An efficient method for N-acetyl-d-neuraminic acid production using coupled bacterial cells with a safe temperature-induced system

Recent advances on N-acetylneuraminic acid: Physiological roles, applications, and biosynthesis

N-Acetylneuraminic acid (Neu5Ac), the most common type of Sia, generally acts as the terminal sugar in cell surface glycans, glycoconjugates, oligosaccharides, lipo-oligosaccharides, and polysaccharides, thus exerting numerous physiological functions. The extensive applications of Neu5Ac in the food, cosmetic, and pharmaceutical industries make large-scale production of this chemical desirable. Biosynthesis which is associated with important application potential and environmental friendliness has become an indispensable approach for large-scale synthesis of Neu5Ac. In this review, the physiological roles of Neu5Ac was first summarized in detail. Second, the safety evaluation, regulatory status, and applications of Neu5Ac were discussed. Third, enzyme-catalyzed preparation, whole-cell biocatalysis, and microbial de novo synthesis of Neu5Ac were comprehensively reviewed. In addition, we discussed the main challenges of Neu5Ac de novo biosynthesis, such as screening and engineering of key enzymes, identifying exporters of intermediates and Neu5Ac, and balancing cell growth and biosynthesis. The corresponding strategies and systematic strategies were proposed to overcome these challenges and facilitate Neu5Ac industrial-scale production.

Constraint-based metabolic control analysis for rational strain engineering

The advancements in genome editing techniques over the past years have rekindled interest in rational metabolic engineering strategies. While Metabolic Control Analysis (MCA) is a well-established method for quantifying the effects of metabolic engineering interventions on flows in metabolic networks and metabolite concentrations, it does not consider the physiological limitations of the cellular environment and metabolic engineering design constraints. We report here a constraint-based framework, Network Response Analysis (NRA), for rational genetic strain design. NRA is cast as a Mixed-Integer Linear Programming problem that integrates MCA, Thermodynamically-based Flux Analysis (TFA), biologically relevant constraints, as well as genome editing restrictions into a comprehensive platform for identifying metabolic engineering targets. We show that the NRA formulation and its core constraints are equivalent to the ones of Flux Balance Analysis (FBA) and TFA, which allows it to be used for a wide range of optimization criteria and with various physiological constraints. We also show how the parametrization and introduction of biological constraints enhance the NRA formulation compared to the classical MCA approach, and we demonstrate its features and its ability to generate multiple alternative optimal strategies given several user-defined boundaries and objectives. In summary, NRA is a sophisticated alternative to classical MCA for rational metabolic engineering that accommodates the incorporation of physiological data at metabolic flux, metabolite concentration, and enzyme expression levels.

Biocatalytic routes to anti-viral agents and their synthetic intermediates

An assessment of biocatalytic strategies for the synthesis of anti-viral agents, offering guidelines for the development of sustainable production methods for a future COVID-19 remedy.

Application of a Pyruvate-Producing Escherichia coli Strain LAFCPCPt-accBC-aceE: A Case Study for d-Lactate Production

Pyruvate, a potential precursor of various chemicals, is one of the fundamental chemicals produced by the fermentation process. We previously reported a pyruvate-producing Escherichia coli strain LAFCPCPt-accBC-aceE (PYR) that has the potential to be applied to the industrial production of pyruvate. In this study, the availability of the PYR strain for the production of pyruvate-derivative chemicals was evaluated using a d-lactate-producing strain (LAC) based on the PYR strain. The LAC strain expresses a d-lactate dehydrogenase-encoding gene from Lactobacillus bulgaricus under the control of a T7 expression system. The d-lactate productivity of the LAC strain was further improved by limiting aeration and changing the induction period for the expression of d-lactate dehydrogenase-encoding gene expression. Under combined conditions, the LAC strain produced d-lactate at 21.7 ± 1.4 g·L−1, which was compatible with the pyruvate production by the PYR strain (26.1 ± 0.9 g·L−1). These results suggest that we have succeeded in the effective conversion of pyruvate to d-lactate in the LAC strain, demonstrating the wide versatility of the parental PYR strain as basal strain for various chemicals production.

