Engineering of an N-acetylneuraminic acid synthetic pathway in Escherichia coli

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.

One-pot biosynthesis of N-acetylneuraminic acid from chitin via combination of chitin-degrading enzymes, N-acetylglucosamine-2-epimerase, and N-neuraminic acid aldolase

N-acetylneuraminic acid (Neu5Ac) possesses the ability to promote mental health and enhance immunity and is widely used in both medicine and food fields as a supplement. Enzymatic production of Neu5Ac using N-acetyl-D-glucosamine (GlcNAc) as substrate was significant. However, the high-cost GlcNAc limited its development. In this study, an in vitro multi-enzyme catalysis was built to produce Neu5Ac using affordable chitin as substrate. Firstly, exochitinase SmChiA from Serratia proteamaculans and N-acetylglucosaminosidase CmNAGase from Chitinolyticbacter meiyuanensis SYBC-H1 were screened and combined to produce GlcNAc, effectively. Then, the chitinase was cascaded with N-acetylglucosamine-2-epimerase (AGE) and N-neuraminic acid aldolase (NanA) to produce Neu5Ac; the optimal conditions of the multi-enzyme catalysis system were 37°C and pH 8.5, the ratio of AGE to NanA (1:4) and addition of pyruvate (70 mM), respectively. Finally, 9.2 g/L Neu5Ac could be obtained from 20 g/L chitin within 24 h along with two supplementations with pyruvate. This work will lay a good foundation for the production of Neu5Ac from cheap chitin resources.

Engineering Cupriavidus necator H16 for heterotrophic and autotrophic production of myo-inositol

Bioconversion of sustainable feedstocks to commodity chemicals is considered as an effective solution for transforming the fossil-based economy into a carbon-neutral model. Here, the CO₂-fixing bacterium Cupriavidus necator H16 was exploited for myo-inositol production from renewable substrates. First, by introducing the glucose transportation system, the glucose consumption route was established. Second, two key enzymes involved in myo-inositol biosynthesis were screened and evaluated. A myo-inositol-producing strain was constructed via overexpression of myo-inositol-3-phosphate synthase from Saccharomyces cerevisiae and inositol monophosphatase from Escherichia coli. Finally, carbon flux redirection was achieved through disruption of Entner-Doudoroff pathway and poly(3-hydroxybutyrate) synthesis pathway, resulting in a final myo-inositol production of 520.2, 1076.3 and 1054.8 mg/L from glucose, glycerol and CO₂, respectively. The myo-inositol production level from CO₂ achieved here set up the record. This study underlines the potential of C. necator to be utilized as microbial factory for upcycling the renewable feedstocks and CO₂ to high-value biochemicals.

Metabolic Engineering of Escherichia coli for Increased Bioproduction of N-Acetylneuraminic Acid

N-Acetylneuraminic acid (NeuAc) is widely used in the food and pharmaceutical industries. Therefore, it is important to develop an efficient and eco-friendly method for NeuAc production. Here, we achieved de novo biosynthesis of NeuAc in an engineered plasmid-free Escherichia coli strain, which efficiently synthesizes NeuAc using glycerol as the sole carbon source, via clustered regularly interspaced palindromic repeat (CRISPR)/CRISPR-associated protein 9-based genome editing. NeuAc key precursor, N-acetylmannosamine (ManNAc; 0.40 g/L), was produced by expressing UDP-N-acetylglucosamine-2-epimerase and glucosamine-6-phosphate synthase (GlmS) mutants and blocking the NeuAc catabolic pathway in E. coli BL21 (DE3). The expression levels of GlmM and GlmU-GlmS_A metabolic modules were optimized, significantly increasing the ManNAc titer to 8.95 g/L. Next, the expression levels of NeuAc synthase from different microorganisms were optimized, leading to the production of 6.27 g/L of NeuAc. Blocking the competing pathway of NeuAc biosynthesis increased the NeuAc titer to 9.65 g/L. In fed-batch culture in a 3 L fermenter, NeuAc titer reached 23.46 g/L with productivity of 0.69 g/L/h, which is the highest level achieved by microbial synthesis using glycerol as the sole carbon source in E. coli. The strategies used in our study can aid in the efficient bioproduction of NeuAc and its derivatives.

