Simultaneous synthesis of enantiomerically pure (S)-amino acids and (R)-amines using coupled transaminase reactions

A One-Pot Biocatalytic Cascade to Access Diverse L-Phenylalanine Derivatives from Aldehydes or Carboxylic Acids

Non-standard amino acids (nsAAs) that are L-phenylalanine derivatives with aryl ring functionalization have long been harnessed in natural product synthesis, therapeutic peptide synthesis, and diverse applications of genetic code expansion. Yet, to date these chiral molecules have often been the products of poorly enantioselective and environmentally harsh organic synthesis routes. Here, we reveal the broad specificity of multiple natural pyridoxal 5'-phosphate (PLP)-dependent enzymes, specifically an L-threonine transaldolase, a phenylserine dehydratase, and an aminotransferase, towards substrates that contain aryl side chains with diverse substitutions. We exploit this tolerance to construct a one-pot biocatalytic cascade that achieves high-yield synthesis of 18 diverse L-phenylalanine derivatives from aldehydes under mild aqueous reaction conditions. We demonstrate addition of a carboxylic acid reductase module to this cascade to enable the biosynthesis of L-phenylalanine derivatives from carboxylic acids that may be less expensive or less reactive than the corresponding aldehydes. Finally, we investigate the scalability of the cascade by developing a lysate-based route for preparative-scale synthesis of 4-formyl-L-phenylalanine, a nsAA with a bio-orthogonal handle that is not readily market-accessible. Overall, this work offers an efficient, versatile, and scalable route with the potential to lower manufacturing cost and democratize synthesis for many valuable nsAAs.

One-Pot Biocatalytic Route from Alkanes to α,ω-Diamines by Whole-Cell Consortia of Engineered Yarrowia lipolytica and Escherichia coli

Metabolically engineered microbial consortia can contribute as a promising production platform for the supply of polyamide monomers. To date, the biosynthesis of long-chain α,ω-diamines from n-alkanes is challenging because of the inert nature of n-alkanes and the complexity of the overall synthesis pathway. We combined an engineered Yarrowia lipolytica module with Escherichia coli modules to obtain a mixed strain microbial consortium that could catalyze an efficient biotransformation of n-alkanes into corresponding α,ω-diamines. The engineered Y. lipolytica strain was constructed (YALI10) wherein the two genes responsible for β-oxidation and the five genes responsible for the overoxidation of fatty aldehydes were deleted. This newly constructed YALI10 strain expressing transaminase (TA) could produce 0.2 mM 1,12-dodecanediamine (40.1 mg/L) from 10 mM n-dodecane. The microbial consortia comprising engineered Y. lipolytica strains for the oxidation of n-alkanes (OM) and an E. coli amination module (AM) expressing an aldehyde reductase (AHR) and transaminase (TA) improved the production of 1,12-diamine up to 1.95 mM (391 mg/L) from 10 mM n-dodecane. Finally, combining the E. coli reduction module (RM) expressing a carboxylic acid reductase (CAR) and an sfp phosphopantetheinyl transferase with OM and AM further improved the production of 1,12-diamine by catalyzing the reduction of undesired 1,12-diacids into 1,12-diols, which further undergo amination to give 1,12-diamine as the target product. This newly constructed mixed strain consortium comprising three modules in one pot gave 4.1 mM (41%; 816 mg/L) 1,12-diaminododecane from 10 mM n-dodecane. The whole-cell consortia reported herein present an elegant "greener" alternative for the biosynthesis of various α,ω-diamines (C8, C10, C12, and C14) from corresponding n-alkanes.

One-Pot Two-Stage Biocatalytic Cascade to Produce l-Phosphinothricin by Two Enantioselective Complementary Aminotransferases at High Substrate Loading via a Deracemization Process

The bioderacemization of racemic phosphinothricin (D, L-PPT) is a promising route for the synthesis of l-phosphinothricin (L-PPT). However, the low activity and tolerance of wild-type enzymes restrict their industrial applications. Two stereocomplementary aminotransferases with high activity and substrate tolerance were identified in a metagenomic library, and a one-pot, two-stage artificial cascade biocatalytic system was developed to produce L-PPT through kinetic resolution and asymmetric amination. We observed that 500 mM D, L-PPT (100 g/L) could be converted into L-PPT with 94% final conversion and >99.9% enantiomeric excess (e.e.) within 24 h, with only 0.02 eq amino acceptor pyruvate and 1.2 eq amino donor l-aspartate required. The process could be scaled up to 10 L under sufficient oxygen and stirring. The superior catalytic performance of this system provides an eco-friendly and sustainable approach to the industrial deracemization of D, L-PPT to L-PPT.

