Synthesis of C-N bonds by nicotinamide-dependent oxidoreductase: an overview

Compounds containing chiral C-N bonds play a vital role in the composition of biologically active natural products and small pharmaceutical molecules. Therefore, the development of efficient and convenient methods for synthesizing compounds containing chiral C-N bonds is a crucial area of research. Nicotinamide-dependent oxidoreductases (NDOs) emerge as promising biocatalysts for asymmetric synthesis of chiral C-N bonds due to their mild reaction conditions, exceptional stereoselectivity, high atom economy, and environmentally friendly nature. This review aims to present the structural characteristics and catalytic mechanisms of various NDOs, including imine reductases/ketimine reductases, reductive aminases, EneIRED, and amino acid dehydrogenases. Additionally, the review highlights protein engineering strategies employed to modify the stereoselectivity, substrate specificity, and cofactor preference of NDOs. Furthermore, the applications of NDOs in synthesizing essential medicinal chemicals, such as noncanonical amino acids and chiral amine compounds, are extensively examined. Finally, the review outlines future perspectives by addressing challenges and discussing the potential of utilizing NDOs to establish efficient biosynthesis platforms for C-N bond synthesis. In conclusion, NDOs provide an economical, efficient, and environmentally friendly toolbox for asymmetric synthesis of C-N bonds, thus contributing significantly to the field of pharmaceutical chemical development.

Assembly and engineering of BioBricks to develop an efficient NADH regeneration system

The cofactor regeneration system plays a crucial role in redox biocatalysis for organic synthesis and the pharmaceutical industry. The alcohol dehydrogenase (ADH)-based regeneration system offers a promising solution for the in situ regeneration of NAD(P)H. However, its widespread use is hindered by low activity and poor expression of ADH in Escherichia coli. Herein, the BioBricks (promoter, ribosome binding site [RBS], functional gene, and terminator) were assembled and engineered to constitute an efficient NADH regeneration system. The semi-rational design was employed to enhance the catalytic efficiency of GstADH (an ADH from Geobacillus stearothermophilus), resulting in a beneficial GstADH variant with a 2.1-fold increase in catalytic efficiency. Furthermore, the RBS optimization was used to increase the expression of ADH genes, leading to the identification of an RBS with a 3.2-fold increased translation rate. Using this developed system, the NADH generating velocity reached more than 2 s-1 even toward 0.1 mM NAD+, indicating that it is the most promising NADH regeneration so far. Finally, the engineered system was utilized for the asymmetric biosynthesis of l-phosphinothricin (a chiral herbicide), with a high yield (>95%).

IMPORTANCE

The alcohol dehydrogenase (ADH)-based coenzyme regeneration system serves as a useful tool in redox biocatalysis. This system effectively replenishes NAD(P)H by utilizing isopropanol as a substrate, with the added advantage of easily separable acetone as a by-product. Previous studies focused on discovering new adh genes and engineering the ADH protein for higher catalytic efficiency, neglecting the optimization of other gene components. In this study, a remarkably efficient NADH regeneration system was developed using BioBricks assembly for system initialization. The ADH engineering was used to enhance catalytic efficiency, and RBS optimization for elevated ADH expression, which resulted in not only a 2.1-fold increase in catalytic efficiency but also a 3.2-fold increase in translation rate. Together, these improvements resulted in an overall 6.7-fold enhancement in performance. This system finds application in a wide range of NADH-dependent biocatalysis processes and is particularly advantageous for the biosynthesis of fine chemicals.

