Multi-enzyme cascade for improving β-hydroxy-α-amino acids production by engineering L-threonine transaldolase and combining acetaldehyde elimination system

Discovery and directed evolution of C-C bond formation enzymes for the biosynthesis of β-hydroxy-α-amino acids and derivatives

β-Hydroxy-α-amino acids (β-HAAs) have extensive applications in the pharmaceutical, chemical synthesis, and food industries. The development of synthetic methodologies aimed at producing optically pure β-HAAs has been driven by practical applications. Among the various synthetic methods, biocatalytic asymmetric synthesis is considered a sustainable approach due to its capacity to generate two stereogenic centers from simple prochiral precursors in a single step. Therefore, extensive efforts have been made in recent years to search for effective enzymes which enable such biotransformation. This review provides an overview on the discovery and engineering of C-C bond formation enzymes for the biocatalytic synthesis of β-HAAs. We highlight examples where the use of threonine aldolases, threonine transaldolases, serine hydroxymethyltransferases, α-methylserine aldolases, α-methylserine hydroxymethyltransferases, and engineered alanine racemases facilitated the synthesis of β-HAAs. Additionally, we discuss the potential future advancements and persistent obstacles in the enzymatic synthesis of β-HAAs.

Enhanced synthesis of chloramphenicol intermediate L-threo-p-nitrophenylserine using engineered L-threonine transaldolase and by-product elimination

L-threo-p-nitrophenylserine (component 2) is an important intermediate during synthesis of chloramphenicol. However, its biosynthesis is limited by enzyme activity and stereoselectivity. In this study, we achieved a breakthrough in the high-efficiency production of 2 by employing engineered Chitiniphilus shinanonensis L-threonine transaldolase (ChLTTA) in conjunction with a by-product elimination system within a one-pot reaction. Notably, a novel visual stepwise high-throughput screening method was developed for the directed evolution of ChLTTA, leveraging its characteristic color. The engineered mutant F70D/F59A (Mu6 variant) emerged as a star performer, exhibiting a remarkable 2.6-fold increase in catalytic efficiency over the wild-type ChLTTA, coupled with an outstanding 91.5 % diastereoisomer excess (de). Molecular dynamics (MD) simulations unraveled the mechanism responsible for the enhanced catalytic performance observed in the Mu6 variant. Meanwhile, the Mu6 variant was coupled with Saccharomyces cerevisiae ethanol dehydrogenase (ScADH) and Candida boidinii formate dehydrogenase (CbFDH) to create a high-efficiency cascade system (E.coli/pRSF-Mu6-ScADH-CbFDH). Under optimized conditions, this cascade system demonstrated unparalleled performance, yielding 201.5 mM of 2 with an impressive conversion of 95.9 % and a de value of 94.5 %. This achievement represents the highest reported yield to date. This study offers a novel insight into the sustainable and efficient production of chloramphenicol intermediate.

An artificial biocatalytic cascade for efficient synthesis of norepinephrine by combination of engineered L-threonine transaldolase with multi-enzyme expression fine-tuning

Norepinephrine, a kind of β-adrenergic receptor agonist, is commonly used for treating shocks and hypotension caused by a variety of symptoms. The development of a straightforward, efficient and environmentally friendly biocatalytic route for manufacturing norepinephrine remains a challenge. Here, we designed and realized an artificial biocatalytic cascade to access norepinephrine starting from 3, 4-dihydroxybenzaldehyde and L-threonine mediated by a tailored-made L-threonine transaldolase PsLTTA-Mu1 and a newly screened tyrosine decarboxylase ErTDC. To overcome the imbalance of multi-enzymes in a single cell, engineering of PsLTTA for improved activity and fine-tuning expression mode of multi-enzymes in single E.coli cells were combined, leading to a robust whole cell biocatalyst ES07 that could produce 100 mM norepinephrine with 99% conversion, delivering a highest time-space yield (3.38 g/L/h) ever reported. To summarized, the current study proposed an effective biocatalytic approach for the synthesis of norepinephrine from low-cost substrates, paving the way for industrial applications of enzymatic norepinephrine production.

Engineering of human tryptophan hydroxylase 2 for efficient synthesis of 5-hydroxytryptophan

L-Tryptophan hydroxylation catalyzed by tryptophan hydroxylase (TPH) presents a promising method for synthesizing 5-hydroxytryptophan (5-HTP), yet the limited activity of wild-type human TPH2 restricts its application. A high-activity mutant, MT10 (H318E/H323E), was developed through semi-rational active site saturation testing (CAST) of wild-type TPH2, exhibiting a 2.85-fold increase in kcat/Km over the wild type, thus enhancing catalytic efficiency. Two biotransformation systems were developed, including an in vitro one-pot system and a Whole-Cell Catalysis System (WCCS). In the WCCS, MT10 achieved a conversion rate of only 31.5 % within 32 h. In the one-pot reaction, MT10 converted 50 mM L-tryptophan to 44.5 mM 5-HTP within 8 h, achieving an 89 % conversion rate, outperforming the M1 (NΔ143/CΔ26) variant. Molecular dynamics simulations indicated enhanced interactions of MT10 with the substrate, suggesting improved binding affinity and system stability. This study offers an effective approach for the efficient production of 5-HTP.

