyruvate using enzymes from different sources: a mandelate synthase (Amycolatopsis orientalis), a mandelate oxidase (Streptomyces coelicolor), a benzoylformate decarboxylase (Pseudomonas putida), and an aminotransferase (Salicibacter pomeroyi). This pathway produces benzylamine at 24 mg/L in 15 h (4.5% yield) in cultures of unoptimized cells supplemented with phenylpyruvate. Because the yield is low, supplementation with pathway intermediates is used to troubleshoot the design. This identifies conversion inefficiencies in the mandelate synthase-mediated synthesis of (S)-mandelic acid, and subsequent genome mining identifies a new mandelate synthase (Streptomyces sp. 1114.5) with improved yield. Supplementation experiments also reveal native redirection of ambient phenylpyruvate away from the pathway to phenylalanine. Overall, this work illustrates how retrosynthetic design can dramatically reduce the number of enzymes in a pathway, potentially reducing its draw on cellular resources. However, it also shows that such benefits can be abrogated by inefficiencies of individual conversions. Addressing these barriers can provide an alternative approach to green production of benzylamine, eliminating upstream dependence on chlorination chemistry.

Redesign of (R)-Omega-Transaminase and Its Application for Synthesizing Amino Acids with Bulky Side Chain

ω-Transaminase (ω-TA) is an attractive biocatalyst for stereospecific preparation of amino acids and derivatives, but low catalytic efficiency and unfavorable substrate specificity hamper their industrial application. In this work, to obtain applicable (R)-ω-TA responsible for amination of α-keto acids substrates, the reactivities of eight previously synthesized ω-TAs toward pyruvate using (R)-α-methylbenzylamine ((R)-α-MBA) as amine donor were investigated, and Gibberella zeae TA (GzTA) with the highest (R)-TA activity and stereoselectivity was selected as starting scaffold for engineering. Site-directed mutagenesis around enzymatic active pocket and access tunnel identified three positive mutation sites, S214A, F113L, and V60A. Kinetic analysis synchronously with molecular docking revealed that these mutations afforded desirable alleviation of steric hindrance for pyruvate and α-MBA. Furthermore, the constructed single-, double-, and triple-mutant exhibited varying degrees of improved specificities toward bulkier α-keto acids. Using 2-oxo-2-phenylacetic acid (1d) as substrate, the conversion rate of triple-mutant F113L/V60A/S214A increased by 3.8-fold relative to that of wide-type GzTA. This study provided a practical engineering strategy for improving catalytic efficiency and substrate specificity of (R)-ω-TA. The obtained experience shed light on creating more industrial ω-TAs mutants that can accommodate structurally diverse substrates.

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.

The Effect of Visible Light on the Catalytic Activity of PLP-Dependent Enzymes

Pyridoxal 5'-phosphate (PLP)-dependent enzymes are a versatile class of biocatalysts and feature a variety of industrial applications. However, PLP is light sensitive and can cause inactivation of enzymes in certain light conditions. As most of the PLP-dependent enzymes are usually not handled in dark conditions, we evaluated the effect of visible light on the activity of PLP-dependent enzymes during production as well as transformation. We tested four amine transaminases, from Chromobacterium violaceum, Bacillus megaterium, Vibrio fluvialis and a variant from Arthrobacter species as well as two lysine decarboxylases, from Selenomonas ruminantium and the LDCc from Escherichia coli. It appeared that five of these six enzymes suffered from a significant decrease in activity by up to 90 % when handled in laboratory light conditions. Surprisingly, only the amine transaminase variant from Arthrobacter species appeared to be unaffected by light exposure and even showed an activation to 150 % relative activity over the course of 6 h regardless of the light conditions.

Structural and Functional Analysis of the Only Two Pyridoxal 5′-Phosphate-Dependent Fold Type IV Transaminases in Bacillus altitudinis W3

Aminotransferases are employed as industrial biocatalysts to produce chiral amines with high enantioselectivity and yield. BpTA-1 and BpTA-2 are the only two pyridoxal 5′-phosphate-dependent fold type IV transaminase enzymes in Bacillus altitudinis W3. Herein, we compared the structures and biochemical characteristics of BpTA-1 and BpTA-2 using bioinformatic analysis, circular dichroism spectroscopy, atomic force microscopy and other approaches. BpTA-1 and BpTA-2 are similar overall; both form homodimers and utilize a catalytic lysine. However, there are distinct differences in the substrate cofactor-binding pocket, molecular weight and the proportion of the secondary structure. Both enzymes have the same stereoselectivity but different enzymatic properties. BpTA-2 is more active under partial alkaline and ambient temperature conditions and BpTA-1 is more sensitive to pH and temperature. BpTA-2 as novel enzyme not only fills the building blocks of transaminase but also has broader industrial application potential for (R)-α-phenethylamines than BpTA-1. Structure-function relationships were explored to assess similarities and differences. The findings lay the foundation for modifying these enzymes via protein engineering to enhance their industrial application potential.