In vivo evolutionary engineering of riboswitch with high-threshold for N-acetylneuraminic acid production

Riboswitches with desired properties, such as sensitivity, threshold, dynamic range, is important for its application. However, the property change of a natural riboswitch is difficult due to the lack of the understanding of aptamer ligand binding properties and a proper screening method for both rational and irrational design. In this study, an effective method to change the threshold of riboswitch was established in vivo based on growth coupled screening by combining both positive and negative selections. The feasibility of the method was verified by the model library. Using this method, an N-acetylneuraminic acid (NeuAc) riboswitch was evolved and modified riboswitches with high threshold and large dynamic range were obtained. Then, using a new NeuAc riboswitch, both ribosome binding sites and key gene in NeuAc biosynthesis pathway were optimized. The highest NeuAc production of 14.32 g/l that has been reported using glucose as sole carbon source was obtained.

Production of N-Acetyl-d-neuraminic Acid by Whole Cells Expressing Bacteroides thetaiotaomicron N-Acetyl-d-glucosamine 2-Epimerase and Escherichia coli N-Acetyl-d-neuraminic Acid Aldolase

N-Acetyl-d-neuraminic acid (Neu5Ac) is a potential baby nutrient and the key precursor of antiflu medicine Zanamivir. The Neu5Ac chemoenzymatic synthesis consists of N-acetyl-d-glucosamine epimerase (AGE)-catalyzed epimerization of N-acetyl-d-glucosamine (GlcNAc) to N-acetyl-d-mannosamine (ManNAc) and aldolase-catalyzed condensation between ManNAc and pyruvate. Herein, we cloned and characterized BT0453, a novel AGE, from a human gut symbiont Bacteroides thetaiotaomicron. BT0453 shows the highest soluble fraction among the AGEs tested. With GlcNAc and sodium pyruvate as substrates, Neu5Ac production by coupling whole cells expressing BT0453 and Escherichia coli N-acetyl-d-neuraminic acid aldolase was explored. After 36 h, a 53.6% molar yield, 3.6 g L^-1 h^-1 productivity and 42.9 mM titer of Neu5Ac were obtained. Furthermore, for the first time, the T7- BT0453-T7- nanA polycistronic unit was integrated into the E. coli genome, generating a chromosome-based biotransformation system. BT0453 protein engineering and metabolic engineering studies hold potential for the industrial production of Neu5Ac.

Production of N-acetyl-D-neuraminic Acid by Recombinant Single Whole Cells Co-expressing N-acetyl-D-glucosamine-2-epimerase and N-acetyl-D-neuraminic Acid Aldolase

N-acetyl-D-neuraminic acid (Neu5Ac) is a costly precursor for many drugs such as anti-influenza antivirals. In a previous study, a whole-cell process for Neu5Ac production was developed using a combination of two Escherichia coli cells expressing Anabaena sp. CH1 N-acetyl-D-glucosamine-2-epimerase (bage) and E. coli N-acetyl-D-neuraminic acid aldolase (nanA), respectively. In this study, we constructed a bAGE and NanA co-expression system to improve Neu5Ac production. Two recombinant E. coli strains, E. coli BL21 (DE3) pET-bage-nanA (HA) and E. coli BL21 (DE3) pET-bage-2nanA (HAA), synchronously expressing bAGE and NanA were used as biocatalysts to generate Neu5Ac from N-acetyl-D-glucosamine (GlcNAc) and pyruvate. The HA biocatalysts produced 187.5 mM Neu5Ac within 8 h. The yield of GlcNAc was 15.6%, and the Neu5Ac production rate was 7.25 g/L/h. The most active HAA biocatalysts generated 412.6 mM Neu5Ac and a GlcNAc yield of 34.4%. HAA achieved a Neu5Ac production rate of 15.9 g/L/h, which surpassed those for all reported Neu5Ac production processes so far. The present study demonstrates that using recombinant E. coli cells synchronously expressing bAGE and NanA as biocatalysts could potentially be used in the industrial mass production of Neu5Ac.

Conversion of glucose-xylose mixtures to pyruvate using a consortium of metabolically engineered Escherichia coli

Two strains of Escherichia coli were engineered to accumulate pyruvic acid from two sugars found in lignocellulosic hydrolysates by knockouts in the aceE, ppsA, poxB, and ldhA genes. Additionally, since glucose and xylose are typically consumed sequentially due to carbon catabolite repression in E. coli, one strain (MEC590) was engineered to grow only on glucose while a second strain (MEC589) grew only on xylose. On a single substrate, each strain generated pyruvate at a yield of about 0.60 g/g in both continuous culture and batch culture. In a glucose-xylose mixture under continuous culture, a consortium of both strains maintained a pyruvate yield greater than 0.60 g/g when three different concentrations of glucose and xylose were sequentially fed into the system. In a fed-batch process, both sugars in a glucose-xylose mixture were consumed simultaneously to accumulate 39 g/L pyruvate in less than 24 h at a yield of 0.59 g/g.