Enhancing Stability and Catalytic Activity of d-Allulose 3-Epimerase through Multistrategy Computational Design and Cross-Regional Advantageous Mutations

d-Allulose 3-epimerase (DAEase) derived from Clostridium bolteae has excellent properties in the catalytic production of d-allulose, a rare sugar with unique biological functions. However, the industrial application of C. bolteae DAEase (Cb-DAEase) for d-allulose production is hindered by its low enzyme activity, poor long-term thermostability, and pH tolerance. In this study, we identified potential noncatalytic residues in Cb-DAEase using methods such as proline substitution, surface charge engineering, and surface residue prediction. The effects of these residues were experimentally validated, followed by structural analysis, which led to the generation of multisite mutants through cross-regional structural combinations. The obtained mutant Cb-R2P-E6P-D137C showed 155.6% of the enzyme activity of the wild type, and the Kcat/Km increased by 1.3-fold, an elevated half-life of 15.7 min, and an elevated Tm value of 1.1 °C. The mutant Cb-R2P-E6P-A83D-D137C had 139.7% of the enzyme activity of the wild type, the Kcat/Km increased by 1.2-fold, with an elevated half-life of 12.3 min, an elevated Tm value of 0.8 °C, and maintained 68% of the enzyme activity at pH 5.0. The findings outlined a feasible approach for the rational design of multiple preset functions of target enzymes simultaneously.

Molecular evolution of nucleoside deoxyribosyl transferase to enhance the activity toward 2'-fluoro-2'-deoxynucleoside

Nucleoside deoxyribosyl transferase (NDT) is an enzyme that catalyzes the transfer of purine and pyrimidine bases between 2'-deoxyribonucleosides and is widely used for synthesizing nucleoside analogs in various biotechnological applications. While NDT exhibits high activity toward natural nucleosides, its activity toward unnatural nucleoside analogs is significantly lower. Previously, the NDT mutant named fNDT(L59Q) was identified displaying 4.4-fold higher activity toward 2'-fluoro-2'-deoxyuridine (2FDU). In this study, molecular evolution strategies using error-prone PCR were employed to further generate mutant enzymes with enhanced activity toward 2FDU. After two rounds of mutational screening, two mutant clones that exhibited high activity against 2FDU were identified as fNDT-i1 (V52A) and fNDT-i2 (L28I), respectively. A double mutant, fNDT-i4, was subsequently constructed by combining the V52A and L28I mutations. Whole-cell-based activity measurements showed that fNDT-i4 exhibited 4.0- and 20.6-fold higher activity at 40°C and 50°C, respectively, compared to the wild-type NDT. The detailed characterization of the purified enzymes conducted under various conditions, including temperature, pH, thermal stability, and enzyme kinetics experiments, showed that fNDT-i1 and fNDT-i4 exhibited 3.1- and 3.7-fold higher catalytic efficiency, respectively than wild-type NDT. The L59Q mutation was identified as a key factor in improving the thermal stability, whereas the V52A and L28I mutations were critical for improving substrate affinity and reaction efficiency. These findings provide the potential of fNDT-i1 and fNDT-i4 as highly efficient biocatalysts for developing industrially relevant nucleoside analog synthesis.

ONE-SENTENCE SUMMARY

The nucleoside deoxyribosyl transferase mutant were engineered to enhance biological activity and physical resistance for production of fluorinated deoxynucleoside as a raw material of oligonucleotide therapeutics.

Computer-Aided Flexible Loops Engineering of Glutamate Dehydrogenase for Asymmetric Synthesis of Chiral Pesticides l-phosphinothricin

The access to the enantiopure noncanonical amino acid l-phosphinothricin (l-PPT) by applying biocatalysts is highly appealing in organic chemistry. In this study, a NADH-dependent glutamate dehydrogenase from Lachnospiraceae bacterium (LbGluDH) was chosen for the asymmetric synthesis of l-PPT. Three flexible loops undergoing big conformational shifts during the catalysis were identified and rationally engineered following the initial mutagenesis. The enzyme's specific activity toward the key precursor of l-PPT, 2-oxo-4-[(hydroxy) (methyl) phosphinyl] butyric acid (PPO), was improved from negligible to 9 U/mg, and the Km value was reduced to 17 mM. The computational analysis showed that the modified loops broadened the enzyme's narrow tunnels, allowing the substrate to access the binding pocket and get closer to the crucial residue D165, thereby enhancing the catalytic process. Utilizing the variant as the catalyst, the preparation of l-PPT achieved a 100% conversion rate within 60 min, coupled with a stereoselectivity exceeding 99.9%, demonstrating its practical capacity for industrial application. Similar enhancement in catalytic activity was obtained applying the same strategy to a typical NADH-dependent GluDH from Pyrobaculum islandicum (PisGluDH), indicating the effectiveness of our strategy for the protein engineering of GluDHs targeted to the biosynthesis of unnatural compounds.