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.

Effects of pH and temperature on (S)-amine activity of transaminase from the cold-adapted bacterium Psychrobacter cryohalolentis

(7R,8S)-diaminopelargonic acid transaminase from the cold-adapted Gram-negative bacterium Psychrobacter cryohalolentis (Pcryo361) is able to react with unnatural substrates including (S)-( +)-1-phenylethylamine, aldehydes and α-diketones. Additionally, Pcryo361 is active at 0-50 °C and retains up to 10% of the maximum activity at 0 °C. Here, we report a detailed study on the stability and low temperature activity of Pcryo361. At the optimal pH for (S)-amine activity (pH 10.0), the enzyme was stable at 0-10 °C and no decrease in the enzyme activity was observed within 24 h in a slightly alkaline medium, pH 8.0, at 35 °C. Pcryo361 was solvent stable and was activated in 10% DMSO and DMFA at 35 °C. An analysis of the efficiency of catalysis of Pcryo361 at 35 °C and 10 °C showed that the specificity towards (S)-( +)-1-phenylethylamine dropped at 10 °C; however, the specificity towards 2,3-butanedione remained unchanged. Inhibition analysis showed that Pcryo361 activity was not inhibited by acetophenone but inhibited by amines (products of aldehyde amination). The observed pH stability and low temperature activity of Pcryo361 with activated keto substrates are attractive features in the field of development of stereoselective amination at low temperatures.

Genome mining and characterisation of a novel transaminase with remote stereoselectivity

Microbial enzymes from pristine niches can potentially deliver disruptive opportunities in synthetic routes to Active Pharmaceutical Ingredients and intermediates in the Pharmaceutical Industry. Advances in green chemistry technologies and the importance of stereochemical control, further underscores the application of enzyme-based solutions in chemical synthesis. The rich tapestry of microbial diversity in the oceanic ecosystem encodes a capacity for novel biotransformations arising from the chemical complexity of this largely unexplored bioactive reservoir. Here we report a novel ω-transaminase discovered in a marine sponge Pseudovibrio sp. isolate. Remote stereoselection using a transaminase has been demonstrated for the first time using this novel protein. Application to the resolution of an intermediate in the synthesis of sertraline highlights the synthetic potential of this novel biocatalyst discovered through genomic mining. Integrated chemico-genomics revealed a unique substrate profile, while molecular modelling provided structural insights into this ‘first in class’ selectivity at a remote chiral centre.

Characterization of a novel thermostable (S)-amine-transaminase from an Antarctic moderately-thermophilic bacterium Albidovulum sp. SLM16

Amine-transaminases (ATAs) are enzymes that catalyze the reversible transfer of an amino group between primary amines and carbonyl compounds. They have been widely studied in the last decades for their application in stereoselective synthesis of chiral amines, which are one of the most valuable building blocks in pharmaceuticals manufacturing. Their excellent enantioselectivity, use of low-cost substrates and no need for external cofactors has turned these enzymes into a promising alternative to the chemical synthesis of chiral amines. Nevertheless, its application at industrial scale remains limited mainly because most of the available ATAs are scarcely tolerant to harsh reaction conditions such as high temperatures and presence of organic solvents. In this work, a novel (S)-ATA was discovered in a thermophilic bacterium, Albidovulum sp. SLM16, isolated from a geothermal Antarctic environmental sample, more specifically from a shoreline fumarole in Deception Island. The transaminase-coding gene was identified in the genome of the microorganism, cloned and overexpressed in Escherichia coli for biochemical characterization. The activity of the recombinant ATA was optimal at 65 °C and pH 9.5. Molecular mass estimates suggest a 75 kDa homodimeric structure. The enzyme turned out to be highly thermostable, maintaining 80% of its specific activity after 5 days of incubation at 50 °C. These results indicate that ATA_SLM16 is an excellent candidate for potential applications in biocatalytic synthesis. To the best of our knowledge, this would be the first report of the characterization of a thermostable (S)-ATA discovered by means of in vivo screening of thermophilic microorganisms.