Pre-optimization and one-step preparation of cascade enzymes system with broad substrates by model guidance: Application of chiral L-norvaline and L-phenylglycine biosynthesis

Cascade biocatalyst systems with catalytic promiscuity can be used for synthesis of a class of chiral chemicals but the optimization of these systems by model guidance is poorly explored. In this study, a cascade system with broad substrate spectrum was characterized and simulated by kinetic model with substrates of DL-Norvaline (DL-Nor) and DL-Phenylglycine (DL-Phg) as examples. To evaluate the optimal cascade system, maximum accumulation of intermediate products and conversion rate in the process were investigated by simultaneous solution of the rate equations for varying enzyme quantities. According to the simulation results, the cascade system was optimized by regulating the expression of D-amino acid oxidase and formate dehydrogenase and was prepared by one-step. The conversion efficiency of DL-Nor and DL-Phg have been significantly improved compared with that of before optimization. Moreover, the total of L-Nor and L-Phg were reached 498.2 mM and 79.5 mM through a gradient fed-batch conversion strategy, respectively.

Rational Design of l-Threonine Transaldolase-Mediated System for Enhanced Florfenicol Intermediate Production

l-threo-p-methylsulfonylphenylserine (compound 1b) is the main intermediate of florfenicol, and its efficient synthesis has been the subject of current research. Herein, Burkholderia diffusa l-threonine transaldolase (BuLTTA) was rationally designed based on the sequence-structure-function relationship. A mutant M4 (Asn35Ser/Thr352Asn) could produce 35.5 mM 1b with 88.8% conversion and 93.8% diastereoselectivity, 314 and 129% of the values observed for wild-type BuLTTA. Molecular dynamics simulations indicated that the shortened distance between key active site residues and the transition state (PLP-1b) and the improved hydrogen bond force enhanced the catalytic performance of the M4 variant. Then, the mutant M4 was combined with K. kurtzmanii alcohol dehydrogenase (KkADH) to eliminate the BuLTTA-inhibiting byproduct acetaldehyde, and a cosubstrate was added to regenerate the ADH cofactor NADH. Under optimized conditions, the yield of 1b reached 115.2 mM with a conversion of 96% and a diastereoselectivity of 95.5%. This work provides a new strategy for the efficient and sustainable production of 1b.

Multidimensional engineering of Escherichia coli for efficient synthesis of L-tryptophan

Development of microbial cell factory for L-tryptophan (L-trp) production has received widespread attention but still requires extensive efforts due to weak metabolic flux distribution and low yield. Here, the riboswitch-based high-throughput screening (HTS) platform was established to construct a powerful L-trp-producing chassis cell. To facilitate L-trp biosynthesis, gene expression was regulated by promoter and N-terminal coding sequences (NCS) engineering. Modules of degradation, transport and by-product synthesis related to L-trp production were also fine-tuned. Next, a novel transcription factor YihL was excavated to negatively regulate L-trp biosynthesis. Self-regulated promoter-mediated dynamic regulation of branch pathways was performed and cofactor supply was improved for further L-trp biosynthesis. Finally, without extra addition, the yield of strain Trp30 reached 42.5 g/L and 0.178 g/g glucose after 48 h of cultivation in 5-L bioreactor. Overall, strategies described here worked up a promising method combining HTS and multidimensional regulation for developing cell factories for products in interest.

Discovery of L-threonine transaldolases for enhanced biosynthesis of beta-hydroxylated amino acids

Beta-hydroxy non-standard amino acids (β-OH-nsAAs) have utility as small molecule drugs, precursors for beta-lactone antibiotics, and building blocks for polypeptides. While the L-threonine transaldolase (TTA), ObiH, is a promising enzyme for β-OH-nsAA biosynthesis, little is known about other natural TTA sequences. We ascertained the specificity of the TTA enzyme class more comprehensively by characterizing 12 candidate TTA gene products across a wide range (20-80%) of sequence identities. We found that addition of a solubility tag substantially enhanced the soluble protein expression level within this difficult-to-express enzyme family. Using an optimized coupled enzyme assay, we identified six TTAs, including one with less than 30% sequence identity to ObiH that exhibits broader substrate scope, two-fold higher L-Threonine (L-Thr) affinity, and five-fold faster initial reaction rates under conditions tested. We harnessed these TTAs for first-time bioproduction of β-OH-nsAAs with handles for bio-orthogonal conjugation from supplemented precursors during aerobic fermentation of engineered Escherichia coli, where we observed that higher affinity of the TTA for L-Thr increased titer. Overall, our work reveals an unexpectedly high level of sequence diversity and broad substrate specificity in an enzyme family whose members play key roles in the biosynthesis of therapeutic natural products that could benefit from chemical diversification.

