Elimination of Byproduct Generation and Enhancement of 2'-Fucosyllactose Synthesis by Expressing a Novel α1,2-Fucosyltransferase in Engineered Escherichia coli

Highly efficient biosynthesis of 3'-sialyllactose in engineered Escherichia coli

3'-Sialyllactose (3'-SL), one of the abundant and important sialylated human milk oligosaccharides, is an emerging food ingredient used in infant formula milk. We previously developed an efficient route for 3'-SL biosynthesis in metabolically engineered Escherichia coli BL21(DE3). Here, several promising α2,3-sialyltransferases were re-evaluated from the byproduct synthesis perspective. The α2,3-sialyltransferase from Neisseria meningitidis MC58 (NST) with great potential and the least byproducts was selected for subsequent molecular modification. Computer-assisted mutation sites combined with a semi-rational modification were designed and performed. A combination of two mutation sites (P120H/N113D) of NST was finally confirmed as the best one, which significantly improved 3'-SL biosynthesis, with extracellular titers of 24.5 g/L at 5-L fed-batch cultivations. When NST-P120H/N113D was additionally integrated into the genome of host EZAK (E. coli BL21(DE3)ΔlacZΔnanAΔnanT), the final strain generated 32.1 g/L of extracellular 3'-SL in a 5-L fed-batch fermentation. Overall, we underscored the existence of by-products and improved 3'-SL production by engineering N. meningitidis α2,3-sialyltransferase.

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.

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.

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.

Semi-rationally designed site-saturation mutation of Helicobacter pylori α-1,2-fucosyltransferase for improved catalytic activity and thermostability

Helicobacter pylori HpfutC, a glycosyltransferase (GT) 11 family glycoprotein, has great potential for industrial 2'-fucosyllactose (2'-FL) production. However, its limited catalytic activity, low expression, and poor thermostability hinder practical applications. Herein, a semi-rationally designed site-saturation mutation was applied to engineer the catalytic activity and thermostability of HpfutC. The 6 single point mutants (K102T, R105C, D115S, Y251F, A255G and K282E) and 6 combined mutants (V1, V2, V3, V4, V5, and V6) with enhanced enzyme activity were obtained by mutant library screening and ordered recombination mutation. The optimal mutant V6, with an optimum temperature of 40 °C, was not a metal-dependent enzyme, yet the reaction was facilitated by Mn2+. Compared to wild-type HpfutC, mutant V6 exhibited a 2.3-fold increase in specific activity and a 2.18-fold increase in half-life at 40 °C, respectively. Kinetic parameters indicated that the Km values of mutant V6 were 34.5 % (lactose) and 25.0 % (GDP-L-fucose) lower than those of the wild enzyme, whereas the kcat/Km values were 1.20 and 1.25-fold higher than those of the wild enzyme. Further, 3D-structure analysis revealed that the highly rigid structure, formation of new hydrogen bonds, increased hydrophobic residues and redistribution of electrostatic charges on the surface may be responsible for the elevated enzyme activity and thermostability. The strategy adopted in this study is of great significance to the solution of the technical bottleneck of HpfutC and the industrial application of 2'-FL.

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.

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.