Biochemical properties of a Pseudomonas aminotransferase involved in caprolactam metabolism

The biodegradation of the nylon-6 precursor caprolactam by a strain of Pseudomonas jessenii proceeds via ATP-dependent hydrolytic ring opening to 6-aminohexanoate. This non-natural ω-amino acid is converted to 6-oxohexanoic acid by an aminotransferase (PjAT) belonging to the fold type I pyridoxal 5'-phosphate (PLP) enzymes. To understand the structural basis of 6-aminohexanoatate conversion, we solved different crystal structures and determined the substrate scope with a range of aliphatic and aromatic amines. Comparison with the homologous aminotransferases from Chromobacterium violaceum (CvAT) and Vibrio fluvialis (VfAT) showed that the PjAT enzyme has the lowest K_M values (highest affinity) and highest specificity constant (k_cat /K_M ) with the caprolactam degradation intermediates 6-aminohexanoate and 6-oxohexanoic acid, in accordance with its proposed in vivo function. Five distinct three-dimensional structures of PjAT were solved by protein crystallography. The structure of the aldimine intermediate formed from 6-aminohexanoate and the PLP cofactor revealed the presence of a narrow hydrophobic substrate-binding tunnel leading to the cofactor and covered by a flexible arginine, which explains the high activity and selectivity of the PjAT with 6-aminohexanoate. The results suggest that the degradation pathway for caprolactam has recruited an aminotransferase that is well adapted to 6-aminohexanoate degradation. DATABASE: The atomic coordinates and structure factors P. jessenii 6-aminohexanoate aminotransferase have been deposited in the PDB as entries 6G4B (E∙succinate complex), 6G4C (E∙phosphate complex), 6G4D (E∙PLP complex), 6G4E (E∙PLP-6-aminohexanoate intermediate), and 6G4F (E∙PMP complex).

Asymmetric synthesis of l-phosphinothricin using thermostable alpha-transaminase mined from Citrobacter koseri

α-Transaminase (α-TA) responsible for catalyzing the reversible transfer of amino groups between amine donors and amine acceptors, is applicable to enzymatic route for asymmetric synthesis of herbicide l-phosphinothricin (l-PPT). In the search for α-TAs with better catalysis performance, three α-TAs were discovered by genome mining approach using a known sequence encoding Escherichia coli tyrosine TA (TyrB) as probe. Through detailed comparison of their expression amount, activities and characteristics, Citrobacter koseri TA (CkTA) exhibited better activity and thermostability, which retain 65.9% of initial activity after incubation at 57 °C for 4 h. The K_m and k_cat/K_m values of CkTA were 36.75 mM and 34.29 mM^-1 min^-1, respectively. In addition, recombinant CkTA cells were immobilized onto Celite 545 using tris(hydroxymethyl)phosphine as crosslinker. During five repetitive asymmetric synthesis of l-PPT from 20 g/L prostereogenic ketone using l-Glu as amine donor, all the yields of l-PPT reached up to 91.2% (＞99% ee). These characteristics made CkTA a valuable addition to the currently scarce α-TA library for stereospecific synthesis of l-PPT.

Designing of a Cofactor Self-Sufficient Whole-Cell Biocatalyst System for Production of 1,2-Amino Alcohols from Epoxides

Optically pure 1,2-amino alcohols are highly valuable products as intermediates for chiral pharmaceutical products. Here we designed an environmentally friendly non-natural biocatalytic cascade for efficient synthesis of 1,2-amino alcohols from cheaper epoxides. A redesignated ω-transaminase PAKω-TA was tested and showed good bioactivity at a lower pH than other reported transaminases. The cascade was efficiently constructed as a single one-pot E. coli recombinant, by coupling SpEH (epoxide hydrolase), MnADH (alcohol dehydrogenase), and PAKω-TA. Furthermore, RBS regulation strategy was used to overcome the rate limiting step by increasing expression of MnADH. For cofactor regeneration and amino donor source, an interesting point was involved as that a cofactor self-sufficient system was designed by expression of GluDH. It established a "bridge" between the cofactor and the cosubstrate, such that the cofactor self-sufficient system could release cofactor (NADP⁺) and cosubstrate (l-Glutamine) regenerated simultaneously. The recombinant E. coli BL21 (SGMP) with cofactor self-sufficient whole-cell cascade biocatalysis showed high ee value (>99%) and high yield, with 99.6% conversion of epoxide ( S)-1a to 1,2-amino alcohol ( S)-1d in 10 h. It further converted ( S)-2a-5a to ( S)-2d-5d with varying conversion rates ranging between 65-96.4%. This study first provides one-step synthesis of optically pure 1,2-amino alcohols from ( S)-epoxides employing a synthetic redox-self-sufficient cascade.

