De novo biosynthesis of 2'-fucosyllactose in a metabolically engineered Escherichia coli using a novel ɑ1,2-fucosyltransferase from Azospirillum lipoferum

Combinatorial metabolic engineering of Escherichia coli for de novo production of structurally defined and homogeneous Amino oligosaccharides

Amino oligosaccharides (AOs) possess various biological activities and are valuable in the pharmaceutical, food industries, and agriculture. However, the industrial manufacturing of AOs has not been realized yet, despite reports on physical, chemical, and biological approaches. In this study, the de novo production of chitin oligosaccharides (CHOS), a type of structurally defined AOs, was achieved in Escherichia coli through combinatorial pathway engineering. The most suitable glycosyltransferase for CHOS production was found to be NodCL from Mesorhizobium Loti. Then, by knocking out the nagB gene to block the flow of N-acetyl-d-glucosamine (NAG) to the glycolytic pathway in E. coli and adjusting the copy number of NodCL-coding gene, the CHOS yield was increased by 6.56 times. Subsequently, by introducing of UDP-N-acetylglucosamine (UDP-GlcNAc) salvage pathway for and optimizing fermentation conditions, the yield of CHOS reached 207.1 and 468.6 mg/L in shake-flask cultivation and a 5-L fed-batch bioreactor, respectively. Meanwhile, the concentration of UDP-GlcNAc was 91.0 mg/L, the highest level reported in E. coli so far. This study demonstrated, for the first time, the production of CHOS with distinct structures in plasmid-free E. coli, laying the groundwork for the biosynthesis of CHOS and providing a starting point for further engineering and commercial production.

Rcs signal transduction system in Escherichia coli: Composition, related functions, regulatory mechanism, and applications

The regulator of capsule synthesis (Rcs) system, an atypical two-component system prevalent in numerous gram-negative bacteria, serves as a sophisticated regulatory phosphorylation cascade mechanism. It plays a pivotal role in perceiving environmental stress and regulating the expression of downstream genes to ensure host survival. During the signaling transduction process, various proteins participate in phosphorylation to further modulate signal inputs and outputs. Although the structure of core proteins related to the Rcs system has been partially well-defined, and two models have been proposed to elucidate the intricate molecular mechanisms underlying signal sensing, a systematic characterization of the signal transduction process of the Rcs system remains challenging. Furthermore, exploring its corresponding regulator outputs is also unremitting. This review aimed to shed light on the regulation of bacterial virulence by the Rcs system. Moreover, with the assistance of the Rcs system, biosynthesis technology has developed high-value target production. Additionally, via this review, we propose designing chimeric Rcs biosensor systems to expand their application as synthesis tools. Finally, unsolved challenges are highlighted to provide the basic direction for future development of the Rcs system.

Construction and application of 3-fucosyllactose whole-cell biosensor for high-throughput screening of overproducers

Biosensor-based high-throughput screening is efficient for improving industrial microorganisms. There is a severe shortage of human milk oligosaccharides (HMOs) biosensors. This study established a 3-fucosyllactose (3-FL, a kind of HMOs) whole-cell biosensor by coupling cell growth with production. To construct and optimize the biosensor, an Escherichia coli 3-FL producer was engineered by deleting the manA, yihS and manX genes, directing the mannose flux solely to 3-FL synthesis. Then, an α-L-fucosidase was introduced to hydrolyze 3-FL to fucose which was used as the only carbon source for cell growth. Using the biosensor, the 3-FL production of a screened mutant was improved by 25 % to 42.05 ± 1.28 g/L. The productivity reached 1.17 g/L/h, the highest level reported by now. The csrB mutant obtained should be a new clue for the 3-FL overproduction mechanism. In summary, this study provided a novel approach to construct HMOs biosensors for strain improvement.

