Elimination of Residual Lacto-N-triose II for Lacto-N-tetraose Biosynthesis in Engineered Escherichia coli

Comprehensive review on human Milk oligosaccharides: Biosynthesis, structure, intestinal health benefits, immune regulation, neuromodulation mechanisms, and applications

This review provides a comprehensive analysis of the biosynthetic pathways of various oligosaccharides in Escherichia coli, structural characteristics, and bioactive mechanisms of human milk oligosaccharides (HMOs), with a particular emphasis on their roles in gut health, immune modulation, and neurodevelopment. HMOs primarily function as prebiotics, facilitating the growth of beneficial bacteria such as Bifidobacterium to maintain microbial homeostasis, with a discussion on the synergistic role of carbohydrate-binding modules (CBMs). In immune modulation, HMOs interact with lectins on immune and epithelial cells, influencing immune responses via pathways such as Toll-like receptors (TLRs). Additionally, HMOs have been linked to enhanced cognitive, motor, and language development in infants, influencing genes such as GABRB2, SLC1A7, GLRA4, and CHRM3. The review also examines commercially available HMO-containing products and highlights future research directions and potential applications in nutrition and healthcare.

Regulation of Metabolic Pathways to Enhance Difucosyllactose Biosynthesis in Escherichia coli

Difucosyllactose (DFL), an important kind of fucosylated human milk oligosaccharides (HMOs), has garnered considerable attention due to its excellent physiological activities in infants. Previously, we achieved de novo biosynthesis of DFL; however, substantial residual intermediates of fucosyllactoses (FL) were detected. In this study, DFL biosynthesis was optimized, and residual FL were reduced by regulating metabolic pathways. Different plasmid combinations were used to regulate gene expression, achieving an optimal flux balance between 2'-FL and DFL. The expression level of key enzyme α-1,3-fucosyltransferase (α-1,3-FT, FucTa) was then enhanced by increasing plasmid copy number and integrating fucTa gene into the chromosome. Exocytosis of 2'-FL was reduced by deleting the sugar efflux transporter setA gene, thereby minimizing residual FL. Finally, strain BSF41 produced 55.3 g/L of DFL with only 2.59 g/L of residual FL in a 5 L fermentor, representing the highest reported titer to date. This study provides an important foundation for advancing the biosynthesis of fucosylated HMOs.

Engineered β1-3-N-Acetylglucosaminyltransferase Facilitating the One-Pot Multienzyme Synthesis of Human Milk Oligosaccharides

β1-3-linked N-acetylglucosaminide is a prevalent carbohydrate motif found in oligosaccharides, polysaccharides, glycoproteins, and glycolipids. It is a crucial component of human milk oligosaccharides (HMOs). Neisseria meningitidis β1-3-N-acetylglucosaminyltransferase (NmLgtA) catalyzes the formation of a glycosidic bond and has the potential for use in synthesizing HMOs. However, this application is hindered by challenges such as low levels of enzyme expression, poor stability, and significant aggregation. Since there is no available crystal structure for NmLgtA, we used its AlphaFold 2 predicted structure to identify potential unfavorable factors. We then modified the enzyme by removing the 17 N-terminal amino acids and substituting nine specific residues. The engineered NmLgtA-Opti exhibited improved thermal stability, increased soluble protein expression, complete relief from aggregation, and enhanced catalysis while maintaining its catalytic specificity and substrate promiscuity. Furthermore, NmLgtA-Opti maximizes substrate utilization and can be employed in a sequential one-pot multienzyme platform for high-yield production of HMOs.

Recent Progress in Physiological Significance and Biosynthesis of Lacto-N-triose II: Insights into a Crucial Biomolecule

Lacto-N-triose II (LNTri II), an important precursor for human milk oligosaccharide (HMOs) synthesis, has garnered significant attention due to its structural features and physiological properties. Composed of galactose (Gal), N-acetylglucosamine (GlcNAc), and glucose (Glc), with the chemical structure GlcNAcβ1,3Galβ1,4Glc, the distinctive structure of LNTri II confers various physiological functions such as promoting the growth of beneficial bacteria, regulating the infant immune system, and preventing certain gastrointestinal diseases. Extensive research efforts have been dedicated to elucidating efficient enzymatic synthesis pathways for LNTri II production, with particular emphasis on the transglycosylation activity of β-N-acetylhexosaminidases and the action of β-1,3-N-acetylglucosaminyltransferases. Additionally, metabolic engineering and cell factory approaches have been explored, harnessing the potential of engineered microbial hosts for the large-scale biosynthesis of LNTri II. This review summarizes the structure, derivatives, physiological effects, and biosynthesis of LNTri II.

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.