Highly efficient production of chiral amines in batch and continuous flow by immobilized ω-transaminases on controlled porosity glass metal-ion affinity carrier

In this study, two stereocomplementary ω-transaminases from Arthrobacter sp. (AsR-ωTA) and Chromobacterium violaceum (Cv-ωTA) were immobilized via iron cation affinity binding onto polymer-coated controlled porosity glass beads (EziG™). The immobilization procedure was studied with different types of carrier materials and immobilization buffers of varying compositions, concentrations, pHs and cofactor (PLP) concentrations. Notably, concentrations of PLP above 0.1 mM were correlated with a dramatic decrease of the immobilization yield. The highest catalytic activity, along with quantitative immobilization, was obtained in MOPS buffer (100 mM, pH 8.0, PLP 0.1 mM, incubation time 2 h). Leaching of the immobilized enzyme was not observed within 3 days of incubation. EziG-immobilized AsR-ωTA and Cv-ωTA retained elevated activity when tested for the kinetic resolution of rac-α-methylbenzylamine (rac-α-MBA) in single batch experiments. Recycling studies demonstrated that immobilized EziG3-AsR-ωTA could be recycled for at least 16 consecutive cycles (15 min per cycle) and always affording quantitative conversion (TON ca. 14,400). Finally, the kinetic resolution of rac-α-MBA with EziG3-AsR-ωTA was tested in a continuous flow packed-bed reactor (157 μL reactor volume), which produced more than 5 g of (S)-α-MBA (>49% conversion, >99% ee) in 96 h with no detectable loss of catalytic activity. The calculated TON was more than 110,000 along with a space-time yield of 335 g L-1 h-1.

Thermostable Branched-Chain Amino Acid Transaminases From the Archaea Geoglobus acetivorans and Archaeoglobus fulgidus: Biochemical and Structural Characterization

Two new thermophilic branched chain amino acid transaminases have been identified within the genomes of different hyper-thermophilic archaea, Geoglobus acetivorans, and Archaeoglobus fulgidus. These enzymes belong to the class IV of transaminases as defined by their structural fold. The enzymes have been cloned and over-expressed in Escherichia coli and the recombinant enzymes have been characterized both biochemically and structurally. Both enzymes showed high thermostability with optimal temperature for activity at 80 and 85°C, respectively. They retain good activity after exposure to 50% of the organic solvents, ethanol, methanol, DMSO and acetonitrile. The enzymes show a low activity to (R)-methylbenzylamine but no activity to (S)-methylbenzylamine. Both enzymes have been crystallized and their structures solved in the internal aldimine form, to 1.9 Å resolution for the Geoglobus enzyme and 2.0 Å for the Archaeoglobus enzyme. Also the Geoglobus enzyme structure has been determined in complex with the amino acceptor α-ketoglutarate and the Archaeoglobus enzyme in complex with the inhibitor gabaculine. These two complexes have helped to determine the conformation of the enzymes during enzymatic turnover and have increased understanding of their substrate specificity. A comparison has been made with another (R) selective class IV transaminase from the fungus Nectria haematococca which was previously studied in complex with gabaculine. The subtle structural differences between these enzymes has provided insight regarding their different substrate specificities.

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.

Characterization of a Putrescine Transaminase From Pseudomonas putida and its Application to the Synthesis of Benzylamine Derivatives

The reductive amination of prochiral ketones using biocatalysts has been of great interest to the pharmaceutical industry in the last decade for integrating novel strategies in the production of chiral building blocks with the intent of minimizing impact on the environment. Amongst the enzymes able to catalyze the direct amination of prochiral ketones, pyridoxal 5'-phosphate (PLP) dependent ω-transaminases have shown great promise as versatile industrial biocatalysts with high selectivity, regioselectivity, and broad substrate scope. Herein the biochemical characterization of a putrescine transaminase from Pseudomonas putida (Pp-SpuC) was performed, which showed an optimum pH and temperature of 8.0 and 60°C, respectively. To gain further structural insight of this enzyme, we crystallized the protein in the apo form and determined the structure to 2.1 Å resolution which revealed a dimer that adopts a class I transaminase fold comparable to other class III transaminases. Furthermore we exploited its dual substrate recognition for biogenic diamines (i.e., cadaverine) and readily available monoamines (i.e., isopropylamine) for the synthesis of benzylamine derivatives with excellent product conversions and extremely broad substrate tolerance.