The inhibitory mechanism of 2-amino-3,8-dimethylimidazo [4,5-f] quinoxaline (MeIQx) formation by ultraviolet-gallic acid (UV-GA) during the oil-frying process of squid

The inhibitory effect of ultraviolet-gallic acid (UV-GA) on carbonyl valence and intermediates and precursors of 2-amino-3,8-dimethylimidazo [4,5-f] quinoxaline (MeIQx) was investigated to futher clarify the inhibitory mechanism for safety control the quality of oil-fried squid. Ultraviolet C-treated gallic acid (UVC-GA) and ultraviolet B-treated gallic acid (UVB-GA) were produced by ultraviolet 225 nm of band C and 300 nm of band B, respectively. The MeIQx contents in oil-fried squid were significantly higher, and UVC-GA and UVB-GA could significantly inhibit the MeIQx formation and the formation rates of carbonyl valence and precursors (threonine (Thr), creatinine, and glucose). The UVB-GA inhibited formaldehyde formation, while UVC-GA significantly reduced the formaldehyde, acetaldehyde, and 2,5-dimethyl pyrazine contents. In conculsion, UV-GA reduced carbonyl produced from the lipid oxidation to further weaken the catalysis of carbonyl, rendering the MeIQx precursor degrading into the intermediates during Strecker degradation. Thus, the MeIQx formation was inhibited.

Beyond 20 in the 21st Century: Prospects and Challenges of Non-canonical Amino Acids in Peptide Drug Discovery

Life is constructed primarily using a toolbox of 20 canonical amino acids-relying upon these building blocks for the assembly of proteins and peptides that regulate nearly every cellular task, including cell structure, function, and maintenance. While Nature continues to be a source of inspiration for drug discovery, medicinal chemists are not beholden to only 20 canonical amino acids and have begun to explore non-canonical amino acids (ncAAs) for the construction of designer peptides with improved drug-like properties. However, as our toolbox of ncAAs expands, drug hunters are encountering new challenges in approaching the iterative peptide design-make-test-analyze cycle with a seemingly boundless set of building blocks. This Microperspective focuses on new technologies that are accelerating ncAA interrogation in peptide drug discovery (including HELM notation, late-stage functionalization, and biocatalysis) while shedding light on areas where further investment could not only accelerate the discovery of new medicines but also improve downstream development.

Combinatorial gene inactivation of aldehyde dehydrogenases mitigates aldehyde oxidation catalyzed by E. coli resting cells

Aldehydes are attractive chemical targets both as end products in the flavors and fragrances industry and as intermediates due to their propensity for C-C bond formation. Here, we identify and address unexpected oxidation of a model collection of aromatic aldehydes, including many that originate from biomass degradation. When diverse aldehydes are supplemented to E. coli cells grown under aerobic conditions, as expected they are either reduced by the wild-type MG1655 strain or stabilized by a strain engineered for reduced aromatic aldehyde reduction (the E. coli RARE strain). Surprisingly, when these same aldehydes are supplemented to resting cell preparations of either E. coli strain, under many conditions we observe substantial oxidation. By performing combinatorial inactivation of six candidate aldehyde dehydrogenase genes in the E. coli genome using multiplexed automatable genome engineering (MAGE), we demonstrate that this oxidation can be substantially slowed, with greater than 50% retention of 6 out of 8 aldehydes when assayed 4 h after their addition. Given that our newly engineered strain exhibits reduced oxidation and reduction of aromatic aldehydes, we dubbed it the E. coli ROAR strain. We applied the new strain to resting cell biocatalysis for two kinds of reactions - the reduction of 2-furoic acid to furfural and the condensation of 3-hydroxy-benzaldehyde and glycine to form a beta hydroxylated non-standard amino acid. In each case, we observed substantial improvements in product titer 20 h after reaction initiation (9-fold and 10-fold, respectively). Moving forward, the use of this strain to generate resting cells should allow aldehyde product isolation, further enzymatic conversion, or chemical reactivity under cellular contexts that better accommodate aldehyde toxicity.