In Vivo Multienzyme Complex Coconstruction of N-Acetylneuraminic Acid Lyase and N-Acetylglucosamine-2-epimerase for Biosynthesis of N-Acetylneuraminic Acid

Metabolic channeling enables efficient transfer of the intermediates by forming a multienzyme complex. To leverage the metabolic channeling for improved biosynthesis, we coexpressed N-acetylneuraminic acid lyase from C. glutamicum ATCC 13032 (CgNal) and N-acetylglucosamine-2-epimerase from Anabaena sp. CH1 (anAGE) in Escherichia coli and used the whole cell to synthesize N-acetylneuraminic acid (Neu5Ac) from N-acetylglucosamine (GlcNAc) and pyruvate. To get the multienzyme complex, polycistronic plasmid with high levels of CgNal and anAGE expression was constructed by tuning the orders of the genes. The Shine-Dalgarno (SD) sequence and aligned spacing (AS) distance were optimized. The E. coli Rosetta harboring the polycistronic plasmid pET-28a-SD₂-AS₁-CgNal-SD-AS-anAGE increased the production of Neu5Ac by 58.7% to 92.5 g/L in 36 h by whole-cell catalysis and by 21.9% up to 112.8 g/L in 24 h with the addition of Triton X-100.

Pathway optimization and key enzyme evolution of N-acetylneuraminate biosynthesis using an in vivo aptazyme-based biosensor

N-acetylneuraminate (NeuAc) biosynthesis has drawn much attention owing to its wide applications in many aspects. Previously, we engineered for the first time an artificial NeuAc biosynthetic pathway in Escherichia coli using glucose as sole substrate. However, rigorous requirements for the flux and cofactor balance make subsequent strain improvement rather difficult. In this study, an in vivo NeuAc biosensor was designed and applied for genetic screening the mutant library of NeuAc producer. Its NeuAc responsive manner was demonstrated using sfgfp as a reporter and a Ni²⁺-based selection system was developed to couple the cell growth with in vivo NeuAc concentration. Employing this selection system, the NeuAc biosynthesis pathway was optimized and the key enzyme NeuAc synthase was evolved, which improved the titer by 34% and 23%, respectively. The final strain produced up to 8.31g/L NeuAc in minimal medium using glucose as sole carbon source. This work demonstrated the effectiveness of NeuAc biosensor in genetic screening and great potential in metabolic engineering of other organisms.

Whole-cell biocatalysts by design

Whole-cell biocatalysts provide unique advantages and have been widely used for the efficient biosynthesis of value-added fine and bulk chemicals, as well as pharmaceutically active ingredients. What is more, advances in synthetic biology and metabolic engineering, together with the rapid development of molecular genetic tools, have brought about a renaissance of whole-cell biocatalysis. These rapid advancements mean that whole-cell biocatalysts can increasingly be rationally designed. Genes of heterologous enzymes or synthetic pathways are increasingly being introduced into microbial hosts, and depending on the complexity of the synthetic pathway or the target products, they can enable the production of value-added chemicals from cheap feedstock. Metabolic engineering and synthetic biology efforts aimed at optimizing the existing microbial cell factories concentrate on improving heterologous pathway flux, precursor supply, and cofactor balance, as well as other aspects of cellular metabolism, to enhance the efficiency of biocatalysts. In the present review, we take a critical look at recent developments in whole-cell biocatalysis, with an emphasis on strategies applied to designing and optimizing the organisms that are increasingly modified for efficient production of chemicals.

Pyruvate production using engineered Escherichia coli

Pyruvate plays an essential role in the central carbon metabolism of multiple organisms and is used as a raw material in the chemical, biochemical and pharmaceutical industries. To meet demand, large amounts of pyruvate are produced through fermentation processes. Here we describe a simple and efficient method for producing pyruvate in Escherichia coli. To stop carbon flux from pyruvate to fatty acids, the accBC genes, which encode the enzyme that catalyzes the first step of fatty acid biosynthesis and is essential for vegetative growth, were manipulated within the genome; its native promoter was replaced with the tetracycline (or doxycycline)-regulated promoter and the corresponding transcriptional regulator genes. The resulting strain grew normally in the presence of doxycycline, but showed poor growth upon withdrawal of doxycycline. Using this strain, we developed a high pyruvate producing strain (strain LAFCPCPt-accBC-aceE), in which the tetracycline-regulated promoter was also introduced upstream of aceE, and the ackA-pta, adhE, cra, ldhA, pflB and poxB genes were deleted. After determining the optimal culture conditions for this strain, the final pyruvate concentration reached 26.1 g L^-1 after 72 h with a theoretical yield of 55.6 %. These levels are high enough to indicate that the developed strain has the potential for application to industrial production of pyruvate.