De Novo Biosynthesis of Difucosyllactose by Artificial Pathway Construction and α1,3/4-Fucosyltransferase Rational Design in Escherichia coli

Difucosyllactose (DFL) is a significant and plentiful oligosaccharide found in human breast milk. In this study, an artificial metabolic pathway of DFL was designed, focusing on the de novo biosynthesis of GDP-fucose from only glycerol. This was achieved by engineering Escherichia coli to endogenously overexpress genes manB, manC, gmd, and wcaG and heterologously overexpress a pair of fucosyltransferases to produce DFL from lactose. The introduction of α-1,2-fucosyltransferase from Helicobacter pylori (FucT2) along with α-1,3/4-fucosyltransferase (HP3/4FT) addressed rate-limiting challenges in enzymatic catalysis and allowed for highly efficient conversion of lactose into DFL. Based on these results, molecular modification of HP3/4FT was performed based on computer-assisted screening and structure-based rational design. The best-performing mutant, MH5, containing a combination of five mutated sites (F49K/Y131D/Y197N/E338D/R369A) of HP3/4FT was obtained. The best strain BLC09-58 harboring MH5 yielded 45.81 g/L of extracellular DFL in 5-L fed-batch cultures, which was the highest titer reported to date.

Implementation of a Quorum-Sensing System for Highly Efficient Biosynthesis of Lacto-N-neotetraose in Engineered Escherichia coli MG1655

Lacto-N-neotetraose (LNnT), a prominent neutral human milk oligosaccharide (HMO), serves as a pivotal structural element in complex HMO biosynthesis. Given its promising health effects for infants, the biosynthesis of LNnT is garnering greater interest. Using a previously engineered strain as a chassis, a highly effective LNnT producer was constructed. First, LNnT synthesis in Escherichia coli MG1655 was achieved by introducing β1,3-N-acetylglucosaminyltransferase LgtA and β1,4-galactosyltransferase CpsIaJ, coupled with the optimization of enzyme expression levels using various promoters. Subsequently, ugd underwent disruption, and the galE gene was enhanced by replacing its promoter with PJ23119 or Ptac. Then, a lux-type quorum sensing (QS) system was applied to achieve varied metabolic regulation. Additionally, systematic optimization of the QS promoters was conducted to further improve the LNnT titer in the shake flask. Finally, the extracellular titer of LNnT was 20.33 g/L, accompanied by a productivity of 0.41 g/L/h.

High-Yield Synthesis of Lacto-N-Neotetraose from Glycerol and Glucose in Engineered Escherichia coli

Lacto-N-neotetraose (LNnT) is a neutral human milk oligosaccharide with important biological functions. However, the low LNnT productivity and the incomplete conversion of the intermediate lacto-N-tetraose II (LNT II) currently limited the sustainable biosynthesis of LNnT. First, the LNnT biosynthetic module was integrated in Escherichia coli. Next, the LNnT export system was optimized to alleviate the inhibition of intracellular LNnT synthesis. Furthermore, by utilizing rate-limiting enzyme diagnosis, the expressions of LNnT synthesis pathway genes were finely regulated to further enhance the production yield of LNnT. Subsequently, a strategy of cofermentation using a glucose/glycerol (4:6, g/g) mixed feed was employed to regulate carbon flux distribution. Finally, by overexpressing key transferases, LNnT and LNT II titers reached 112.47 and 7.42 g/L, respectively, in a 5 L fermenter, and 107.4 and 2.08 g/L, respectively, in a 1000 L fermenter. These are the highest reported titers of LNnT to date, indicating its significant potential for industrial production.