Recent advances in the utilization of immobilized laccase for the degradation of phenolic compounds in aqueous solutions: A review

Phenolic compounds such as phenol, bisphenol A, 2,4-dichlorophenol, 2,4-dinitrophenol, 4-chlorophenol and 4-nitrophenol are well known to be highly detrimental to both human and living beings. Thus, it is of critical importance that suitable remediation technologies are developed to effectively remove phenolic compounds from aqueous solutions. Biodegradation utilizing enzymatic technologies is a promising biotechnological solution to sustainably address the pollution in the aquatic environment as caused by phenolic compounds under a defined environmentally optimized strategy and thus should be investigated in great detail. This review aims to present the latest developments in the employment of immobilized laccase for the degradation of phenolic compounds in water. The review first succinctly delineates the fundamentals of biological enzyme degradation along with a critical discussion on the myriad types of laccase immobilization techniques, which include physical adsorption, ionic adsorption, covalent binding, entrapment, and self-immobilization. Then, this review presents the major properties of immobilized laccase, namely pH stability, thermal stability, reusability, and storage stability, as well as the degradation efficiencies and associated kinetic parameters. In addition, the optimization of the immobilized enzyme, specifically on laccase immobilization methods and multi-enzyme system are critically discussed. Finally, pertinent future perspectives are elucidated in order to significantly advance the developments of this research field to a higher level.

Computer-aided directed evolution ofl-threonine aldolase for asymmetric biocatalytic synthesis of a chloramphenicol intermediate

l-Threonine aldolases (LTAs) employing pyridoxal phosphate (PLP) as cofactor can convert low-cost achiral substrates glycine and aldehyde directly into valuable β-hydroxy-α-amino acids such as (2R,3S)-2-amino-3-hydroxy-3-(4-nitrophenyl) propanoic acid ((R,S)-AHNPA), which is utilized broadly as crucial chiral intermediates for bioactive compounds. However, LTAs' stereospecificity towards the β carbon is rather moderate and their activity and stability at high substrate load is low, which limits their industrial application. Here, computer-aided directed evolution was applied to improve overall activity, selectivity and stability under desired process conditions of a l-threonine aldolase in the asymmetric synthesis of (R,S)-AHNPA. Selectivity and stability determining regions were computationally identified for structure-guided directed evolution of LTA-variants under efficient biocatalytic process conditions using 40% ethanol as cosolvent. We applied molecular modeling to rationalize selectivity improvement and design focused libraries targeting the substrate binding pocket, and we also used MD simulations in nonaqueous process environment as an effective and promising method to predict potential unstable loop regions near the tetramer interface which are hot-spots for cosolvent resistance. An excellent LTA variant EM-ALDO031 with 18 mutations was obtained, which showed ∼ 30-fold stability improvement in 40% ethanol and diastereoselectivity (de) raised from 31.5% to 85% through a three-phase evolution campaign. Our fast and efficient data-driven methodology utilizing a combination of experimental and computational tools enabled us to evolve an aldolase variant to achieve the target of 90% conversion at up to 150 g/L substrate load in 40% ethanol, enabling the biocatalytic production of β-hydroxy-α-amino acids from cheap achiral precursors at multi-ton scale.

Microbiome engineering for sustainable agriculture: using synthetic biology to enhance nitrogen metabolism in plant-associated microbes

Plants benefit from symbiotic relationships with their microbiomes. Modifying these microbiomes to further promote plant growth and improve stress tolerance in crops is a promising strategy. However, such efforts have had limited success, perhaps because the original microbiomes quickly re-establish. Since the complex biological networks involved are little understood, progress through conventional means is time-consuming. Synthetic biology, with its practical successes in multiple industries, could speed up this research considerably. Some fascinating candidates for production by synthetic microbiomes are organic nitrogen metabolites and related pyridoxal-5'-phosphate-dependent enzymes, which have pivotal roles in microbe-microbe and plant-microbe interactions. This review summarizes recent studies of these metabolites and enzymes and discusses prospective synthetic biology platforms for sustainable agriculture.

Efficient production of inositol from glucose via a tri-enzymatic cascade pathway

Inositol is an essential intermediate in cosmetics, food, medicine and other industries. However, developing an efficient biotransformation system for large-scale production of inositol remains challenging. Herein, a tri-enzymatic cascade route with three novel enzymes including polyphosphate glucokinase (PPGK) from Thermobifida fusca, inositol 3-phosphate synthase (IPS) from Archaeoglobus profundus DSM 5631 and inositol monophosphatase (IMP) from Thermotoga petrophila RKU-1 was designed and reconstructed for the production of inositol from glucose. The problem of poor cooperativity of the cascade reactions was addressed by ribosome binding site (RBS) optimization of PPGK and replication of IPS. Under the optimum biotransformation conditions, the engineered whole-cell immobilized with colloidal chitin transformed 120 g/L glucose to 110.8 g/L inositol with 92.3% conversion in four cycles of reuse, representing the highest titer of inositol to date. Furthermore, this is the first study for inositol production using a three-enzyme coordinated immobilized single-cell.