Production of α-ketobutyrate using engineered Escherichia coli via temperature shift

Alpha-ketobutyrate has been widely used in medicine and food additive industry. Because chemical and enzymatic methods are associated with many deficiencies, the recent focus shifted to fermentation for the production of α-ketobutyrate. In this study, a genetically engineered strain THRDΔrhtAΔilvIH/pWSK29-ilvA was constructed, starting from an L-threonine-producing strain, by overexpressing threonine dehydratase (TD), reducing α-ketobutyrate catabolism and L-threonine export. The shake flask cultivation of THRDΔrhtAΔilvIH/pWSK29-ilvA allowed the production of 16.2 g/L α-ketobutyrate. Accumulation of α-ketobutyrate severely inhibited the cell growth. To develop a better TD expression system and avoid the usage of the expensive inducer IPTG, a temperature-induced plasmid pBV220-ilvA was selected to transform the strain THRDΔrhtAΔilvIH for α-ketobutyrate production. The initial temperature was maintained at 35°C to guarantee normal cell growth, and then elevated to 40°C to induce the expression of TD. Under optimized conditions, the α-ketobutyrate titer reached 40.8 g/L after 28 h of fermentation, with a productivity of 1.46 g/L/h and a yield of 0.19 g/g glucose, suggesting large-scale production potential. Biotechnol. Bioeng. 2016;113: 2054-2059. © 2016 Wiley Periodicals, Inc.

Characterization of a novel N-acetylneuraminic acid lyase favoring industrial N-acetylneuraminic acid synthesis

N-Acetylneuraminic acid lyase (NAL, E.C. number 4.1.3.3) is a Class I aldolase that catalyzes the reversible aldol cleavage of N-acetylneuraminic acid (Neu5Ac) from pyruvate and N-acetyl-D-mannosamine (ManNAc). Due to the equilibrium favoring Neu5Ac cleavage, the enzyme catalyzes the rate-limiting step of two biocatalytic reactions producing Neu5Ac in industry. We report the biochemical characterization of a novel NAL from a “GRAS” (General recognized as safe) strain C. glutamicum ATCC 13032 (CgNal). Compared to all previously reported NALs, CgNal exhibited the lowest k_cat/K_m value for Neu5Ac and highest k_cat/K_m values for ManNAc and pyruvate, which makes CgNal favor Neu5Ac synthesis the most. The recombinant CgNal reached the highest expression level (480 mg/L culture), and the highest reported yield of Neu5Ac was achieved (194 g/L, 0.63 M). All these unique properties make CgNal a promising biocatalyst for industrial Neu5Ac biosynthesis. Additionally, although showing the best Neu5Ac synthesis activity among the NAL family, CgNal is more related to dihydrodipicolinate synthase (DHDPS) by phylogenetic analysis. The activities of CgNal towards both NAL's and DHDPS' substrates are fairly high, which indicates CgNal a bi-functional enzyme. The sequence analysis suggests that CgNal might have adopted a unique set of residues for substrates recognition.

New N-acyl-D-glucosamine 2-epimerases from cyanobacteria with high activity in the absence of ATP and low inhibition by pyruvate

N-Acetylneuraminic acid, an important component of glycoconjugates with various biological functions, can be produced from N-acetyl-D-glucosamine (GlcNAc) and pyruvate using a one-pot, two-enzyme system consisting of N-acyl-D-glucosamine 2-epimerase (AGE) and N-acetylneuraminate lyase (NAL). In this system, the epimerase catalyzes the conversion of GlcNAc into N-acetyl-D-mannosamine (ManNAc). However, all currently known AGEs have one or more disadvantages, such as a low specific activity, substantial inhibition by pyruvate and strong dependence on allosteric activation by ATP. Therefore, four novel AGEs from the cyanobacteria Acaryochloris marina MBIC 11017, Anabaena variabilis ATCC 29413, Nostoc sp. PCC 7120, and Nostoc punctiforme PCC 73102 were characterized. Among these enzymes, the AGE from the Anabaena strain showed the most beneficial characteristics. It had a high specific activity of 117±2 U mg(-1) at 37 °C (pH 7.5) and an up to 10-fold higher inhibition constant for pyruvate as compared to other AGEs indicating a much weaker inhibitory effect. The investigation of the influence of ATP revealed that the nucleotide has a more pronounced effect on the Km for the substrate than on the enzyme activity. At high substrate concentrations (≥200 mM) and without ATP, the enzyme reached up to 32% of the activity measured with ATP in excess.