Enzymatic asymmetric synthesis of chiral amino acids

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

New generation of biocatalysts for organic synthesis

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

Enzyme catalysed tandem reactions

To transfer to the laboratory, the excellent efficiency shown by enzymes in Nature, biocatalysis, had to mimic several synthetic strategies used by the living organisms. Biosynthetic pathways are examples of tandem catalysis and may be assimilated in the biocatalysis field for the use of isolated multi-enzyme systems in the homogeneous phase. The concurrent action of several enzymes that work sequentially presents extraordinary advantages from the synthetic point of view, since it permits a reversible process to become irreversible, to shift the equilibrium reaction in such a way that enantiopure compounds can be obtained from prochiral or racemic substrates, reduce or eliminate problems due to product inhibition or prevent the shortage of substrates by dilution or degradation in the bulk media, etc. In this review we want to illustrate the developments of recent studies involving in vitro multi-enzyme reactions for the synthesis of different classes of organic compounds.

Crystallization and preliminary X-ray crystallographic studies of ω-transaminase from Vibrio fluvialis JS17

Omega-transaminase (ω-TA) catalyzes the transfer of an amino group from a non-alpha-position amino acid or an amine compound with no carboxylic group to an amino acceptor. ω-TA from Vibrio fluvialis JS17 (ω-TAVf) is a novel amine:pyruvate transaminase that is capable of stereoselective transamination of aryl chiral amines. In this study, omega-TAVf was overexpressed in Escherichia coli with engineered C-terminal His tags. ω-TAVf was then purified to homogeneity and crystallized at 292 K. X-ray diffraction data were collected to a resolution of 2.5 A from a crystal belonging to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a=78.43, b=95.95, c=122.89 A.

omega-Transaminases for the synthesis of non-racemic alpha-chiral primary amines

Optically pure amines are highly valuable products or key intermediates for a vast number of bioactive compounds; however, efficient methods for their preparation are rare. omega-Transaminases (TAs) can be applied either for the kinetic resolution of racemic amines or for the asymmetric synthesis of amines from the corresponding ketones. The latter process is more advantageous because it leads to 100% product, and is therefore a major focus of this review. This review summarizes various methodologies for transamination reactions, and provides an overview of omega-TAs that have the potential to be used for the preparation of a broad spectrum of alpha-chiral amines. Recent methodological developments as well as some recently identified novel omega-TAs warrant an update on this topic.

Biotransformation of D-methionine into L-methionine in the cascade of four enzymes

D-Methionine was converted to L-methionine in a reaction system where four enzymes were used. D-amino acid oxidase (D-AAO) from Arthrobacter protophormiae was used for the complete conversion of D-methionine to 2-oxo-4-methylthiobutyric acid. Catalase was added to prevent 2-oxo-4-methylthiobutyric acid decarboxylation. In the second reaction step, L-phenylalanine dehydrogenase (L-PheDH) from Rhodococcus sp. was used to convert 2- oxo-4-methylthiobutyric acid to L-methionine, and formate dehydrogenase (FDH) from Candida boidinii was added for NADH regeneration. Enzyme kinetics of all enzymes was analyzed in detail. Mathematical models for separate reactions steps, as well as for the complete system were developed and validated in the batch reactor experiments. Complete conversion of D-methionine to L-methionine was achieved. Considering that both enzymes act on different substrates, such a system could be easily employed for the synthesis of other amino acids from D-isomer, as well as from the racemate of a certain amino acid (DL-amino acid).