Construction of an engineered Escherichia coli for effective synthesis of 2'-fucosyllactose via the salvage pathway

2'-Fucosyllactose (2'-FL) is one of the important functional oligosaccharides in breast milk. So far, few attempts on biosynthesis of 2'-FL by the salvage pathway have been reported. Herein, the salvage pathway enzyme genes were introduced into the E. coli BL21star(DE3) for synthesis of 2'-FL. The 2'-FL titer increased from 1.56 to 2.13 g/L by deleting several endogenous genes on competitive pathways. The α-1,2-fucosyltransferase (WbgL) was selected, and improved the 2'-FL titer to 2.88 g/L. Additionally, the expression level of pathway enzyme genes was tuned through optimizing the plasmid copy number. Furthermore, the spatial distribution of WbgL was enhanced by fusing with the MinD C-tag. After optimizing the fermentation conditions, the 2'-FL titer reached to 7.13 g/L. The final strain produced 59.22 g/L of 2'-FL with 95% molar conversion rate of lactose and 92% molar conversion rate of fucose in a 5 L fermenter. These findings will contribute to construct a highly efficient microbial cell factory to produce 2'-FL or other HMOs.

Efficient Biosynthesis of Difucosyllactose via De Novo GDP-l-Fucose Pathway in Metabolically Engineered Escherichia coli

Difucosyllactose (DFL) is an important component of human milk oligosaccharides (HMOs) and has significant benefits for the growth and development of infants. So far, a few microbial cell factories have been constructed for the production of DFL, which still have problems of low production and high cost. Herein, a high-level de novo pathway DFL-producing strain was constructed by multistep optimization strategies in Escherichia coli BL21star(DE3). We first efficiently synthesized the intermediate 2'-fucosyllactose (2'-FL) in E. coli BL21star(DE3) by the advisable stepwise strategy. The truncated α-1,3/4-fucosyltransferase (Hp3/4FT) was then introduced into the engineered strain to achieve de novo biosynthesis of DFL. ATP-dependent protease (Lon) and GDP-mannose hydrolase (NudK) were deleted, and mannose-6-phosphate isomerase (ManA) was overexpressed to improve GDP-l-fucose accumulation. The regulator RcsA was overexpressed to fine-tune the expression level of pathway genes, thereby increasing the synthesis of DFL. The final strain produced 6.19 g/L of DFL in the shake flask and 33.45 g/L of DFL in the 5 L fermenter, which were the highest reported titers so far. This study provides a more economical, sustainable, and effective strategy to produce the fucosylated human milk oligosaccharides (HMOs).

Microbial Synthesis of Lacto-N-fucopentaose I with High Titer and Purity by Screening of Specific Glycosyltransferase and Elimination of Residual Lacto-N-triose II and Lacto-N-tetraose

Lacto-N-fucopentaose I (LNFP I) has recently been approved as generally recognized as safe, demonstrating its great commercial potential in the food industry. Microbial synthesis through metabolic engineering strategies is an effective approach for large-scale production of LNFP I. Biosynthesis of LNFP I requires consideration of two key points: high titer with low byproduct 2'-fucosyllactose (2'-FL) generation and high purity with low lacto-N-triose II (LNTri II) and lacto-N-tetraose (LNT) residues. Herein, α1,2-fucosyltransferase from Thermoanaerobacterium sp. RBIITD was screened from 16 selected LNFP I-producing glycosyltransferase candidates, showing the highest in vivo LNFP I productivity. Chromosomal integration of wbgO enhanced the LNFP I production by improving the precursor conversion from LNTri II to LNT. The best engineered strain produced 4.42 and 35.1 g/L LNFP I in shake-flask and fed-batch cultivation, respectively. The residual LNTri II and LNT were eliminated by further cultivation with a recombinant strain coexpressing Bifidobacterium bifidum β-N-acetylhexosaminidase and lacto-N-biosidase. A strategy for LNFP I biosynthesis with high yield and purity was finally realized, providing support for its practical application in large-scale production.

Efficient production of 2'-fucosyllactose from fructose through metabolically engineered recombinant Escherichia coli

BACKGROUND

The biosynthesis of human milk oligosaccharides (HMOs) using several microbial systems has garnered considerable interest for their value in pharmaceutics and food industries. 2'-Fucosyllactose (2'-FL), the most abundant oligosaccharide in HMOs, is usually produced using chemical synthesis with a complex and toxic process. Recombinant E. coli strains have been constructed by metabolic engineering strategies to produce 2'-FL, but the low stoichiometric yields (2'-FL/glucose or glycerol) are still far from meeting the requirements of industrial production. The sufficient carbon flux for 2'-FL biosynthesis is a major challenge. As such, it is of great significance for the construction of recombinant strains with a high stoichiometric yield.