Construction and expression of a polycistronic plasmid encoding N-acetylglucosamine 2-epimerase and N-acetylneuraminic acid lyase simultaneously for production of N-acetylneuraminic acid

Synthesis of N-acetylneuraminic acid (Neu5Ac) from N-acetylglucosamine (GlcNAc) and pyruvate was carried out by constructing and expressing a polycistronic plasmid encoding an N-acetylglucosamine 2-epimerase (AGE) gene and an N-acetylneuraminic acid lyase (Nal) gene simultaneously. Nal from Escherichia coli K12 and AGEs from Synechocystis sp. PCC 6803 (snAGE) and Anabaena sp. CH1 (anAGE) were used. And four polycistronic plasmids were constructed in which the positions of AGE gene differed with respect to Nal gene. Among these plasmids, pET-28a-Nal-anAGE with anAGE gene located next to Nal gene caused the production of the highest amount of Neu5Ac, generating 61.3g/L in 60h by whole-cell catalysis without the addition of ATP as AGE activator. And pET-28a-Nal-anAGE lowered anAGE's expression level, allowing it to fold properly. Thus, an inclusion-body-free E. coli strain capable of producing Neu5Ac by whole-cell catalysis with high yield and low cost was constructed in the present study.

One-pot bio-synthesis: N-acetyl-D-neuraminic acid production by a powerful engineered whole-cell catalyst

Whole cell biocatalysis is an important tool for pharmaceutical intermediates synthesis, although it is hindered by some shortcomings, such as high cost and toxicity of inducer, mass transfer resistance caused by cell membrane and side reactions. Whole-cell catalysis using N-acetyl-d-glucosamine 2-epimerase (EC 5.1.3.8) and N-acetyl-d-neuraminic acid (Neu5Ac) aldolase (EC 4.1.3.3) is a promising approach for the production of Neu5Ac, a potential precursor of many anti-viral drugs. A powerful catalyst was developed by packaging the enzymes in an engineered bacterium and using a safe temperature-induced vector. Since the mass transfer resistance and the side reactions were substantially reduced, a high Neu5Ac amount (191 mM) was achieved. An efficient method was also presented, which allows one-pot synthesis of Neu5Ac with a safe and economic manner. The results highlight the promise of large-scale Neu5Ac synthesis and point at a potential of our approach as a general strategy to improve whole-cell biocatalysis.

Chemoenzymatic synthesis of N-acetyl-D-neuraminic acid from N-acetyl-D-glucosamine by using the spore surface-displayed N-acetyl-D-neuraminic acid aldolase

Chemoenzymatic synthesis of N-acetyl-d-neuraminic acid from N-acetyl-d-glucosamine using the spore surface-displayed N-acetyl-d-neuraminic acid aldolase at a high concentration (53.9 g liter(-1)) was achieved in this study. Thus, displaying a target enzyme on the surface of spores might be an alternative for integration of biocatalytic conversion into chemical synthesis.

Production of N-acetyl-D-neuraminic acid by use of an efficient spore surface display system

Production of N-acetyl-D-neuraminic acid (Neu5Ac) via biocatalysis is traditionally conducted using isolated enzymes or whole cells. The use of isolated enzymes is restricted by the time-consuming purification process, whereas the application of whole cells is limited by the permeability barrier presented by the microbial cell membrane. In this study, a novel type of biocatalyst, Neu5Ac aldolase presented on the surface of Bacillus subtilis spores, was used for the production of Neu5Ac. Under optimal conditions, Neu5Ac at a high concentration (54.7 g liter⁻¹) and a high yield (90.2%) was obtained under a 5-fold excess of pyruvate over N-acetyl-D-mannosamine. The novel biocatalyst system, which is able to express and immobilize the target enzyme simultaneously on the surface of B. subtilis spores, represents a suitable alternative for value-added chemical production.