Enhancement of Edwardsiella piscicida infection, biofilm formation, and motility caused by N-acetylneuraminate lyase

Sialic acid and its catabolism are involved in bacterial pathogenicity. N-acetylneuraminate lyase (NAL), which catalyzes the reversible aldol cleavage of sialic acid to form N-acetyl-D-mannosamine in the first step of sialic acid degradation, has been recently investigated to elucidate whether NAL enhances bacterial virulence; however, the role of NAL in bacterial pathogenicity remains unclear. In the present study, we demonstrated that the existence of two enzymes in Edwardsiella piscicida, referred to as dihydrodipicolinate synthase (DHDPS) and NAL, induced the cleavage/condensation activity toward sialic acids such as N-acetylneuraminic acid, N-glycolylneuraminic acid and 3-deoxy-D-glycero-D-galacto-non-2-ulopyranosonic acid. NAL enhanced cellular infection in vitro and suppressed the survival rate in zebrafish larvae in bath-infection in vivo, whereas DHDPS did not. Furthermore, NAL strongly activated the expression of E. piscicida phenotypes such as biofilm formation and motility, whereas DHDPS did not. Besides, the gene expression level of nanK, nanE, and glmU were up-regulated in the NAL-overexpressing strain, along with an increase in the total amount of N-acetylglucosamine.

Engineering of Synthetic Multiplexed Pathways for High-Level N-Acetylneuraminic Acid Bioproduction

N-Acetylneuraminic acid (NeuAc) is widely used as a supplement to promote brain health and enhance immunity. However, the low efficiency of de novo NeuAc synthesis limits its cost-efficient bioproduction. Herein, a synthetic multiplexed pathway engineering (SMPE) strategy is proposed to improve NeuAc synthesis. First, we compare the key enzyme sources and optimize the expression levels of three NeuAc synthesis pathways in Bacillus subtilis; the AGE, NeuC, and NanE pathways, for which NeuAc production reached 3.94, 5.67, and 0.19 g/L, respectively. Next, these synthesis pathways were combined and modularly optimized via the SMPE strategy, with production reaching 7.87 g/L. Finally, fed-batch fermentation in a 5 L fermenter reached 30.10 g/L NeuAc production, the highest reported production using glucose as the sole carbon source. Using a generally regarded as safe strain as a production host, the developed NeuAc-producing approach should be favorable for efficient bioproduction, without the need for plasmids, antibiotics, or chemical inducers.

Designing and Development of FRET-Based Nanosensor for Real Time Analysis of N-Acetyl-5-Neuraminic Acid in Living Cells

N-acetyl-5-neuraminic acid (NeuAc) plays crucial role in improving the growth, brain development, brain health maintenance, and immunity enhancement of infants. Commercially, it is used in the production of antiviral drugs, infant milk formulas, cosmetics, dietary supplements, and pharmaceutical products. Because of the rapidly increasing demand, metabolic engineering approach has attracted increasing attention for NeuAc biosynthesis. However, knowledge of metabolite flux in biosynthetic pathways is one of the major challenges in the practice of metabolic engineering. So, an understanding of the flux of NeuAc is needed to determine its cellular level at real time. The analysis of the flux can only be performed using a tool that has the capacity to measure metabolite level in cells without affecting other metabolic processes. A Fluorescence Resonance Energy Transfer (FRET)-based genetically-encoded nanosensor has been generated in this study to monitor the level of NeuAc in prokaryotic and eukaryotic cells. Sialic acid periplasmic binding protein (SiaP) from Haemophilus influenzae was exploited as a sensory element for the generation of nanosensor. The enhanced cyan fluorescent protein (ECFP) and Venus were used as Fluroscence Resonance Energy Transfer (FRET) pair. The nanosensor, which was termed fluorescent indicator protein for sialic acid (FLIP-SA), was successfully transformed into, and expressed in Escherichia coli BL21 (DE3) cells. The expressed protein of the nanosensor was isolated and purified. The purified nanosensor protein was characterized to assess the affinity, specificity, and stability in the pH range. The developed nanosensor exhibited FRET change after addition to NeuAc. The developed nanosensor was highly specific, exhibited pH stability, and detected NeuAc levels in the nanomolar to milimolar range. FLIP-SA was successfully introduced in bacterial and yeast cells and reported the real-time intracellular levels of NeuAc non-invasively. The FLIP-SA is an excellent tool for the metabolic flux analysis of the NeuAc biosynthetic pathway and, thus, may help unravel the regulatory mechanism of the metabolic pathway of NeuAc. Furthermore, FLIP-SA can be used for the high-throughput screening of E. coli mutant libraries for varied NeuAc production levels.