RESULTS

In the present study, we designed a 2'-FL biosynthesis pathway from fructose with a theoretical stoichiometric yield of 0.5 mol 2'-FL/mol fructose. The biosynthesis of 2'-FL involves five key enzymes: phosphomannomutase (ManB), mannose-1-phosphate guanylytransferase (ManC), GDP-D-mannose 4,6-dehydratase (Gmd), and GDP-L-fucose synthase (WcaG), and α-1,2-fucosyltransferase (FucT). Based on starting strain SG104, we constructed a series of metabolically engineered E. coli strains by deleting the key genes pfkA, pfkB and pgi, and replacing the original promoter of lacY. The co-expression systems for ManB, ManC, Gmd, WcaG, and FucT were optimized, and nine FucT enzymes were screened to improve the stoichiometric yields of 2'-FL. Furthermore, the gene gapA was regulated to further enhance 2'-FL production, and the highest stoichiometric yield (0.498 mol 2'-FL/mol fructose) was achieved by using recombinant strain RFL38 (SG104ΔpfkAΔpfkBΔpgi119-lacYΔwcaF::119-gmd-wcaG-manC-manB, 119-AGGAGGAGG-gapA, harboring plasmid P30). In the scaled-up reaction, 41.6 g/L (85.2 mM) 2'-FL was produced by a fed-batch bioconversion, corresponding to a stoichiometric yield of 0.482 mol 2'-FL/mol fructose and 0.986 mol 2'-FL/mol lactose.

CONCLUSIONS

The biosynthesis of 2'-FL using recombinant E. coli from fructose was optimized by metabolic engineering strategies. This is the first time to realize the biological production of 2'-FL production from fructose with high stoichiometric yields. This study also provides an important reference to obtain a suitable distribution of carbon flux between 2'-FL synthesis and glycolysis.

De novo biosynthesis of 2'-fucosyllactose by bioengineered Corynebacterium glutamicum

2'-Fucosyllactose (2'-FL) which is well-known human milk oligosaccharide was biotechnologically synthesized using engineered Corynebacterium glutamicum, a GRAS microbial workhorse. By construction of the complete de novo pathway for GDP-L-fucose supply and heterologous expression of Escherichia coli lactose permease and Helicobacter pylori α-1,2-fucosyltransferase, bioengineered C. glutamicum BCGW_TL successfully biosynthesized 0.25 g L-1 2'-FL from glucose. The additional genetic perturbations including the expression of a putative 2'-FL exporter and disruption of the chromosomal pfkA gene allowed C. glutamicum BCGW_cTTLEΔP to produce 2.5 g L-1 2'-FL batchwise. Finally, optimized fed-batch cultivation of the BCGW_cTTLEΔP using glucose, fructose, and lactose resulted in 21.5 g L-1 2'-FL production with a productivity of 0.12 g L-1 •h, which were more than 3.3 times higher value relative to the batch culture of the BCGW_TL. Conclusively, it would be a groundwork to adopt C. glutamicum for biotechnological production of other food additives including human milk oligosaccharides.

Efficient production of 2'-fucosyllactose in unconventional yeast Yarrowia lipolytica

2'-Fucosyllactose (2'-FL) has great application value as a nutritional component and the whole cell biosynthesis of 2'-FL has become the focus of current research. Yarrowia lipolytica has great potential in oligosaccharide synthesis and large-scale fermentation. In this study, systematic engineering of Y. lipolytica for efficient 2'-FL production was performed. By fusing different protein tags, the synthesis of 2'-FL was optimized and the ubiquitin tag was demonstrated to be the best choice to increase the 2'-FL production. By iterative integration of the related genes, increasing the precursor supply, and promoting NADPH regeneration, the 2'-FL synthesis was further improved. The final 2'-FL titer, 41.10 g/L, was obtained in the strain F5-1. Our work reports the highest 2'-FL production in Y. lipolytica, and demonstrates that Y. lipolytica is an efficient microbial chassis for the synthesis of oligosaccharides.