Efficient and easible biocatalysts: Strategies for enzyme improvement. A review

Owing to the environmental friendliness and vast advantages that enzymes offer in the biotechnology and industry fields, biocatalysts are a prolific investigation field. However, the low catalytic activity, stability, and specific selectivity of the enzyme limit the range of the reaction enzymes involved in. A comprehensive understanding of the protein structure and dynamics in terms of molecular details enables us to tackle these limitations effectively and enhance the catalytic activity by enzyme engineering or modifying the supports and solvents. Along with different strategies including computational, enzyme engineering based on DNA recombination, enzyme immobilization, additives, chemical modification, and physicochemical modification approaches can be promising for the wide spread of industrial enzyme usage. This is attributed to the successful application of biocatalysts in industrial and synthetic processes requires a system that exhibits stability, activity, and reusability in a continuous flow process, thereby reducing the production cost. The main goal of this review is to display relevant approaches for improving enzyme characteristics to overcome their industrial application.

Engineering substrate specificity of quinone-dependent dehydrogenases for efficient oxidation of deoxynivalenol to 3-keto-deoxynivalenol

The oxidative reaction of Fusarium mycotoxin deoxynivalenol (DON) using the dehydrogenase is a desirable strategy and environmentally friendly to mitigate its toxicity. However, a critical issue for these dehydrogenases shows widespread substrate promiscuity. In this study, we conducted pocket reshaping of Devosia strain A6-243 pyrroloquinoline quinone (PQQ)-dependent dehydrogenase (DADH) on the basis of protein structure and kinetic analysis of substrate libraries to improve preference for particular substrate DON (10a). The variant presented an increased preference for substrate 10a and enhanced catalytic efficiency. A 4.7-fold increase in preference for substrate 10a was observed. Kinetic profiling and molecular dynamics (MD) simulations provided insights into the enhanced substrate specificity and activity. Moreover, the variant exhibited stronger conversion of substrate 10a to 3-keto-DON compared to the wild DADH. Overall, this study provides a feasible protocol for the redesign of PQQ-dependent dehydrogenases with favourable substrate specificity and catalytic activity, which is desperately needed for DON antidote development.

Heterologous expression of a deacetylase and its application in L-glufosinate preparation

With potent herbicidal activity, biocatalysis synthesis of L-glufosinate has drawn attention. In present research, NAP-Das2.3, a deacetylase capable of stereoselectively resolving N-acetyl-L-glufosinate to L-glufosinate mined from Arenimonas malthae, was heterologously expressed and characterized. In Escherichia coli, NAP-Das2.3 activity only reached 0.25 U/L due to the formation of inclusive bodies. Efficient soluble expression of NAP-Das2.3 was achieved in Pichia pastoris. In shake flask and 5 L bioreactor fermentation, NAP-Das2.3 activity by recombinant P. pastoris reached 107.39 U/L and 1287.52 U/L, respectively. The optimum temperature and pH for N-acetyl-glufosinate hydrolysis by NAP-Das2.3 were 45 °C and pH 8.0, respectively. The Km and Vmax of NAP-Das2.3 towards N-acetyl-glufosinate were 25.32 mM and 19.23 μmol mg-1 min-1, respectively. Within 90 min, 92.71% of L-enantiomer in 100 mM racemic N-acetyl-glufosinate was converted by NAP-Das2.3. L-glufosinate with high optical purity (e.e.P above 99.9%) was obtained. Therefore, the recombinant NAP-Das2.3 might be an alternative for L-glufosinate biosynthesis.

