KfoA, the UDP-glucose-4-epimerase of Escherichia coli strain O5:K4:H4, shows preference for acetylated substrates

Reprogramming Metabolic Flux in Escherichia Coli to Enhance Chondroitin Production

Reprogramming metabolic flux is a promising approach for constructing efficient microbial cell factories (MCFs) to produce chemicals. However, how to boost the transmission efficiency of metabolic flux is still challenging in complex metabolic pathways. In this study, metabolic flux is systematically reprogrammed by regulating flux size, flux direction, and flux rate to build an efficient MCF for chondroitin production. The ammoniation pool for UDP-GalNAc synthesis and the carbonization pool for UDP-GlcA synthesis are first enlarged to increase flux size for providing enough precursors for chondroitin biosynthesis. Then, the ammoniation pool and the carbonization pool are rematched using molecular valves to shift flux direction from cell growth to chondroitin biosynthesis. Next, the adaptability of polymerization pool with the ammoniation and carbonization pools is fine-tuned by dynamic and static valve-based adapters to accelerate flux rate for polymerizing UDP-GalNAc and UDP-GlcA to produce chondroitin. Finally, the engineered strain E. coli F51 is able to produce 9.2 g L-1 chondroitin in a 5-L bioreactor. This strategy shown here provides a systematical approach for regulating metabolic flux in complex metabolic pathways for efficient biosynthesis of chemicals.

A Review of Chondroitin Sulfate's Preparation, Properties, Functions, and Applications

Chondroitin sulfate (CS) is a natural macromolecule polysaccharide that is extensively distributed in a wide variety of organisms. CS is of great interest to researchers due to its many in vitro and in vivo functions. CS production derives from a diverse number of sources, including but not limited to extraction from various animals or fish, bio-synthesis, and fermentation, and its purity and homogeneity can vary greatly. The structural diversity of CS with respect to sulfation and saccharide content endows this molecule with distinct complexity, allowing for functional modification. These multiple functions contribute to the application of CS in medicines, biomaterials, and functional foods. In this article, we discuss the preparation of CS from different sources, the structure of various forms of CS, and its binding to other relevant molecules. Moreover, for the creation of this article, the functions and applications of CS were reviewed, with an emphasis on drug discovery, hydrogel formation, delivery systems, and food supplements. We conclude that analyzing some perspectives on structural modifications and preparation methods could potentially influence future applications of CS in medical and biomaterial research.

Chromosome evolution of Escherichia coli Nissle 1917 for high-level production of heparosan

Heparosan is a crucial-polysaccharide precursor for the chemoenzymatic synthesis of heparin, a widely used anticoagulant drug. Presently, heparosan is mainly extracted with the potential risk of contamination from Escherichia coli strain K5, a pathogenic bacterium causing urinary tract infection. Here, a nonpathogenic probiotic, E. coli strain Nissle 1917 (EcN), was metabolically engineered to carry multiple copies of the 19-kb kps locus and produce heparosan to 9.1 g/L in fed-batch fermentation. Chromosome evolution driven by antibiotics was employed to amplify the kps locus, which governed the synthesis and export of heparosan from EcN at 21 mg L^-1  OD^-1 . The average copy number of kps locus increased from 1 to 24 copies per cell, which produced up to 104 mg L^-1  OD^-1 of heparosan in the shaking flask cultures of engineered strains. The following in-frame deletion of recA stabilized the recombinant duplicates of chromosomal kps locus and the productivity of heparosan in continuous culture for at least 56 generations. Fed-batch fermentation of the engineered strain EcN8 was carried out to bring the yield of heparosan up to 9.1 g/L. Heparosan from the fermentation culture was further purified at a 75% overall recovery. The structure of purified heparosan was characterized and further modified by N-sulfotransferase with 3'-phosphoadenosine-5'-phosphosulfate as the sulfo-donor. The analysis of element composition showed that heparosan was N-sulfated by over 80%. These results indicated that duplicating large DNA cassettes up to 19-kb, followed by high-cell-density fermentation, was promising in the large-scale preparation of chemicals and could be adapted to engineer other industrial-interest bacteria metabolically.

Simultaneous production of N-acetylheparosan and recombinant chondroitin using gene-engineered Escherichia coli K5

N-Acetylheparosan and chondroitin are increasingly needed as alternative sources of animal-derived sulfated glycosaminoglycans (GAGs) and as inert substances in medical devices and pharmaceuticals. The N-acetylheparosan productivity of E. coli K5 has achieved levels of industrial applications, whereas E.coli K4 produces a relatively lower amount of fructosylated chondroitin. In this study, the K5 strain was gene-engineered to co-express K4-derived, chondroitin-synthetic genes, namely kfoA and kfoC. The productivities of total GAG and chondroitin in batch culture were 1.2 g/L and 1.0 g/L respectively, which were comparable to the productivity of N-acetylheparosan in the wild K5 strain (0.6-1.2 g/L). The total GAG of the recombinant K5 was partially purified by DEAE-cellulose chromatography and was subjected to degradation tests with specific GAG-degrading enzymes combined with HPLC and 1H NMR analyses. The results indicated that the recombinant K5 simultaneously produced both 100-kDa chondroitin and 45-kDa N-acetylheparosan at a weight ratio of approximately 4:1. The content of chondroitin in total GAG partially purified was 73.2%. The molecular weight of recombinant chondroitin (100 kDa) was 5-10 times higher than that of commercially available chondroitin sulfate. These results indicated that the recombinant K5 strain acquired the chondroitin-producing ability without altering the total GAG productivity of the host.

