Supported liquid membrane as a novel tool for driving the equilibrium of ω-transaminase catalyzed asymmetric synthesis

Crystallization-Assisted Asymmetric Synthesis of Enantiopure Amines Using Membrane-Immobilized Transaminase

The production of active pharmaceutical ingredients (APIs) requires enantiopure chiral amines, for which greener synthesis processes are needed. Transaminases (TAs) are enzymes that catalyze the enantioselective production of chiral amines from prochiral ketones through transamination under mild conditions. Yet, industrial applications of biocatalytic transamination remain currently hindered by the limited stability of soluble enzymes and by the unfavorable thermodynamic equilibrium of targeted asymmetric reactions. Enzyme immobilization can be applied to address stability, recoverability, and reusability issues. In the perspective of process intensification, we chose to immobilize TAs on polymeric (polypropylene) membranes. In the asymmetric synthesis of (R)-2-fluoro-α-methylbenzylamine ((R)-FMBA), such membrane-immobilized TAs exhibited superior specific activity and stability compared with soluble TAs; they also outperformed TAs immobilized on resins. The reaction yield remained, however, limited by thermodynamics. To further enhance the synthesis yield, the reaction was coupled with the in situ crystallization of (R)-FMBA with 3,3-diphenylpropionic acid (DPPA). By doing so, the theoretical equilibrium conversion was pushed from ∼44% to ∼83%. In fact, a 72% overall recovery yield of crystallized (R)-FMBA was demonstrated. The enantioselectivity of the reaction mixture was preserved. Importantly, purification was greatly facilitated since the target enantiopure amine was readily recovered as high-purity (R)-FMBA:DPPA crystals. The biocatalytic membranes were found to be fully reusable, performing successive high-yield asymmetric syntheses with only minor deactivation. Overall, the crystallization-assisted strategy proposed herein offers a greener path for the biocatalytic production of valuable chiral targets.

Enhanced catalytic activity of recombinant transaminase by molecular modification to improve L-phosphinothricin production

Transaminases catalyze the transfer of an amino group from a donor to a keto group of an acceptor substrate and are applicable to the asymmetric synthesis of herbicide L-phosphinothricin (L-PPT). Here, the important residue sites (C390, I22, V52, R141, Y138 and D239) of transaminase from Salmonella enterica (SeTA) were modified at the adjacency of the substrate-binding pocket to improve the enzyme activity. Among the constructed mutant library, the SeTA-Y138F mutant displayed higher activity than the wild-type enzyme. Compared to the wild-type, SeTA-Y138F showed improved catalytic efficiency with a 4.36-fold increase. The K_m and k_cat of SeTA -Y138F toward 4-(hydroxy(methyl) phosphoryl)-2-oxobutanoic acid (PPO) were 26.39 mM and 34.28 s^-1, respectively. Subsequently, the three-enzyme co-expression system of E. coli BL21 (DE3)/pACYCDuet-SeTA-Y138F/pETDuet-AlaDH-BsGDH was developed by combining a alanine dehydrogenase (AlaDH) to recycle the byproduct of amino donor, a glucose dehydrogenase (BsGDH) for cofactor recycling. Under the optimized conditions, an excellent L-PPT yield of 90.8% was achieved by the whole-cell biotransformation with 500 mM PPO. It exhibited the tri-enzymatic coupling system was potential for effective production of target L-PPT.

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.

Biocatalytic Reduction Reactions from a Chemist's Perspective

Reductions play a key role in organic synthesis, producing chiral products with new functionalities. Enzymes can catalyse such reactions with exquisite stereo-, regio- and chemoselectivity, leading the way to alternative shorter classical synthetic routes towards not only high-added-value compounds but also bulk chemicals. In this review we describe the synthetic state-of-the-art and potential of enzymes that catalyse reductions, ranging from carbonyl, enone and aromatic reductions to reductive aminations.

ω-Transaminase-Mediated Asymmetric Synthesis of (S)-1-(4-Trifluoromethylphenyl)Ethylamine

The pivotal role played by ω-transaminases (ω-TAs) in the synthesis of chiral amines used as building blocks for drugs and pharmaceuticals is widely recognized. However, chiral bulky amines are challenging to produce. Herein, a ω-TA (TR8) from a marine bacterium was used to synthesize a fluorine chiral amine from a bulky ketone. An analysis of the reaction conditions for process development showed that isopropylamine concentrations above 75 mM had an inhibitory effect on the enzyme. Five different organic solvents were investigated as co-solvents for the ketone (the amine acceptor), among which 25–30% (v/v) dimethyl sulfoxide (DMSO) produced the highest enzyme activity. The reaction reached equilibrium after 18 h at 30% of conversion. An in situ product removal (ISPR) approach using an aqueous organic two-phase system was tested to mitigate product inhibition. However, the enzyme activity initially decreased because the ketone substrate preferentially partitioned into the organic phase, n-hexadecane. Consequently, DMSO was added to the system to increase substrate mass transfer without affecting the ability of the organic phase to prevent inhibition of the enzyme activity by the product. Thus, the enzyme reaction was maintained, and the product amount was increased for a 62 h reaction time. The investigated ω-TA can be used in the bioconversion of bulky ketones to chiral amines for future bioprocess applications.