Single point mutations reveal amino acid residues important for Chromobacterium violaceum transaminase activity in the production of unnatural amino acids

Unnatural amino acids (UAAs) are chiral amines with high application potential in drug discovery and synthesis of other valuable chemicals. Biocatalysis offers the possibility to synthesise novel optically pure UAAs with different physical and chemical properties. While the biocatalytic potential of transaminases in the synthesis of UAAs has been demonstrated, there is still a need to improve the activity with non-native substrates and to understand which amino acids residues are important for activity with these UAAs. Using a rational design approach, six variants of Chromobacterium violaceum DSM30191 transaminase (CV_TA) carrying a single and one variant carrying two substitutions were generated. Among the variants with a single substitution, CV_Y168F showed a 2 to 2.6-fold increased affinity for 2-oxooctanoic acid (2-OOA) and 3-oxobutyric acid (3-OBA) methyl ester used to synthesise an α- and β-UAA. Analysis of the first half of the transaminase reaction showed no change in the activity with the donor (S)-1-phenylethylamine. The combination of W60C and Y168F substitutions improved the CV_TA affinity for 2-OOA 10-fold compared to the wild type. Other substitutions showed no change, or reduced activity with the tested substrates. Our findings provide structural information on CV_TA and demonstrate the potential of rational design for biosynthesis of UAAs.

Characterization of a novel ω-transaminase from a Triassic salt mine metagenome

Chiral amines are valuable building blocks for the pharmaceutical industry, and are increasingly synthesized by transaminase-mediated (TAm) synthesis. Currently available TAms, primarily isolated from the genomes of cultured mesophilic bacteria, often suffer from a number of drawbacks, including poor substrate range and an inability to tolerate the harsh conditions often demanded by industrial processes. These characteristics have, in part, driven the search for novel biocatalysts from both metagenomic sources and extreme environments. Herein, we report the isolation and characterization of an ω-TAm from a metagenome of a Triassic salt mine in Kilroot, N. Ireland, an extremely hypersaline environment formed circa 220-250 mya. The gene sequence was identified based on homology with existing bacterial TAms, synthesized within a pET28a(+) plasmid and expressed in E. coli BL21 DE3 cells. The resultant 49 kDa protein accepted (S)-methylbenzylamine (MBA) as amino donor and had a specific activity of 0.54 U/mg using α-ketoglutarate (ΑKG) as substrate. Molecular modeling and substrate docking indicated the presence of key residues, conserved in a number of other TAms. Despite the hypersaline environment from which it was isolated, the enzyme displayed low halotolerance, highlighting that not all biocatalysts will demonstrate the extreme characteristics associated with their source environment. This study does however reinforce the viability of mining metagenomic datasets as a means of discovering novel and functional biocatalysts, and adds to a currently scant list of such examples in the field of TAms.

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.

Metagenomic discovery of a novel transaminase for valorization of monoaromatic compounds

The profitability of next-generation biorefineries is acutely contingent on the discovery and utilization of biocatalysts that can valorize lignin. To this end, the metabolic catalogues of diverse microbiota have been mined previously using functional metagenomics in order to identify biocatalysts that can selectively degrade lignin into monoaromatic compounds. Herein, we have further improved the valorization factor of biorefining by deploying functional metagenomics toward the identification of a novel transaminase that can selectively functionalize lignin-derived monoaromatics to produce value-added feedstocks for pharmaceutical synthesis. We implemented a high-throughput colorimetric assay using o-xylylenediamine as the amino donor and successfully identified a transaminase that utilizes the canonical cofactor, pyridoxal 5'-phosphate, to aminate as many as 14 monoaromatic aldehydes and ketones. We subsequently identified the optimal conditions for enzyme activity towards the most favoured amino acceptor, benzaldehyde, including temperature, pH and choice of co-solvent. We also evaluated the specificity of the enzyme towards a variety of amino donors, as well as the optimal concentration of the most favoured amino donor. Significantly, the novel enzyme is markedly smaller than typical transaminases, and it is stably expressed in E. coli without any modifications to its amino acid sequence. Finally, we developed and implemented a computational methodology to assess the activity of the novel transaminase. The methodology is generalizable for assessing any transaminase and facilitates in silico screening of enzyme-substrate combinations in order to develop efficient biocatalytic routes to value-added amines. The computational pipeline is an ideal complement to metagenomics and opens new possibilities for biocatalyst discovery.