Imaging of Escherichia coli K5 and glycosaminoglycan precursors via targeted metabolic labeling of capsular polysaccharides in bacteria

The introduction of unnatural chemical moieties into glycosaminoglycans (GAGs) has enormous potential to facilitate studies of the mechanism and application of these critical, widespread molecules. Unnatural N-acetylhexosamine analogs were metabolically incorporated into the capsule polysaccharides of Escherichia coli and Bacillus subtilis via bacterial metabolism. Targeted metabolic labeled hyaluronan and the precursors of heparin and chondroitin sulfate were obtained. The azido-labeled polysaccharides (purified or in capsules) were reacted with dyes, via bioorthogonal chemistry, to enable detection and imaging. Site-specific introduction of fluorophores directly onto cell surfaces affords another choice for observing and quantifying bacteria in vivo and in vitro. Furthermore, azido-polysaccharides retain similar biological properties to their natural analogs, and reliable and predictable introduction of functionalities, such as fluorophores, onto azido-N-hexosamines in the disaccharide repeat units provides chemical tools for imaging and metabolic analysis of GAGs in vivo and in vitro.

Cell-based high-throughput screening of polysaccharide biosynthesis hosts

Valuable polysaccharides are usually produced using wild-type or metabolically-engineered host microbial strains through fermentation. These hosts act as cell factories that convert carbohydrates, such as monosaccharides or starch, into bioactive polysaccharides. It is desirable to develop effective in vivo high-throughput approaches to screen cells that display high-level synthesis of the desired polysaccharides. Uses of single or dual fluorophore labeling, fluorescence quenching, or biosensors are effective strategies for cell sorting of a library that can be applied during the domestication of industrial engineered strains and metabolic pathway optimization of polysaccharide synthesis in engineered cells. Meanwhile, high-throughput screening strategies using each individual whole cell as a sorting section are playing growing roles in the discovery and directed evolution of enzymes involved in polysaccharide biosynthesis, such as glycosyltransferases. These enzymes and their mutants are in high demand as tool catalysts for synthesis of saccharides in vitro and in vivo. This review provides an introduction to the methodologies of using cell-based high-throughput screening for desired polysaccharide-biosynthesizing cells, followed by a brief discussion of potential applications of these approaches in glycoengineering.

Complete biosynthesis of a sulfated chondroitin in Escherichia coli

Sulfated glycosaminoglycans (GAGs) are a class of important biologics that are currently manufactured by extraction from animal tissues. Although such methods are unsustainable and prone to contamination, animal-free production methods have not emerged as competitive alternatives due to complexities in scale-up, requirement for multiple stages and cost of co-factors and purification. Here, we demonstrate the development of single microbial cell factories capable of complete, one-step biosynthesis of chondroitin sulfate (CS), a type of GAG. We engineer E. coli to produce all three required components for CS production–chondroitin, sulfate donor and sulfotransferase. In this way, we achieve intracellular CS production of ~27 μg/g dry-cell-weight with about 96% of the disaccharides sulfated. We further explore four different factors that can affect the sulfation levels of this microbial product. Overall, this is a demonstration of simple, one-step microbial production of a sulfated GAG and marks an important step in the animal-free production of these molecules.

Chondroitin sulfate (CS) is a type of sulfated glycosaminoglycan that is manufactured by extraction from animal tissues for the treatment of osteoarthritis and in drug delivery applications. Here, the authors report the development of single microbial cell factories capable of compete, one-step biosynthesis of animal-free CS production in E. coli.

Misincorporation of Galactose by Chondroitin Synthase of Escherichia coli K4: From Traces to Synthesis of Chondbiuronan, a Novel Chondroitin-Like Polysaccharide

Chondroitin synthase KfoC is a bifunctional enzyme which polymerizes the capsular chondroitin backbone of Escherichia coli K4, composed of repeated β3N-acetylgalactosamine (GalNAc)-β4-glucuronic acid (GlcA) units. Sugar donors UDP-GalNAc and UDP-GlcA are the natural precursors of bacterial chondroitin synthesis. We have expressed KfoC in a recombinant strain of Escherichia coli deprived of 4-epimerase activity, thus incapable of supplying UDP-GalNAc in the bacterial cytoplasm. The strain was also co-expressing mammal galactose β-glucuronyltransferase, providing glucuronyl-lactose from exogenously added lactose, serving as a primer of polymerization. We show by the mean of NMR analyses that in those conditions, KfoC incorporates galactose, forming a chondroitin-like polymer composed of the repeated β3-galactose (Gal)-β4-glucuronic acid units. We also show that when UDP-GlcNAc 4-epimerase KfoA, encoded by the K4-operon, was co-expressed and produced UDP-GalNAc, a small proportion of galactose was still incorporated into the growing chain of chondroitin.