A novel NADH-dependent leucine dehydrogenase for multi-step cascade synthesis of L-phosphinothricin

L-Phosphinothricin (L-PPT) is the effective constituent in racemic PPT (a high-efficiency and broad-spectrum herbicide), and the exploitation of green and sustainable synthesis route for L-PPT has always been the focus in pesticide industry. In recent years, "one-pot, two-step" enzyme-mediated cascade strategy is a mainstream pathway to obtain L-PPT. Herein, RgDAAO and BsLeuDH were applied to expand "one-pot, two-step" process. Notably, a NADH-dependent leucine dehydrogenase from Bacillus subtilis (BsLeuDH) was firstly characterized and attempted to generate L-PPT, achieving an excellent enantioselectivity (99.9% ee). Meanwhile, a formate dehydrogenase from Pichia pastoris (PpFDH) was utilized to implement NADH cofactor regeneration and only CO₂ was by-product. Sufficient amount of the corresponding keto acid precursor PPO was obtained by oxidation of D-PPT relying on a D-amino acid oxidase from Rhodotorula gracilis (RgDAAO) with content conversion (46.1%). L-PPT was ultimately prepared from racemized PPT via oxidative deamination catalyzed by RgDAAO and reductive amination catalyzed by BsLeuDH, achieving 80.3% overall yield and > 99.9% ee value.

Rational design of enzyme activity and enantioselectivity

The strategy of rational design to engineer enzymes is to predict the potential mutants based on the understanding of the relationships between protein structure and function, and subsequently introduce the mutations using the site-directed mutagenesis. Rational design methods are universal, relatively fast and have the potential to be developed into algorithms that can quantitatively predict the performance of the designed sequences. Compared to the protein stability, it was more challenging to design an enzyme with improved activity or selectivity, due to the complexity of enzyme molecular structure and inadequate understanding of the relationships between enzyme structures and functions. However, with the development of computational force, advanced algorithm and a deeper understanding of enzyme catalytic mechanisms, rational design could significantly simplify the process of engineering enzyme functions and the number of studies applying rational design strategy has been increasing. Here, we reviewed the recent advances of applying the rational design strategy to engineer enzyme functions including activity and enantioselectivity. Five strategies including multiple sequence alignment, strategy based on steric hindrance, strategy based on remodeling interaction network, strategy based on dynamics modification and computational protein design are discussed and the successful cases using these strategies are introduced.

Substrate-Specific Engineering of Amino Acid Dehydrogenase Superfamily for Synthesis of a Variety of Chiral Amines and Amino Acids

Amino acid dehydrogenases (AADHs) are a group of enzymes that catalyze the reversible reductive amination of keto acids with ammonia to produce chiral amino acids using either nicotinamide adenine dinucleotide (NAD+) or nicotinamide adenine dinucleotide phosphate (NADP+) as cofactors. Among them, glutamate dehydrogenase, valine dehydrogenase, leucine dehydrogenase, phenylalanine dehydrogenase, and tryptophan dehydrogenase have been classified as a superfamily of amino acid dehydrogenases (s-AADHs) by previous researchers because of their conserved structures and catalytic mechanisms. Owing to their excellent stereoselectivity, high atom economy, and low environmental impact of the reaction pathway, these enzymes have been extensively engineered to break strict substrate specificities for the synthesis of high value-added chiral compounds (chiral amino acids, chiral amines, and chiral amino alcohols). Substrate specificity engineering of s-AADHs mainly focuses on recognition engineering of the substrate side chain R group and substrate backbone carboxyl group. This review summarizes the reported studies on substrate specificity engineering of s-AADHs and reports that this superfamily of enzymes shares substrate specificity engineering hotspots (the inside of the pocket, substrate backbone carboxyl anchor sites, substrate entrance tunnel, and hinge region), which sheds light on the substrate-specific tailoring of these enzymes.