Redesigning the substrate specificity of omega-aminotransferase for the kinetic resolution of aliphatic chiral amines

Substrate specificity of the omega-aminotransferase obtained from Vibrio fluvialis (omega-ATVf) was rationally redesigned for the kinetic resolution of aliphatic chiral amines. omega-ATVf showed unique substrate specificity toward aromatic amines with a high enantioselectivity (E > 100) for (S)-enantiomers. However, the substrate specificity of this enzyme was much narrower toward aliphatic amines. To overcome the narrow substrate specificity toward aliphatic amines, we redesigned the substrate specificity of omega-ATVf using homology modeling and the substrate structure- activity relationship. The homology model and the substrate structure-activity relationship showed that the active site of omega-ATVf consists of one large substrate-binding site and another small substrate-binding site. The key determinant in the small substrate-binding site was D25, whose role was expected to mask R415 and to generate the electrostatic repulsion with the substrate's alpha-carboxylate group. In the large substrate-binding site, R256 was predicted to recognize the alpha-carboxylate group of substrate thus obeying the dual substrate recognition mechanism of aminotransferase subgroup II enzymes. Among the several amino acid residues in the large substrate-binding site, W57 and W147, with their bulky side chains, were expected to restrict the recognition of aliphatic amines. Two mutant enzymes, W57G and W147G, showed significant changes in their substrate specificity such that they catalyzed transamination of a broad range of aliphatic amines without losing the original activities toward aromatic amines and enantioselectivity.

An integrated microfluidic chip for the analysis of biochemical reactions by MALDI mass spectrometry

Using an integrated microfluidic chip combined with mass spectrometry is an attractive method for parallel and multiple analyses because of its inherent simplicity, low sample consumption, and high sensitivity. To realize an effective microfluidic chip for the rapid analysis of biochemical reactions by matrix assisted laser desorption/ionization (MALDI)-mass spectrometry (MS), the basic operations on microfluids, namely loading, metering, cutting, transporting, mixing, and injecting, must be integrated. This study describes an integrated microfluidic chip with MALDI-MS that performs the on-chip analysis of biochemical reactions, such as enzymatic reactions. For on-chip multiple reactions, we present sequential fluidic manipulations with nanoliter-sized droplets, based on the precise control of wettability and the capillary pressure of a microchannel. The microfluidic chip we have developed successfully performed biochemical reactions and can dispense a droplet of a few hundred nanoliters on the MALDI target plate according to the designed multiple reaction procedure. Finally, the MS spectrum showed accurate and clear characteristic peaks for reaction products. Our investigations into reaction efficiency showed that the microfluidic chip could reduce the reaction time to one third, and the volume to one hundredth, of off-chip methods using conventional labware such as the micropipette and Eppendorf tube.

Cloning and characterization of a novel beta-transaminase from Mesorhizobium sp. strain LUK: a new biocatalyst for the synthesis of enantiomerically pure beta-amino acids

A novel beta-transaminase gene was cloned from Mesorhizobium sp. strain LUK. By using N-terminal sequence and an internal protein sequence, a digoxigenin-labeled probe was made for nonradioactive hybridization, and a 2.5-kb gene fragment was obtained by colony hybridization of a cosmid library. Through Southern blotting and sequence analysis of the selected cosmid clone, the structural gene of the enzyme (1,335 bp) was identified, which encodes a protein of 47,244 Da with a theoretical pI of 6.2. The deduced amino acid sequence of the beta-transaminase showed the highest sequence similarity with glutamate-1-semialdehyde aminomutase of transaminase subgroup II. The beta-transaminase showed higher activities toward d-beta-aminocarboxylic acids such as 3-aminobutyric acid, 3-amino-5-methylhexanoic acid, and 3-amino-3-phenylpropionic acid. The beta-transaminase has an unusually broad specificity for amino acceptors such as pyruvate and alpha-ketoglutarate/oxaloacetate. The enantioselectivity of the enzyme suggested that the recognition mode of beta-aminocarboxylic acids in the active site is reversed relative to that of alpha-amino acids. After comparison of its primary structure with transaminase subgroup II enzymes, it was proposed that R43 interacts with the carboxylate group of the beta-aminocarboxylic acids and the carboxylate group on the side chain of dicarboxylic alpha-keto acids such as alpha-ketoglutarate and oxaloacetate. R404 is another conserved residue, which interacts with the alpha-carboxylate group of the alpha-amino acids and alpha-keto acids. The beta-transaminase was used for the asymmetric synthesis of enantiomerically pure beta-aminocarboxylic acids. (3S)-Amino-3-phenylpropionic acid was produced from the ketocarboxylic acid ester substrate by coupled reaction with a lipase using 3-aminobutyric acid as amino donor.