N-glycolyl chondroitin synthesis using metabolically engineered E. coli

N-glycolyl chondroitin (Gc-CN) is a metabolite of N-glycolylneuraminic acid (Neu5Gc), a sialic acid that is commonly found in mammals, but not humans. Humans can incorporate exogenous Neu5Gc into their tissues from eating red meat. Neu5Gc cannot be biosynthesized by humans due to an evolutionary mutation and has been implicated in causing inflammation causing human diseases, such as cancer. The study Neu5Gc is important in evolutionary biology and the development of potential cancer biomarkers. Unfortunately, there are several limitations to detecting Neu5Gc. The elimination of Neu5Gc involves a degradative pathway leading to the incorporation of N-glycolyl groups into glycosaminoglycans (GAGs), such as Gc-CN. Gc-CN has been found in humans and in animals including mice, lamb and chimpanzees. Here, we present the biosynthesis of Gc-CN in bacteria by feeding chemically synthesized N-glycolylglucosamine to Escherichia coli. A metabolically engineered strain of E. coli K4, fed with glucose supplemented with GlcNGc, converted it to N-glycolylgalactosamine (GalNGc) that could then be utilized as a substrate in the chondroitin biosynthetic pathway. The final product, Gc-CN was converted to disaccharides using chondroitin lyase ABC and analyzed by liquid chromatography-tandem mass spectrometry with multiple reaction monitoring detection. This analysis showed the incorporation of GalNGc into the backbone of the chondroitin oligosaccharide.

Microbial production and metabolic engineering of chondroitin and chondroitin sulfate

Several commercial uses and potential novel applications have recently been described for chondroitin sulfate (CS). However, the currently applied animal extractive procedure has a high environmental impact, which may become more profound especially in relation to the forecasted expansion of the CS market for applications as a food supplement, pharmaceutical ingredient, and biopolymer in materials for regenerative medicine. This issue, together with religious and consumer concerns, has prompted the good manufacturing practice (GMP) of chondroitin and CS. This is achievable by combining the design of metabolically engineered microorganisms and tailor-made fermentation processes with semi-synthetic or enzyme-based approaches. The final target is to obtain molecules with specific sulfation patterns that resemble those occurring in natural products and improve the sulfation motif or introduce specific substitutions, such as fucosylation, to tune the biological function. The frontier that is currently triggering attention is related to evaluating the bioactivity of unsulfated chondroitin. Due to recent advancements in the field, a brief survey of the most recent patent and research literature is discussed here.

Advances in enzyme substrate analysis with capillary electrophoresis

Capillary electrophoresis provides a rapid, cost-effective platform for enzyme and substrate characterization. The high resolution achievable by capillary electrophoresis enables the analysis of substrates and products that are indistinguishable by spectroscopic techniques alone, while the small volume requirement enables analysis of enzymes or substrates in limited supply. Furthermore, the compatibility of capillary electrophoresis with various detectors makes it suitable for KM determinations ranging from nanomolar to millimolar concentrations. Capillary electrophoresis fundamentals are discussed with an emphasis on the separation mechanisms relevant to evaluate sets of substrate and product that are charged, neutral, and even chiral. The basic principles of Michaelis-Menten determinations are reviewed and the process of translating capillary electrophoresis electropherograms into a Michaelis-Menten curve is outlined. The conditions that must be optimized in order to couple off-line and on-line enzyme reactions with capillary electrophoresis separations, such as incubation time, buffer pH and ionic strength, and temperature, are examined to provide insight into how the techniques can be best utilized. The application of capillary electrophoresis to quantify enzyme inhibition, in the form of KI or IC50 is detailed. The concept and implementation of the immobilized enzyme reactor is described as a means to increase enzyme stability and reusability, as well as a powerful tool for screening enzyme substrates and inhibitors. Emerging techniques focused on applying capillary electrophoresis as a rapid assay to obtain structural identification or sequence information about a substrate and in-line digestions of peptides and proteins coupled to mass spectrometry analyses are highlighted.

Enhancing fructosylated chondroitin production in Escherichia coli K4 by balancing the UDP-precursors

Microbial production of chondroitin and chondroitin-like polysaccharides from renewable feedstock is a promising and sustainable alternative to extraction from animal tissues. In this study, we attempted to improve production of fructosylated chondroitin in Escherichia coli K4 by balancing intracellular levels of the precursors UDP-GalNAc and UDP-GlcA. To this end, we deleted pfkA to favor the production of Fru-6-P. Then, we identified rate-limiting enzymes in the synthesis of UDP-precursors. Third, UDP-GalNAc synthesis, UDP-GlcA synthesis, and chondroitin polymerization were combinatorially optimized by altering the expression of relevant enzymes. The ratio of intracellular UDP-GalNAc to UDP-GlcA increased from 0.17 in the wild-type strain to 1.05 in a 30-L fed-batch culture of the engineered strain. Titer and productivity of fructosylated chondroitin also increased to 8.43 g/L and 227.84 mg/L/h; the latter represented the highest productivity level achieved to date.