Asymmetric Synthesis of N-Substituted 1,2-Amino Alcohols from Simple Aldehydes and Amines by One-Pot Sequential Enzymatic Hydroxymethylation and Asymmetric Reductive Amination

The chiral N-substituted 1,2-amino alcohol motif is found in many natural and synthetic bioactive compounds. In this study, enzymatic asymmetric reductive amination of α-hydroxymethyl ketones with enantiocomplementary imine reductases (IREDs) enabled the synthesis of chiral N-substituted 1,2-amino alcohols with excellent ee values (91-99 %) in moderate to high yields (41-84 %). Furthermore, a one-pot, two-step enzymatic process involving benzaldehyde lyase-catalyzed hydroxymethylation of aldehydes and subsequent asymmetric reductive amination was developed, offering an environmentally friendly and economical way to produce N-substituted 1,2-amino alcohols from readily available simple aldehydes and amines. This methodology was then applied to rapidly access a key synthetic intermediate of anti-malaria and cytotoxic tetrahydroquinoline alkaloids.

Scalable and Selective β-Hydroxy-α-Amino Acid Synthesis Catalyzed by Promiscuous l-Threonine Transaldolase ObiH

Enzymes from secondary metabolic pathways possess broad potential for the selective synthesis of complex bioactive molecules. However, the practical application of these enzymes for organic synthesis is dependent on the development of efficient, economical, operationally simple, and well-characterized systems for preparative scale reactions. We sought to bridge this knowledge gap for the selective biocatalytic synthesis of β-hydroxy-α-amino acids, which are important synthetic building blocks. To achieve this goal, we demonstrated the ability of ObiH, an l-threonine transaldolase, to achieve selective milligram-scale synthesis of a diverse array of non-standard amino acids (nsAAs) using a scalable whole cell platform. We show how the initial selectivity of the catalyst is high and how the diastereomeric ratio of products decreases at high conversion due to product re-entry into the catalytic cycle. ObiH-catalyzed reactions with a variety of aromatic, aliphatic and heterocyclic aldehydes selectively generated a panel of β-hydroxy-α-amino acids possessing broad functional-group diversity. Furthermore, we demonstrated that ObiH-generated β-hydroxy-α-amino acids could be modified through additional transformations to access important motifs, such as β-chloro-α-amino acids and substituted α-keto acids.

Dual enzyme co-immobilization on reversibly soluble polymers for the one-pot conversion of ferulic acid from wheat bran

The difficulty of using immobilized enzyme to decompose wheat bran to produce ferulic acid lies in the recovery of enzyme from solid-rich wheat bran hydrolysates. In this study, two enzymes...

Preparation of ZIF@ADH/NAD-MSN/LDH Core Shell Nanocomposites for the Enhancement of Coenzyme Catalyzed Double Enzyme Cascade

The field of enzyme cascades in limited microscale or nanoscale environments has undergone a quick growth and attracted increasing interests in the field of rapid development of systems chemistry. In this study, alcohol dehydrogenase (ADH), lactate dehydrogenase (LDH), and mesoporous silica nanoparticles (MSN) immobilized nicotinamide adenine dinucleotide (NAD⁺) were successfully immobilized on the zeolitic imidazolate frameworks (ZIFs). This immobilized product was named ZIF@ADH/NAD-MSN/LDH, and the effect of the multi-enzyme cascade was studied by measuring the catalytic synthesis of lactic acid. The loading efficiency of the enzyme in the in-situ co-immobilization method reached 92.65%. The synthesis rate of lactic acid was increased to 70.10%, which was about 2.82 times that of the free enzyme under the optimal conditions (40 °C, pH = 8). Additionally, ZIF@ADH/NAD-MSN/LDH had experimental stability (71.67% relative activity after four experiments) and storage stability (93.45% relative activity after three weeks of storage at 4 °C; 76.89% relative activity after incubation in acetonitrile-aqueous solution for 1 h; 27.42% relative activity after incubation in 15% N, N-Dimethylformamide (DMF) solution for 1 h). In summary, in this paper, the cyclic regeneration of coenzymes was achieved, and the reaction efficiency of the multi-enzyme biocatalytic cascade was improved due to the reduction of substrate diffusion.

Whole-cell biocatalytic synthesis of S-(4-chlorophenyl)-(pyridin-2-yl) methanol in a liquid-liquid biphasic microreaction system

This work aims to synthesize S-(4-chlorophenyl)-(pyridin-2-yl) methanol (S-CPMA) in a green, economic, and efficient way. In the water-cyclohexane liquid-liquid system, recombinant Escherichia coli (E. coli) was used as a whole-cell catalyst and retained > 60% of its catalytic activity after five reuse cycles. In situ accumulation of the substrate/product in the organic phase effectively improves substrate tolerance and reduces product inhibition and toxicity. Meanwhile, a microreaction system consisting of membrane dispersion and three-dimensional (3D) bending-microchannel was developed to successfully generate droplet swarms with an average diameter of 30 μm. Large specific surface area provided high mass transfer efficiency between phases. While the analogous reaction in a traditional stirred tank required > 270 min to achieve a yield of > 99%, in this biphasic microreaction system, the yield reached 99.6% with a high enantiomeric excess (ee) of > 99% in only 80 min. Efficient synthesis was achieved by reducing the time by 70%.