Metabolic Engineering of Escherichia coli for Lacto-N-triose II Production with High Productivity

Lacto-N-triose II (LNT II), a core structural unit of human milk oligosaccharides (HMOs), has attracted substantial attention for its nutraceutical potentials and applications in the production of complex HMOs. In this study, Escherichia coli was metabolically engineered to efficiently produce LNT II using glycerol as a carbon source and lactose as a substrate. The UDP-N-acetylglucosamine (UDP-GlcNAc) biosynthesis pathway was strengthened, and β-1,3-N-acetylglucosaminyltransferase (LgtA) was introduced to construct an LNT II-producing base strain. To increase the titer and yield of LNT II, combinatorial optimization of the copy number and the ribosomal binding site sequence was performed to tune the gene expression strength and translation rates of the pathway enzymes. Next, multipoint mutations were introduced to glucosamine-6-phosphatesynthase (GlmS) to relieve the feedback inhibition. Then, a series of engineered strains were constructed by blocking the futile pathways by the deletion of the relevant genes. Finally, the culture conditions were optimized. LNT II titer was improved step-by-step from 0.53 to 5.52 g/L in shake-flask cultivations. Fed-batch culture of the final engineered strain produced 46.2 g/L of LNT II, with an LNT II productivity and content of 0.77 g/(L·h) and 0.95 g/g dry cell weight, respectively.

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.

An overview and future prospects of sialic acids

Sialic acids (Sias) are negatively charged functional monosaccharides present in a wide variety of natural sources (plants, animals and microorganisms). Sias play an important role in many life processes, which are widely applied in the medical and food industries as intestinal antibacterials, antivirals, anti-oxidative agents, food ingredients, and detoxification agents. Most Sias are composed of N-acetylneuraminic acid (Neu5Ac, >99%), and Sia is its most commonly used name. In this article, we review Sias in terms of their structures, applications, determination methods, metabolism, and production strategies. In particular, we summarise and compare different production strategies, including extraction from natural sources, chemical synthesis, polymer decomposition, enzymatic synthesis, whole-cell catalysis, and de novo biosynthesis via microorganism fermentation. We also discuss research on their physiological functions and applications, barriers to efficient production, and strategies for overcoming these challenges. We focus on efficient de novo biosynthesis strategies for Neu5Ac via microbial fermentation using novel synthetic biology tools and methods that may be applied in future. This work provides a comprehensive overview of recent advances on Sias, and addresses future challenges regarding their functions, applications, and production.

Degradation Kinetics and Shelf Life of N-acetylneuraminic Acid at Different pH Values

The objective of this study was to investigate the stability and degradation kinetics of N-acetylneuraminic acid (Neu5Ac). The pH of the solution strongly influenced the stability of Neu5Ac, which was more stable at neutral pH and low temperatures. Here, we provide detailed information on the degradation kinetics of Neu5Ac at different pH values (1.0, 2.0, 11.0 and 12.0) and temperatures (60, 70, 80 and 90 °C). The study of the degradation of Neu5Ac under strongly acidic conditions (pH 1.0–2.0) is highly pertinent for the hydrolysis of polysialic acid. The degradation kinetics of alkaline deacetylation were also studied. Neu5Ac was highly stable at pH 3.0–10.0, even at high temperature, but the addition of H₂O₂ greatly reduced its stability at pH 5.0, 7.0 and 9.0. Although Neu5Ac has a number of applications in products of everyday life, there are no reports of rigorous shelf-life studies. This research provides kinetic data that can be used to predict product shelf lives at different temperatures and pH values.