Asymmetric synthesis of primary amines catalyzed by thermotolerant fungal reductive aminases

Chiral primary amines are important intermediates in the synthesis of pharmaceutical compounds. Fungal reductive aminases (RedAms) are NADPH-dependent dehydrogenases that catalyse reductive amination of a range of ketones with short-chain primary amines supplied in an equimolar ratio to give corresponding secondary amines. Herein we describe structural and biochemical characterisation as well as synthetic applications of two RedAms from Neosartorya spp. (NfRedAm and NfisRedAm) that display a distinctive activity amongst fungal RedAms, namely a superior ability to use ammonia as the amine partner. Using these enzymes, we demonstrate the synthesis of a broad range of primary amines, with conversions up to >97% and excellent enantiomeric excess. Temperature dependent studies showed that these homologues also possess greater thermal stability compared to other enzymes within this family. Their synthetic applicability is further demonstrated by the production of several primary and secondary amines with turnover numbers (TN) up to 14 000 as well as continous flow reactions, obtaining chiral amines such as (R)-2-aminohexane in space time yields up to 8.1 g L⁻¹ h⁻¹. The remarkable features of NfRedAm and NfisRedAm highlight their potential for wider synthetic application as well as expanding the biocatalytic toolbox available for chiral amine synthesis.

Fungal reductive aminases as effective biocatalysts for the preparation of chiral primary amines.

Concept Study for an Integrated Reactor-Crystallizer Process for the Continuous Biocatalytic Synthesis of (S)-1-(3-Methoxyphenyl)ethylamine

An integrated biocatalysis-crystallization concept was developed for the continuous amine transaminase-catalyzed synthesis of (S)-1-(3-methoxyphenyl)ethylamine, which is a valuable intermediate for the synthesis of rivastigmine, a highly potent drug for the treatment of early stage Alzheimer’s disease. The three-part vessel system developed for this purpose consists of a membrane reactor for the continuous synthesis of the product amine, a saturator vessel for the continuous supply of the amine donor isopropylammonium and the precipitating reagent 3,3-diphenylpropionate and a crystallizer in which the product amine can continuously precipitate as (S)-1-(3-methoxyphenyl)ethylammonium-3,3-diphenylpropionate.

The challenge of using isopropylamine as an amine donor in transaminase catalysed reactions

Amine transaminases (ATAs) propose an appealing alternative to transition metal catalysts as they can provide chiral amines of high purity from pro-chiral compounds by asymmetric synthesis. Industrial interest on ATAs arises from the fact that chiral amines are present in a wide spectrum of pharmaceutical and other high value-added chiral compounds and building blocks. Despite their potential as useful synthetic tools, several drawbacks such as challenges associated with the thermodynamic equilibrium can still impede their utilization. Several methods have been developed to displace the equilibrium, such as the use of alanine as an amine donor and the subsequent removal of pyruvate with a two-enzyme system, or the use of o-xylylene diamine. To date, the preferred amine donor remains isopropylamine (IPA), as the produced acetone can be removed easily under low pressure or slight heating, without complicating the process with other enzymes. Despite its small size, IPA is not widely accepted from wild-type ATAs, and this fact compromises its wide applicability. Herein, we index the reported biocatalytic aminations with IPA, comparing the sequences, while we discuss significant parameters of the process, such as the effect of temperature and pH, as well as the protein engineering and process development advances in the field. This information is expected to provide an insight for potential designs of tailor-made ATAs and IPA processes.

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.

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.

Development of in situ product removal strategies in biocatalysis applying scaled-down unit operations

An experimental platform based on scaled-down unit operations combined in a plug-and-play manner enables easy and highly flexible testing of advanced biocatalytic process options such as in situ product removal (ISPR) process strategies. In such a platform, it is possible to compartmentalize different process steps while operating it as a combined system, giving the possibility to test and characterize the performance of novel process concepts and biocatalysts with minimal influence of inhibitory products. Here the capabilities of performing process development by applying scaled-down unit operations are highlighted through a case study investigating the asymmetric synthesis of 1-methyl-3-phenylpropylamine (MPPA) using ω-transaminase, an enzyme in the sub-family of amino transferases (ATAs). An on-line HPLC system was applied to avoid manual sample handling and to semi-automatically characterize ω-transaminases in a scaled-down packed-bed reactor (PBR) module, showing MPPA as a strong inhibitor. To overcome the inhibition, a two-step liquid-liquid extraction (LLE) ISPR concept was tested using scaled-down unit operations combined in a plug-and-play manner. Through the tested ISPR concept, it was possible to continuously feed the main substrate benzylacetone (BA) and extract the main product MPPA throughout the reaction, thereby overcoming the challenges of low substrate solubility and product inhibition. The tested ISPR concept achieved a product concentration of 26.5 g_MPPA  · L^-1 , a purity up to 70% g_MPPA  · g_tot^-1 and a recovery in the range of 80% mol · mol^-1 of MPPA in 20 h, with the possibility to increase the concentration, purity, and recovery further. Biotechnol. Bioeng. 2017;114: 600-609. © 2016 Wiley Periodicals, Inc.

Production of chiral compounds using immobilized cells as a source of biocatalysts

The importance of chiral compounds in all fields of technology and life sciences is shown. Small chiral molecules are mainly used as building blocks in the synthesis of more complex and functionalized compounds. Nature creates and imposes stereoselectivity by means of enzymes, which are highly efficient biocatalysts. The use of whole cells as a biocatalyst source is a promising strategy for avoiding some drawbacks associated with the use of pure enzymes, especially their high cost. The use of free cells is also challenging, since cell lysis can also occur under the reaction conditions. However, cell immobilization has been employed to increase the catalytic potential of enzymes by extending their lifetimes in organic solvents and non-natural environments. Besides, immobilized cells maintain their biocatalytic performance for several reaction cycles. Considering the above-mentioned arguments, several authors have synthesized different classes of chiral compounds such as alcohols, amines, carboxylic acids, amides, sulfides and lactones by means of immobilized cells. Our aim was to discuss the main aspects of the production of chiral compounds using immobilized cells as a source of biocatalysts, except under fermentation conditions.