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.

Characterization of the caprolactam degradation pathway in Pseudomonas jessenii using mass spectrometry-based proteomics

Some bacterial cultures are capable of growth on caprolactam as sole carbon and nitrogen source, but the enzymes of the catabolic pathway have not been described. We isolated a caprolactam-degrading strain of Pseudomonas jessenii from soil and identified proteins and genes putatively involved in caprolactam metabolism using quantitative mass spectrometry-based proteomics. This led to the discovery of a caprolactamase and an aminotransferase that are involved in the initial steps of caprolactam conversion. Additionally, various proteins were identified that likely are involved in later steps of the pathway. The caprolactamase consists of two subunits and demonstrated high sequence identity to the 5-oxoprolinases. Escherichia coli cells expressing this caprolactamase did not convert 5-oxoproline but were able to hydrolyze caprolactam to form 6-aminocaproic acid in an ATP-dependent manner. Characterization of the aminotransferase revealed that the enzyme deaminates 6-aminocaproic acid to produce 6-oxohexanoate with pyruvate as amino acceptor. The amino acid sequence of the aminotransferase showed high similarity to subgroup II ω-aminotransferases of the PLP-fold type I proteins. Finally, analyses of the genome sequence revealed the presence of a caprolactam catabolism gene cluster comprising a set of genes involved in the conversion of caprolactam to adipate.

Biocatalytic potential of Streptomyces spp. isolates from rhizosphere of plants and mycorrhizosphere of fungi

Biocatalytic potential of Streptomyces strains isolated from the rhizosphere of plants and from mycorrhizosphere of fungi has been investigated. A total of 118 Streptomyces isolates were selected and functionally screened for 10 different biotechnologically important enzymatic activities: hydrolase (cellulase, cutinase, gelatinase, lipase, protease, polyhydroxyalkanoate (PHA) depolymerase), phenol oxidase and peroxidase (laccase, tyrosinase, and lignin peroxidase), and aminotransferase. Out of 118 tested Streptomyces spp., 90% showed at least one enzymatic activity. The most abundant were enzymes involved in the biomass degradation, as the production of cutinase, cellulase, and lignin peroxidase were detected in 31%, 40%, and 48% of the isolates, respectively. The improved specific activities of lipase (isolates BV315 and BV100) and tyrosinase (isolates BV87 and BV88) were shown in comparison with the industrially relevant activities of Pseudomonas strains. Plant rhizosphere soils were more prolific source of Streptomyces strains with biocatalytic potential in comparison with mycorrhizosphere soils. Overall, 284 enzyme activities among 118 Streptomyces isolates have been detected. This is the first comprehensive screening of Streptomyces isolates from rhizosphere and mycorrhizosphere soils for novel biocatalysts, showing that specific environmental habitats, such as rhizosphere soils, are "treasure troves" of Streptomyces with biocatalytic potential.

The ω-transaminase engineering database (oTAED): A navigation tool in protein sequence and structure space

The ω-Transaminase Engineering Database (oTAED) was established as a publicly accessible resource on sequences and structures of the biotechnologically relevant ω-transaminases (ω-TAs) from Fold types I and IV. The oTAED integrates sequence and structure data, provides a classification based on fold type and sequence similarity, and applies a standard numbering scheme to identify equivalent positions in homologous proteins. The oTAED includes 67 210 proteins (114 655 sequences) which are divided into 169 homologous families based on global sequence similarity. The 44 and 39 highly conserved positions which were identified in Fold type I and IV, respectively, include the known catalytic residues and a large fraction of glycines and prolines in loop regions, which might have a role in protein folding and stability. However, for most of the conserved positions the function is still unknown. Literature information on positions that mediate substrate specificity and stereoselectivity was systematically examined. The standard numbering schemes revealed that many positions which have been described in different enzymes are structurally equivalent. For some positions, multiple functional roles have been suggested based on experimental data in different enzymes. The proposed standard numbering schemes for Fold type I and IV ω-TAs assist with analysis of literature data, facilitate annotation of ω-TAs, support prediction of promising mutation sites, and enable navigation in ω-TA sequence space. Thus, it is a useful tool for enzyme engineering and the selection of novel ω-TA candidates with desired biochemical properties.