High-Yield Synthesis of 2'-Fucosyllactose from Glycerol and Glucose in Engineered Escherichia coli

2'-Fucosyllactose (2'-FL) is vital for the growth and development of newborns. In this study, we developed a synthesis pathway for 2'-FL in Escherichia coli BL21 (DE3). Then, we optimized the solubility of α-1,2-fucosyltransferase, thereby enhancing the production yield of 2'-FL. Based on this finding, we further enhanced the expression of guanosine inosine kinase Gsk and knocked out the isocitrate lyase regulator gene iclR. This strategy reduced the formation of byproduct acetate during the metabolic process and alleviated carbon source overflow effects in the strain, resulting in further improvement of the yield of 2'-FL. In a 3 L bioreactor, employing fed-batch fermentation with glycerol and glucose as substrates, the engineered strain BWLAI-RSZL exhibited impressive 2'-FL titers of 121.9 and 111.56 g/L, along with productivity levels of 1.57 and 1.31 g/L/h, respectively. The reported 2'-FL titers reached a groundbreaking level, irrespective of the carbon source employed (glycerol or glucose), highlighting the significant potential for large-scale industrial synthesis of 2'-FL.

Engineering Escherichia coli MG1655 for Highly Efficient Biosynthesis of 2'-Fucosyllactose by De Novo GDP-Fucose Pathway

2'-Fucosyllactose (2'-FL), the most typical human milk oligosaccharide, is used as an additive in premium infant formula. Herein, we constructed two highly effective 2'-FL synthesis producers via a de novo GDP-fucose pathway from engineered Escherichia coli MG1655. First, lacZ and wcaJ, two competitive pathway genes, were disrupted to block the invalid consumption of lactose and GDP-fucose, respectively. Next, the lacY gene was strengthened by switching its native promoter to PJ23119. To enhance the supply of endogenous GDP-fucose, the promoters of gene clusters manC-manB and gmd-fcl were strengthened individually or in combination. Subsequently, chromosomal integration of a constitutive PJ23119 promoter-based BKHT expression cassette (PJ23119-BKHT) was performed in the arsB and recA loci. The most productive plasmid-based and plasmid-free strains produced 76.9 and 50.1 g/L 2'-FL by fed-batch cultivation, respectively. Neither of them generated difucosyl lactose nor 3-fucosyllactose as byproducts.

An Overview of Sugar Nucleotide-Dependent Glycosyltransferases for Human Milk Oligosaccharide Synthesis

Human milk oligosaccharides (HMOs) have received increasing attention because of their special effects on infant health and commercial value as the new generation of core components in infant formula. Currently, large-scale production of HMOs is generally based on microbial synthesis using metabolically engineered cell factories. Introduction of the specific glycosyltransferases is essential for the construction of HMO-producing engineered strains in which the HMO-producing glycosyltransferases are generally sugar nucleotide-dependent. Four types of glycosyltransferases have been used for typical glycosylation reactions to synthesize HMOs. Soluble expression, substrate specificity, and regioselectivity are common concerns of these glycosyltransferases in practical applications. Screening of specific glycosyltransferases is an important research topic to solve these problems. Molecular modification has also been performed to enhance the catalytic activity of various HMO-producing glycosyltransferases and to improve the substrate specificity and regioselectivity. In this article, various sugar nucleotide-dependent glycosyltransferases for HMO synthesis were overviewed, common concerns of these glycosyltransferases were described, and the future perspectives of glycosyltransferase-related studies were provided.