Expression of L-phosphinothricin synthesis enzymes in Pichia pastoris for synthesis of L-phosphinothricin

With the ban of highly toxic herbicides, such as paraquat and glyphosate, phosphinothricin (PPT) is becoming the most popular broad-spectrum and highly effective herbicide. The current PPT products in the market are usually a racemic mixture with two configurations, the D-type and L-type, of which only the L-PPT has the herbicidal activity. The racemic product is not atom economic, more toxic and may cause soil damage. Asymmetric synthesis of L-PPT has become a research focus in recent years, while biological synthesis methods are preferred for its character of environmental friendly and requiring less reaction steps when being compared to the chemical methods. We have developed a biological synthesis route to produce optically pure L-PPT from D,L-PPT in two steps using 2-carbonyl-4- (hydroxymethyl phosphonyl) butyric acid as the intermediate. In this study, we expressed the glutamate dehydrogenase and glucose dehydrogenase using Pichia pastoris as the first time. After a series of optimization, the total L-PPT yield reached 84%. The developed synthesis system showed a high potential for future industrial application. Compare to the previous plasmid-carrying-E. coli expression system, the established method may avoid antibiotic usage and provided an alternative way for industrial synthesis of optically pure L-PPT.

Combined active pocket and hinge region engineering to develop an NADPH-dependent phenylglycine dehydrogenase

NADPH-dependent amino acid dehydrogenases (AADHs) are favorable enzymes to construct artificial biosynthetic pathways in whole-cell for high-value noncanonical amino acids (NcAAs) production. Glutamate dehydrogenases (GluDHs) represent attractive candidates for the development of novel NADPH-dependent AADHs. Here, we report the development of a novel NADPH-dependent phenylglycine dehydrogenase by combining active pocket engineering and hinge region engineering of a GluDH from Pseudomonas putida (PpGluDH). The active pocket of PpGluDH was firstly tailored to optimize its binding mode with bulky substrate α-oxobenzeneacetic acid (α-OA), and then, the hinge region was further engineered to tune the protein conformational dynamics, which finally resulted in a mutant M3 (T196A/T121I/L123D) with a 103-fold increase of catalytic efficiency (k_cat/K_m) toward α-OA. The M3 mutant exhibited high catalytic performance in both in vitro biocatalysis preparation and in vivo biosynthesis of l-phenylglycine, indicating its promising practical applications. Our results demonstrated that co-engineering of the active pocket and hinge region is an effective strategy for developing novel NADPH-dependent AADHs from GluDHs for NcAAs production.

Structure-guided steric hindrance engineering of Bacillus badius phenylalanine dehydrogenase for efficient L-homophenylalanine synthesis

BACKGROUND

Direct reductive amination of prochiral 2-oxo-4-phenylbutyric acid (2-OPBA) catalyzed by phenylalanine dehydrogenase (PheDH) is highly attractive in the synthesis of the pharmaceutical chiral building block L-homophenylalanine (L-HPA) given that its sole expense is ammonia and that water is the only byproduct. Current issues in this field include a poor catalytic efficiency and a low substrate loading.

RESULTS

In this study, we report a structure-guided steric hindrance engineering of PheDH from Bacillus badius to create an enhanced biocatalyst for efficient L-HPA synthesis. Mutagenesis libraries based on molecular docking, double-proximity filtering, and a degenerate codon significantly increased catalytic efficiency. Seven superior mutants were acquired, and the optimal triple-site mutant, V309G/L306V/V144G, showed a 12.7-fold higher k_cat value, and accordingly a 12.9-fold higher k_cat/K_m value, than that of the wild type. A paired reaction system comprising V309G/L306V/V144G and glucose dehydrogenase converted 1.08 M 2-OPBA to L-HPA in 210 min, and the specific space-time conversion was 30.9 mmol g^-1 L^-1 h^-1. The substrate loading and specific space-time conversion are the highest values to date. Docking simulation revealed increases in substrate-binding volume and additional degrees of freedom of the substrate 2-OPBA in the pocket. Tunnel analysis suggested the formation of new enzyme tunnels and the expansion of existing ones.

CONCLUSIONS

Overall, the results show that the mutant V309G/L306V/V144G has the potential for the industrial synthesis of L-HPA. The modified steric hindrance engineering approach can be a valuable addition to the current enzyme engineering toolbox.