Exploring Amino Sugar and Phosphoenolpyruvate Metabolism to Improve Escherichia coli N-Acetylneuraminic Acid Production

N-acetyl-d-neuraminic acid (NeuAc) has attracted considerable attention because of its wide-ranging applications. The use of cheap carbon sources such as glucose without the addition of any precursor in microbial NeuAc production has many advantages. In this study, improved NeuAc production was attained through the optimization of amino sugar metabolism pathway kinetics and reservation of a phosphoenolpyruvate (PEP) pool in Escherichia coli. N-acylglucosamine 2-epimerase and N-acetylneuraminate synthase from different sources and their best combinations were used to obtain optimized enzyme kinetics and expression intensity, which resulted in a significant increase in NeuAc production. Next, after a design was engineered for enabling the PEP metabolic pathway to retain the PEP pool, the production of NeuAc reached 16.7 g/L, which is the highest NeuAc production rate that has been reported from using glucose as the sole carbon source.

The elucidation of phosphosugar stress response in Bacillus subtilis guides strain engineering for high N-acetylglucosamine production

Bacillus subtilis is a preferred microbial host for the industrial production of nutraceuticals and a promising candidate for the synthesis of functional sugars, such as N-acetylglucosamine (GlcNAc). Previously, a GlcNAc-overproducer B. subtilis SFMI was constructed using glmS ribozyme dual-regulatory tool. Herein, we further engineered to enhance carbon flux from glucose towards GlcNAc synthesis. As a result, the increased flux towards GlcNAc synthesis triggered phosphosugar stress response, which caused abnormal cell growth. Unfortunately, the mechanism of phosphosugar stress response had not been elucidated in B. subtilis. To reveal the stress mechanism and overcome its negative effect in bioproduction, we performed comparative transcriptome analysis. The results indicate that cells slow glucose utilization by repression of glucose import and accelerate catabolic reactions of phosphosugar. To verify these results, we overexpressed the phosphatase YwpJ, which relieved phosphosugar stress and allowed us to identify the enzyme responsible for GlcNAc synthesis from GlcNAc 6-phosphate. In addition, the deletion of nagBB and murQ, responsible for GlcNAc precursor degradation, further improved GlcNAc synthesis. The best engineered strain, B. subtilis FMIP34, increased GlcNAc titer from 11.5 to 26.1 g/L in shake flasks and produced 87.5 g/L GlcNAc in 30-L fed-batch bioreactor. Our results not only elucidate, for the first time, the phosphosugar stress response mechanism in B. subtilis, but also demonstrate how the combination of rational metabolic engineering with novel insights into physiology and metabolism allows the construction of highly efficient microbial cell factories for the production of high-value chemicals.

Enhancement of Production of D-Glucosamine in Escherichia coli by Blocking Three Pathways Involved in the Consumption of GlcN and GlcNAc

D-Glucosamine is a commonly used dietary supplement that promotes cartilage health in humans. Metabolic flux analysis showed that D-glucosamine production could be increased by blocking three pathways involved in the consumption of glucosamine-6-phosphate and acetylglucosamine-6-phosphate. By homologous single-exchange, two key genes (nanE and murQ) of Escherichia coli BL21 were knocked out, respectively. The D-glucosamine yields of the engineered strains E. coli BL21ΔmurQ and E. coli BL21ΔnanE represented increases by factors of 2.14 and 1.79, respectively. Meanwhile, for bifunctional gene glmU, we only knocked out its glucosamine-1-phosphate acetyltransferase domain by 3D structural analysis to keep the engineered strain E. coli BL21glmU-Δgpa survival, which resulted in an increase in the production of D-glucosamine by a factor of 2.16. Moreover, for further increasing D-glucosamine production, two genes encoding rate-limiting enzymes, named glmS and gna1, were coexpressed by an RBS sequence in those engineered strains. The total concentrations of D-glucosamine in E. coli BL21 glmU-Δgpa', E. coli BL21ΔmurQ', and E. coli BL21ΔnanE' were 2.65 g/L, 1.73 g/L, and 1.38 g/L, which represented increases by factors of 8.83, 5.76, and 3.3, respectively.