Enzymatic resolution for the preparation of enantiomerically enrichedD-?-heterocyclic alanine derivatives usingEscherichia coli aromaticL-amino acid transaminase

An enzymatic resolution was carried out for the preparation of enriched beta-heterocyclic D-alanine derivatives using Escherichia coli aromatic L-amino acid transaminase. The excess of pyrazole, imidazole, or 1,2,4-triazole reacted with methyl-2-acetamidoacrylate in acetonitrile in the presence of potassium carbonate at 60 degrees C, directly leading to make the potassium salt of the corresponding N-acetyl-beta-heterocyclic alanine derivatives. After the acidic deprotection of the N-acetyl group, 10 mM of racemic pyrazolylalanine, triazolylalanine, and imidazolylalanine were resolved to D-pyrazolylalanine, D-triazolylalanine, and D-imidazolylalanine with 46% (85% ee), 42% (72% ee), and 48% (95% ee) conversion yield in 18 h, respectively, using E. coli aromatic L-amino acid transaminase (EC 2.6.1.5). Although the three beta-heterocyclic L-alanine derivatives have similar molecular structures, they showed different reaction rates and enantioselectivities. The relative reactivities of the transaminase toward the beta-heterocyclic L-alanine derivatives could be explained by the relationship between the substrate binding energy (E, kcal/mol) to the enzyme active site and the distance (delta, A) from the nitrogen of alpha-amino group of the substrates to the C4' carbon of PLP-Lys258 Schiff base. As the ratio of the substrate binding energy (E) to the distance (delta) becomes indicative value of k(cat)/K(M) of the enzyme to the substrate, the relative reactivities of the beta-heterocyclic L-alanine derivatives were successfully correlated with E/delta, and the relationship was confirmed by our experiments.

Asymmetric synthesis of L-homophenylalanine by equilibrium-shift using recombinant aromatic L-amino acid transaminase

L-Homophenylalanine (L-HPA) was asymmetrically synthesized from 2-oxo-4-phenylbutyric acid (2-OPBA) and L-aspartate using a recombinant aromatic amino acid transaminase (AroAT). To screen microorganisms having such an L-specific AroAT with a relaxed substrate inhibition in the asymmetric synthesis of unnatural amino acids, enrichment cultures were performed in a minimal media containing 50 mM L-HPA as a sole nitrogen source. To reduce the intracellular background synthetic activity by amino acid pools in the cells, a two-step screening method was used. The putative AroAT (i.e., AroATEs) from the screened Enterobacter sp. BK2K-1 was cloned, sequenced, and overexpressed in E. coli cells. The activity of the overexpressed AroATEs was 314-fold higher than that of the wild-type cell. The substrate specificities of the enzyme and homology search revealed that the cloned transaminase is true AroAT. The AroATEs showed a substrate inhibition by 2-OPBA from 40 mM in the asymmetric synthesis, which made it difficult to perform batch asymmetric synthesis of L-HPA at high concentrations of 2-OPBA. To avoid the substrate inhibition by 2-OPBA, intermittent addition of the solid-state substrate was attempted to obtain a high concentration of L-HPA. By using the cell extract (75 U) obtained from the recombinant E. coli harboring the AroATEs gene, the asymmetric synthesis of L-HPA at 840 mM of 2-OPBA resulted in >94% of conversion yield and >99% ee of L-HPA of optical purity. Due to the low solubility (<2 mM) of L-HPA in the reaction buffer, synthesized L-HPA was continuously precipitated in the reaction media, which drives the reaction equilibrium towards the product formation. After full completion of the reaction, L-HPA of high purity (>99% ee) was easily recovered by simple pH shift of the reaction media. This method can permit very efficient asymmetric synthesis of other unnatural amino acids using a single transaminase reaction.