Cascaded Biocatalysis and Bioelectrocatalysis: Overview and Recent Advances

Enzyme cascades are plentiful in nature, but they also have potential in artificial applications due to the possibility of using the target substrate in biofuel cells, electrosynthesis, and biosensors. Cascade reactions from enzymes or hybrid bioorganic catalyst systems exhibit extended substrate range, reaction depth, and increased overall performance. This review addresses the strategies of cascade biocatalysis and bioelectrocatalysis for ( a) CO2 fixation, ( b) high value-added product formation, ( c) sustainable energy sources via deep oxidation, and ( d) cascaded electrochemical enzymatic biosensors. These recent updates in the field provide fundamental concepts, designs of artificial electrocatalytic oxidation-reduction pathways (using a flexible setup involving organic catalysts and engineered enzymes), and advances in hybrid cascaded sensors for sensitive analyte detection.

Simultaneous directed evolution of coupled enzymes for efficient asymmetric synthesis of l-phosphinothricin

The traditional strategy to improve the efficiency of an entire coupled enzyme system relies on separate direction of the evolution of enzymes involved in their respective enzymatic reactions. This strategy can lead to enhanced single-enzyme catalytic efficiency but may also lead to loss of coordination among enzymes. This study aimed to overcome such shortcomings by executing a directed evolution strategy on multiple enzymes in one combined group that catalyzes the asymmetric biosynthesis of l-phosphinothricin. The genes of a glutamate dehydrogenase from Pseudomonas moorei (PmGluDH) and a glucose dehydrogenase from Exiguobacterium sibiricum (EsGDH), along with other gene parts (promoters, ribosomal binding sites (RBSs), and terminators) were simultaneously evolved. The catalytic efficiency of PmGluDH was boosted by introducing the beneficial mutation A164G (from 1.29 s^-1mM^-1 to 183.52 s^-1mM^-1), and the EsGDH expression level was improved by optimizing the linker length between the RBS and the start codon of gdh. The total turnover numbers of the bioreaction increased from 115 (GluDH WT^NADPH) to 5846 (A164G^NADPH coupled with low expression of EsGDH), and to 33950 (A164G^NADPH coupled with high expression of EsGDH). The coupling efficiency was increased from ∼30% (GluDH_WT with low expression of GDH) to 83.3% (GluDH_A164G with high expression of GDH). In the batch production of l-phosphinothricin utilizing whole-cell catalysis, the strongest biocatalytic reaction exhibited a high space-time yield (6410 g·L^-1·d^-1) with strict stereoselectivity (>99% enantiomeric excess).Importance: The traditional strategy to improve multienzyme-catalyzed reaction efficiency may lead to enhanced single-enzyme catalytic efficiency but may also result in loss of coordination among enzymes. We describe a directed evolution strategy of an entire coupled enzyme system to simultaneously enhance enzyme coordination and catalytic efficiency. The simultaneous evolution strategy was applied to a multienzyme-catalyzed reaction for the asymmetric synthesis of l-phosphinothricin, which not only enhanced the catalytic efficiency of GluDH but also improved the coordination between GluDH and GDH. Since this strategy is enzyme-independent, it may be applicable to other coupled enzyme systems for chiral chemical synthesis.

Rational engineering of Acinetobacter tandoii glutamate dehydrogenase for asymmetric synthesis of l-homoalanine through biocatalytic cascades

A high yield of l-homoalanine can be obtained by an engineered dual cofactor-dependent GluDH in a cascade without the addition of NAD(P)H.

Highly Efficient Chemoenzymatic Synthesis of l-Phosphinothricin from N-Phenylacetyl-d,l-phosphinothricin by a Robust Immobilized Amidase

A chemoenzymatic strategy was developed for the highly efficient synthesis of l-phosphinothricin employing a robust immobilized amidase. An enzymatic hydrolysis of 500 mM N-phenylacetyl-d,l-phosphinothricin resulted in 49.9% conversion and 99.9% ee of l-phosphinothricin within 6 h. To further evaluate the bioprocess for l-phosphinothricin production, the biotransformation was performed for 100 batches under a stirred tank reactor with an average productivity of 8.21 g L^-1 h^-1. Moreover, unreacted N-phenylacetyl-d-phosphinothricin was racemized and subjected to the enzymatic hydrolysis, giving l-phosphinothricin with a 22.3% yield. A total yield of 69.4% was achieved after one recycle of N-phenylacetyl-d-phosphinothricin. Significantly, this chemoenzymatic approach shows great potential in the industrial production of l-phosphinothricin.