CRISPR/Cas9-Assisted Seamless Genome Editing in Lactobacillus plantarum and Its Application in N-Acetylglucosamine Production

Lactobacillus plantarum is a potential starter and health-promoting probiotic bacterium. Effective, precise, and diverse genome editing of Lactobacillus plantarum without introducing exogenous genes or plasmids is of great importance. In this study, CRISPR/Cas9-assisted double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) recombineering was established in L. plantarum WCFS1 to seamlessly edit the genome, including gene knockouts, insertions, and point mutations. To optimize our editing method, phosphorothioate modification was used to improve the dsDNA insertion, and adenine-specific methyltransferase was used to improve the ssDNA recombination efficiency. These strategies were applied to engineer L. plantarum WCFS1 toward producing N-acetylglucosamine (GlcNAc). nagB was truncated to eliminate the reverse reaction of fructose-6-phosphate (F6P) to glucosamine 6-phosphate (GlcN-6P). Riboswitch replacement and point mutation in glmS1 were introduced to relieve feedback repression. The resulting strain produced 797.3 mg/liter GlcNAc without introducing exogenous genes or plasmids. This strategy may contribute to the available methods for precise and diverse genetic engineering in lactic acid bacteria and boost strain engineering for more applications.IMPORTANCE CRISPR/Cas9-assisted recombineering is restricted in lactic acid bacteria because of the lack of available antibiotics and vectors. In this study, a seamless genome editing method was carried out in Lactobacillus plantarum using CRISPR/Cas9-assisted double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) recombineering, and recombination efficiency was effectively improved by endogenous adenine-specific methyltransferase overexpression. L. plantarum WCFS1 produced 797.3 mg/liter N-acetylglucosamine (GlcNAc) through reinforcement of the GlcNAc pathway, without introducing exogenous genes or plasmids. This seamless editing strategy, combined with the potential exogenous GlcNAc-producing pathway, makes this strain an attractive candidate for industrial use in the future.

Metabolic characterization of the chitinolytic bacterium Serratia marcescens using a genome-scale metabolic model

Background

Serratia marcescens is a chitinolytic bacterium that can potentially be used for consolidated bioprocessing to convert chitin to value-added chemicals. Currently, S. marcescens is poorly characterized and studies on intracellular metabolic and regulatory mechanisms would expedite development of bioprocessing applications.

Results

In this study, our goal was to characterize the metabolic profile of S. marcescens to provide insight for metabolic engineering applications and fundamental biological studies. Hereby, we constructed a constraint-based genome-scale metabolic model (iSR929) including 929 genes, 1185 reactions and 1164 metabolites based on genomic annotation of S. marcescens Db11. The model was tested by comparing model predictions with experimental data and analyzed to identify essential aspects of the metabolic network (e.g. 138 essential genes predicted). The model iSR929 was refined by integrating RNAseq data of S. marcescens growth on three different carbon sources (glucose, N-acetylglucosamine, and glycerol). Significant differences in TCA cycle utilization were found for growth on the different carbon substrates, For example, for growth on N-acetylglucosamine, S. marcescens exhibits high pentose phosphate pathway activity and nucleotide synthesis but low activity of the TCA cycle.

Conclusions

Our results show that S. marcescens model iSR929 can provide reasonable predictions and can be constrained to fit with experimental values. Thus, our model may be used to guide strain designs for metabolic engineering to produce chemicals such as 2,3-butanediol, N-acetylneuraminic acid, and n-butanol using S. marcescens.

Electronic supplementary material

The online version of this article (10.1186/s12859-019-2826-1) contains supplementary material, which is available to authorized users.

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.