Improving the Cβ Stereoselectivity of l-Threonine Aldolase for the Synthesis of l-threo-4-Methylsulfonylphenylserine by Modulating the Substrate-Binding Pocket To Control the Orientation of the Substrate Entrance

l-Threonine aldolase from Actinocorallia herbida (AhLTA) is an ideal catalyst for producing l-threo-4-methylsulfonylphenylserine [(2S,3R)-1 b], a key chiral precursor for florfenicol and thiamphenicol. The moderate C_β stereoselectivity is the main obstacle to the industrial application of AhLTA. To address this issue, a combinatorial active-site saturation test (CAST) together with sequence conservatism analysis was applied to engineer the AhLTA toward improved C_β stereoselectivity. The optical mutant Y314R could asymmetrically synthesize l-threo-4-methylsulfonylphenylserine with 81 % diastereomeric excess (de), which is 23 % higher than wild-type AhLTA. Molecular dynamic (MD) simulations revealed that the mechanism for the improvement in C_β stereoselectivity of Y314R is due to the acylamino group of residues Arg314 controlling the orientation of substrate 4-methylsulfonyl benzaldehyde (1 a) in the active pocket by directed interaction with the methylsulfonyl group; this leads to asymmetric synthesis of l-threo-4-methylsulfonylphenylserine. The success in this study demonstrates that direct control of substrates in an active pocket is an attract strategy to address the C_β stereoselectivity problem of LTA and contribute to the industrial application of LTA.

Peculiarities of promiscuous L-threonine transaldolases for enantioselective synthesis of β-hydroxy-α-amino acids

The introduction of β-hydroxy-α-amino acids (βHAAs) into organic molecules has received considerable attention as these molecules have often found widespread applications in bioorganic chemistry, medicinal chemistry and biomaterial science. Despite innovation of asymmetric synthesis of βHAAs, stereoselective synthesis to control the two chiral centres at C_α and C_β positions is still challenging, with poor atomic economy and multi protection and deprotection steps. These syntheses are often operated under harsh conditions. Therefore, a biotransformation approach using biocatalysts is needed to selectively introduce these two chiral centres into structurally diverse molecules. Yet, there are few ways that enable one-step synthesis of βHAAs. One is to extend the substrate scope of the existing enzyme inventory. Threonine aldolases have been explored to produce βHAAs. However, the enzymes have poor controlled installation at C_β position, often resulting in a mixture of diastereoisomers which are difficult to be separated. In this respect, L-threonine transaldolases (LTTAs) offer an excellent potential as the enzymes often provide controlled stereochemistry at C_α and C_β positions. Another is to mine LTTA homologues and engineer the enzymes using directed evolution with the aim of finding engineered biocatalysts to accept broad substrates with enhanced conversion and stereoselectivity. Here, we review the development of LTTAs that incorporate various aldehyde acceptors to generate structurally diverse βHAAs and highlight areas for future developments. KEY POINTS: • The general mechanism of the transaldolation reaction catalysed by LTTAs • Recent advances in LTTAs from different biosynthetic pathways • Applications of LTTAs as biocatalysts for production of βHAAs.

Synthesis of γ-Hydroxy-α-amino Acid Derivatives by Enzymatic Tandem Aldol Addition-Transamination Reactions

Three enzymatic routes toward γ-hydroxy-α-amino acids by tandem aldol addition–transamination one-pot two-step reactions are reported. The approaches feature an enantioselective aldol addition of pyruvate to various nonaromatic aldehydes catalyzed by trans-o-hydroxybenzylidene pyruvate hydratase-aldolase (HBPA) from Pseudomonas putida. This affords chiral 4-hydroxy-2-oxo acids, which were subsequently enantioselectively aminated using S-selective transaminases. Three transamination processes were investigated involving different amine donors and transaminases: (i) l-Ala as an amine donor with pyruvate recycling, (ii) a benzylamine donor using benzaldehyde lyase from Pseudomonas fluorescens Biovar I (BAL) to transform the benzaldehyde formed into benzoin, minimizing equilibrium limitations, and (iii) l-Glu as an amine donor with a double cascade comprising branched-chain α-amino acid aminotransferase (BCAT) and aspartate amino transferase (AspAT), both from E. coli, using l-Asp as a substrate to regenerate l-Glu. The γ-hydroxy-α-amino acids thus obtained were transformed into chiral α-amino-γ-butyrolactones, structural motifs found in many biologically active compounds and valuable intermediates for the synthesis of pharmaceutical agents.