A Single-Transaminase-Catalyzed Biocatalytic Cascade for Efficient Asymmetric Synthesis of l-Phosphinothricin

A single-transaminase-catalyzed biocatalytic cascade was developed by employing the desired biocatalyst, ATA-117-Rd11, that showed high activity toward 2-oxo-4-[(hydroxy)(methyl)phosphinoyl] butyric acid (PPO) and α-ketoglutarate, and low activity against pyruvate. The cascade successfully promotes a highly asymmetric amination reaction for the synthesis of l-phosphinothricin (l-PPT) with high conversion (>95 %) and>99 % ee. In a scale-up experiment, using 10 kg pre-frozen E. coli cells harboring ATA-117-Rd11 as catalyst, 80 kg PPO was converted to ≈70 kg l-PPT after 24 hours with a high ee value (>99 %).

Semi-rational hinge engineering: modulating the conformational transformation of glutamate dehydrogenase for enhanced reductive amination activity towards non-natural substrates

The hinge region was identified to be a promising hotspot for activity engineering of GluDHs, providing a potent alternative for developing high-performance biocatalysts toward valuable optically pure l-amino acid production.

Design and engineering of whole-cell biocatalytic cascades for the valorization of fatty acids

This review presents the key factors to construct a productive whole-cell biocatalytic cascade exemplified for the biotransformation of renewable fatty acids.

Efficient synthesis of L-phosphinothricin using a novel aminoacylase mined from Stenotrophomonas maltophilia

L-phosphinothricin (L-PPT) is a competitive and environmentally friendly herbicide. To develop an efficient approach for synthesis of l-PPT, a kinetic resolution route with a novel aminoacylase (SmAcy) mined from Stenotrophomonas maltophilia using N-acetyl-PPT as a substrate was first constructed. This SmAcy exhibited high hydrolytic activity and excellent enantioselectivity (E > 200) toward N-acetyl-PPT. Optically pure l-PPT (> 99.9 % ee_p) was acquired with high conversion (> 49 %) within 4 h by the whole cells. On the basis of the docking analysis, a main reason for high enantioselectivity (E > 200) of SmAcy towards l-enantiomer would be that the D-N-acetyl-PPT cannot interact with the key general acid-base residue and the metal ions. A low-cost and simple preparation process of the substrate from commercially available racemic PPT for production of L-PPT was provided. A chemical racemization method of the unreacted D-enantiomer of substrate was also provided to recycle the unwanted substrate enantiomer. This study provides a potential route for the industrial production of L-PPT.

Identification and characterization of an amidase from Leclercia adecarboxylata for efficient biosynthesis of L-phosphinothricin

L-phosphinothricin (L-PPT) is an important broad-spectrum herbicide with expanding utilization because it is environmentally benign. A strain Leclercia adecarboxylata ZJB-17008 with capability of catalyzing rac-4-(hydroxy(methyl)phosphoryl)-2-(2-phenylacetamido) butanoic acid (rac-S) to L-PPT was screened and identified, from which an amidase (La-Ami) was cloned and secretory expressed in Bacillus subtilis WB 800 for the bioproduction of L-PPT. The recombinant La-Ami exhibited an excellent enantioselectivity (99.9% ee) and remarkable thermostability with a half-life of 19.8 h at 50 °C. Furthermore, La-Ami displaying a high space-time yield of 787.2 g L^-1 d^-1 at 50 °C and pH 8.5 under the rac-S concentration of 500 mM (150 g L^-1). The finally refined L-PPT was obtained with a purity of 99% and a total yield reached 90%. These results implying that this secretory expressed amidase La-Ami is possible to be applied in the large-scale bioproduction of L-PPT.

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.