Protein engineering of a bacterial N-acyl-d-glucosamine 2-epimerase for improved stability under process conditions

Enzymatic cascade reactions, i.e. the combination of several enzyme reactions in one pot without isolation of intermediates, have great potential for the establishment of sustainable chemical processes. However, many cascade reactions suffer from cross-inhibitions and enzyme inactivation by components of the reaction system. This study focuses on the two-step enzymatic synthesis of N-acetylneuraminic acid (Neu5Ac) using an N-acyl-d-glucosamine 2-epimerase from Anabaena variabilis ATCC 29413 (AvaAGE) in combination with an N-acetylneuraminate lyase (NAL) from Escherichia coli. AvaAGE epimerizes N-acetyl-d-glucosamine (GlcNAc) to N-acetyl-d-mannosamine (ManNAc), which then reacts with pyruvate in a NAL-catalyzed aldol condensation to form Neu5Ac. However, AvaAGE is inactivated by high pyruvate concentrations, which are used to push the NAL reaction toward the product side. A biphasic inactivation was observed in the presence of 50-800mM pyruvate resulting in activity losses of the AvaAGE of up to 60% within the first hour. Site-directed mutagenesis revealed that pyruvate modifies one of the four lysine residues in the ATP-binding site of AvaAGE. Because ATP is an allosteric activator of the epimerase and the binding of the nucleotide is crucial for its catalytic properties, saturation mutagenesis at position K160 was performed to identify the most compatible amino acid exchanges. The best variants, K160I, K160N and K160L, showed no inactivation by pyruvate, but significantly impaired kinetic parameters. For example, depending on the mutant, the turnover number kcat was reduced by 51-68% compared with the wild-type enzyme. A mechanistic model of the Neu5Ac synthesis was established, which can be used to select the AvaAGE variant that is most favorable for a given process condition. The results show that mechanistic models can greatly facilitate the choice of the right enzyme for an enzymatic cascade reaction with multiple cross-inhibitions and inactivation phenomena.

High-level soluble expression of a bacterial N-acyl-d-glucosamine 2-epimerase in recombinant Escherichia coli

N-Acyl-d-glucosamine 2-epimerase (AGE) is an important enzyme for the biocatalytic synthesis of N-acetylneuraminic acid (Neu5Ac). Due to the wide range of biological applications of Neu5Ac and its derivatives, there has been great interest in its large-scale synthesis. Thus, suitable strategies for achieving high-level production of soluble AGE are needed. Several AGEs from various organisms have been recombinantly expressed in Escherichia coli. However, the soluble expression level was consistently low with an excessive formation of inclusion bodies. In this study, the effects of different solubility-enhancement tags, expression temperatures, chaperones and host strains on the soluble expression of the AGE from the freshwater cyanobacterium Anabaena variabilis ATCC 29413 (AvaAGE) were examined. The optimum combination of tag, expression temperature, co-expression of chaperones and host strain (His6-tag, 37°C, GroEL/GroES, E. coli BL21(DE3)) led to a 264-fold improvement of the volumetric epimerase activity, a measure of the soluble expression, compared to the starting conditions (His6-maltose-binding protein-tag, 20°C, without chaperones, E. coli BL21(DE3)). A maximum yield of 22.5mg isolated AvaAGE per liter shake flask culture was obtained.

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.

Growth media simulating ileal and colonic environments affect the intracellular proteome and carbon fluxes of enterohemorrhagic Escherichia coli O157:H7 strain EDL933

In this study, the intracellular proteome of Escherichia coli O157:H7 strain EDL933 was analyzed by two-dimensional gel electrophoresis and matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) spectrometry after growth in simulated ileal environment media (SIEM) and simulated colonic environment media (SCEM) under aerobic and microaerobic conditions. Differentially expressed intracellular proteins were identified and allocated to functional protein groups. Moreover, metabolic fluxes were analyzed by isotopologue profiling with [U-(13)C(6)]glucose as a tracer. The results of this study show that EDL933 responds with differential expression of a complex network of proteins and metabolic pathways, reflecting the high metabolic adaptability of the strain. Growth in SIEM and SCEM is obviously facilitated by the upregulation of nucleotide biosynthesis pathway proteins and could be impaired by exposition to 50 µM 6-mercaptopurine under aerobic conditions. Notably, various stress and virulence factors, including Shiga toxin, were expressed without having contact with a human host.