Efficient single whole-cell biotransformation for L-2-aminobutyric acid production through engineering of leucine dehydrogenase combined with expression regulation

Leucine dehydrogenase (LDH) is widely used in the preparation of L-2-aminobutyric acid (L-2-ABA), however its wide application is limited by 2-ketobutyric acid (2-OBA) inhibition. Firstly, a novel high-throughput screening method of LDH was established, specific enzyme activity and 2-OBA tolerance of Lys72Ala mutant were 33.3% higher than those of the wild type. Subsequently, we constructed a single cell comprised of ivlA, Esldh_K72A, fdh and optimized expression through fine-tuning RBS intensity, so that the yield of E. coli BL21/pET28a-R3ivlA-Esldh_K72A-fdh was 2.6 times higher than that of the original strain. As a result, 150 g L-threonine was transformed to 121 g L-2-ABA in 5 L fermenter with 95% molar conversion rate, and a productivity of 5.04 g·L^-1·h^-1, which is the highest productivity of L-2-ABA currently reported by single-cell biotransformation. In summary, our research provided a green synthesis for L-2-ABA, which has potential for industrial production of drug precursors.

Recent advances in artificial enzyme cascades for the production of value-added chemicals

Enzyme cascades are efficient tools to perform multi-step synthesis in one-pot in a green and sustainable manner, enabling non-natural synthesis of valuable chemicals from easily available substrates by artificially combining two or more enzymes. Bioproduction of many high-value chemicals such as chiral and highly functionalised molecules have been achieved by developing new enzyme cascades. This review summarizes recent advances on engineering and application of enzyme cascades to produce high-value chemicals (alcohols, aldehydes, ketones, amines, carboxylic acids, etc) from simple starting materials. While 2-step enzyme cascades are developed for versatile enantioselective synthesis, multi-step enzyme cascades are engineered to functionalise basic chemicals, such as styrenes, cyclic alkanes, and aromatic compounds. New cascade reactions have also been developed for producing valuable chemicals from bio-based substrates, such as ʟ-phenylalanine, and renewable feedstocks such as glucose and glycerol. The challenges in current process and future outlooks in the development of enzyme cascades are also addressed.

Efficient biosynthesis of (2S, 3R)-4-methylsulfonylphenylserine by artificial self-assembly of enzyme complex combined with an intensified acetaldehyde elimination system

(2S, 3R)-4-methylsulfonylphenylserine [(2S, 3R)-MPS], a key chiral precursor for antibiotics florfenicol and thiamphenicol, could be asymmetrically synthesized by l-threonine transaldolase (LTTA) coupled with an acetaldehyde elimination system. The low efficiency of acetaldehyde elimination system blocked further accumulation of (2S, 3R)-MPS. To address this issue, strengthening acetaldehyde elimination system and enzyme self-assembly strategy were combined to accelerate biosynthesis of (2S, 3R)-MPS. The new multi-enzyme cascade with intensified acetaldehyde elimination system BL21 (PsLTTAD2/ScADH/BtGDH) could produce (2S, 3R)-MPS with a titer of 157.6 mM, 1.7-folds than that produced by the original system BL21 (PsLTTAD2/ApADH/CbFDH). Moreover, self-assembly of PsLTTAD2 and ScADH by respective fusion of SpyTag and SpyCatcher were carried out to develop a self-assembled multi-enzyme cascade BL21 (ST-PsLTTAD2/SC-ScADH/BtGDH). As a result, the yield of (2S, 3R)-MPS was up to 248.1 mM with 95% de. As far as we knew, that represented the highest yield of (2S, 3R)-MPS by enzymatic synthesis, and therefore was a promising and green route for industrial production of this valuable compound.

Co-immobilization of multi-enzyme on reversibly soluble polymers in cascade catalysis for the one-pot conversion of gluconic acid from corn straw

The difficulties in the process of cellulose cascade conversion based on immobilization technology lies in the recycling enzymes from rich solid-containing straw hydrolysate and the incompatibility of conventional immobilization with this process. In this study, three types of enzyme (cellulase, glucose oxidase and catalase) were successfully immobilized on a reversible soluble Eudragit L-100. Through the determination of the preparation conditions, enzymatic properties and catalytic conditions, the co-immobilized enzyme was applied to the catalytic reaction of one-pot conversion of corn straw to gluconic acid. The yield of gluconic acid achieved 0.28 mg/mg, conversion rate of cellulose in corn straw to gluconic acid reached 61.41%. The recovery of co-immobilized enzyme from solid substrate was achieved by using reversible and soluble characteristics of the carrier. After 6 times of recycling, the activity of co-immobilized enzyme was maintained at 52.38%, confirming the feasibility of multi-enzyme immobilization strategy using reversible soluble carrier in cascade reactions.

l-Threonine Transaldolase Activity Is Enabled by a Persistent Catalytic Intermediate

l-Threonine transaldolases (lTTAs) are a poorly characterized class of pyridoxal-5'-phosphate (PLP) dependent enzymes responsible for the biosynthesis of diverse β-hydroxy amino acids. Here, we study the catalytic mechanism of ObiH, an lTTA essential for biosynthesis of the β-lactone natural product obafluorin. Heterologously expressed ObiH purifies as a mixture of chemical states including a catalytically inactive form of the PLP cofactor. Photoexcitation of ObiH promotes the conversion of the inactive state of the enzyme to the active form. UV-vis spectroscopic analysis reveals that ObiH catalyzes the retro-aldol cleavage of l-threonine to form a remarkably persistent glycyl quinonoid intermediate, with a half-life of ∼3 h. Protonation of this intermediate is kinetically disfavored, enabling on-cycle reactivity with aldehydes to form β-hydroxy amino acids. We demonstrate the synthetic potential of ObiH via the single step synthesis of (2S,3R)-β-hydroxyleucine. To further understand the structural features underpinning this desirable reactivity, we determined the crystal structure of ObiH bound to PLP as the Schiff's base at 1.66 Å resolution. This high-resolution model revealed a unique active site configuration wherein the evolutionarily conserved Asp that traditionally H-bonds to the cofactor is swapped for a neighboring Glu. Molecular dynamics simulations combined with mutagenesis studies indicate that a structural rearrangement is associated with l-threonine entry into the catalytic cycle. Together, these data explain the basis for the unique reactivity of lTTA enzymes and provide a foundation for future engineering and mechanistic analysis.

Structure-guided evolution of a ketoreductase for efficient and stereoselective bioreduction of bulky α-amino β-keto esters

Chiral vicinal amino alcohols were generated with excellent stereoselectivity and high conversion from bulky α-amino β-keto esters by an engineered ketoreductase called M30.

Design of an efficient whole-cell biocatalyst for the production of hydroxyarginine based on a multi-enzyme cascade

3-Hydroxyarginine (3-OH-Arg) is an important intermediate for the synthesis of viomycin, an important antibiotic for the clinical treatment of tuberculosis. An efficient strategy for 3-OH-Arg production based on protein engineering and recombinant whole-cell biocatalysis was demonstrated for the first time. To avoid challenging product separation due to the generation of α-ketoglutarate (α-KG) in the system, the molar ratio of the substrates L-Arg and L-Glu was optimized to ensure the efficient production of 3-OH-Arg as well as the complete consumption of α-KG. Through the establishment of a fed-batch process, 3-OH-Arg and succinic acid (SA) production reached to 9.9 g/L and 5.98 g/L after 36 h of reaction under the optimized conditions. This is the highest biosynthetic yield of 3-OH-Arg achieved to date, potentially offering a promising strategy for commercial production of hydroxylated amino acids.

Construction and Application of PLP Self-sufficient Biocatalysis System for Threonine Aldolase

A number of organic synthesis involve threonine aldolase (TA), a pyridoxal phosphate (PLP)-dependent enzyme. Although the addition of exogenous PLP is necessary for the reactions, it increases the cost and complicates the purification of the product. This work constructed a PLP self-sufficient biocatalysis system for TA, which included an improvement of the intracellular PLP level and co-immobilization of TA with PLP. Engineered strain BL-ST was constructed by introducing PLP synthase PdxS/T to Escherichia coli BL21(ED3). The intracellular PLP concentration of the strain increased approximately fivefold to 48.5 μmol/g_DCW. l-TA, from Bacillus nealsonii (BnLTA), was co-expressed in the strain BL-ST with PdxS/T, resulting in the engineered strain BL-BnLTA-ST. Compared with the control strain BL-BnLTA (254.1 U/L), the enzyme activity of the strain BL-BnLTA-ST reached 1518.4 U/L without the addition of exogenous PLP. An efficient co-immobilization system was then designed. The epoxy resin LX-1000HFA wrapped by polyethyleneimine (PEI) acted as a carrier to immobilize the crude enzyme solution of the strain BL-BnLTA-ST mixed with an extra 100 μM of exogenous PLP, resulting in the catalyst HFA^PEI-BnLTA-STPLP 100. HFA^PEI-BnLTA-STPLP 100 exhibited a half-life of approximately 450 h, and the application of the catalyst in the continuous biosynthesis of 3-[4-(methylsulfonyl) phenyl] serine had more than 180 batch reactions (>60%_conv) without the extra addition of exogenous PLP. The excellent compatibility and stability of the system were further confirmed by other TAs. This work introduced a PLP self-sufficient biocatalysis system that can reduce the cost of PLP and contribute to the industrial application of TA. In addition, the system may also be applied in other PLP-dependent enzymes.