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

Crystal structure and structure-guided tunnel engineering in a bacterial β-1,4-galactosyltransferase

Lacto-N-neotetraose (LNnT), a representative oligosaccharide found in human milk, has been previously examined for its beneficial traits. However, the LNnT titer is limited by the efficient glycosyltransferase pathway, particularly with respect to the catalysis of rate-limiting steps. As data on the crystal structure of the key enzyme required for synthesizing LNnT are lacking, the synthesis of LNnT remains an uncertainty. Here, for the first time we report the three-dimensional structure of a bacterial β-1,4-galactosyltransferase, Aaβ4GalT, and analyze the critical role played by residues in its catalytic efficacy. Guided by structural insights, we engineered this enzyme to enhance its catalytic efficiency using structure-guided tunnel engineering. The mutant enzyme L5 (K155M/H156D/F157W/K185M/Q216V) so produced, showed a 50-fold enhancement in catalytic activity. Crystal structure analysis revealed that the mechanism underlying the improvement in activity was of the swing door type. The closed conformation formed by dense hydrophobic packing with Q216V-K155M widened and permitted substrate entry. Our results show that altering the tunnel conformation helped appropriately accommodate the substrate for catalysis and provide a structural basis for the modification of other glycosyltransferases.

Intracellular Construction of Organelle-like Compartments Facilitates Metabolic Flux in Escherichia coli

The formation of well-designed synthetic compartments or membraneless organelles for applications in synthetic biology and cellular engineering has aroused enormous interest. However, establishing stable and robust intracellular compartments in bacteria remains a challenge. Here, we use the structured DIX domains derived from Wnt signaling pathway components, more specifically, Dvl2 and Axin1, as building blocks to generate intracellular synthetic compartments in Escherichia coli. Moreover, the aggregation behaviors and physical properties of the DIX-based compartments can be tailored by genetically embedding a specific dimeric domain into the DIX domains. Then, a pair of interacting motifs, consisting of the aforementioned dimeric domain and its corresponding binding ligand, was incorporated to modify the client recruitment pattern of the synthetic compartments. As a proof of concept, the human milk oligosaccharide lacto-N-tetraose (LNT) biosynthesis pathway was selected as a model metabolic pathway. The fermentation results demonstrated that the co-compartmentalization of sequential pathway enzymes into intracellular compartments created by DIX domain, or by the DIX domain in conjunction with interacting motifs, prominently enhanced the metabolic flux and increased LNT production. These synthetic protein compartments may provide a feasible and effective tool to develop versatile organelle-like compartments in bacteria for applications in cellular engineering and synthetic biology.

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.

Compartmentalization of pathway sequential enzymes into synthetic protein compartments for metabolic flux optimization in Escherichia coli

Advancing the formation of artificial membraneless compartments with organizational complexity and diverse functionality remains a challenge. Typically, synthetic compartments or membraneless organelles are made up of intrinsically disordered proteins featuring low-complexity sequences or polypeptides with repeated distinctive short linear motifs. In order to expand the repertoire of tools available for the formation of synthetic membraneless compartments, here, a range of DIshevelled and aXin (DIX) or DIX-like domains undergoing head-to-tail polymerization were demonstrated to self-assemble into aggregates and generate synthetic compartments within E. coli cells. Then, synthetic complex compartments with diverse intracellular morphologies were generated by coexpressing different DIX domains. Further, we genetically incorporated a pair of interacting motifs, comprising a homo-dimeric domain and its anchoring peptide, into the DIX domain and cargo proteins, respectively, resulting in the alteration of both material properties and client recruitment of synthetic compartments. As a proof-of-concept, several human milk oligosaccharide biosynthesis pathways were chosen as model systems. The findings indicated that the recruitment of pathway sequential enzymes into synthetic compartments formed by DIX-DIX heterotypic interactions or by DIX domains embedded with specific interacting motifs efficiently boosted metabolic pathway flux and improved the production of desired chemicals. We propose that these synthetic compartment systems present a potent and adaptable toolkit for controlling metabolic flux and facilitating cellular engineering.

Combinatorial Optimization Strategies for 3-Fucosyllactose Hyperproduction in Escherichia coli

3-Fucosyllactose (3-FL), an important fucosylated human milk oligosaccharide in breast milk, offers numerous health benefits to infants. Previously, we metabolically engineered Escherichia coli BL21(DE3) for the in vivo biosynthesis of 3-FL. In this study, we initially optimized culture conditions to double 3-FL production. Competing pathway genes involved in in vivo guanosine 5'-diphosphate-fucose biosynthesis were subsequently inactivated to redirect fluxes toward 3-FL biosynthesis. Next, three promising transporters were evaluated using plasmid-based or chromosomally integrated expression to maximize extracellular 3-FL production. Additionally, through analysis of α1,3-fucosyltransferase (FutM2) structure, we identified Q126 residues as a highly mutable residue in the active site. After site-saturation mutation, the best-performing mutant, FutM2-Q126A, was obtained. Structural analysis and molecular dynamics simulations revealed that small residue replacement positively influenced helical structure generation. Finally, the best strain BD3-A produced 6.91 and 52.1 g/L of 3-FL in a shake-flask and fed-batch cultivations, respectively, highlighting its potential for large-scale industrial applications.

CRISPR-Cas tools for simultaneous transcription & translation control in bacteria

Robust control over gene translation at arbitrary mRNA targets is an outstanding challenge in microbial synthetic biology. The development of tools that can regulate translation will greatly expand our ability to precisely control genes across the genome. In Escherichia coli, most genes are contained in multi-gene operons, which are subject to polar effects where targeting one gene for repression leads to silencing of other genes in the same operon. These effects pose a challenge for independently regulating individual genes in multi-gene operons. Here, we use CRISPR-dCas13 to address this challenge. We find dCas13-mediated repression exhibits up to 6-fold lower polar effects compared to dCas9. We then show that we can selectively activate single genes in a synthetic multi-gene operon by coupling dCas9 transcriptional activation of an operon with dCas13 translational repression of individual genes within the operon. We also show that dCas13 and dCas9 can be multiplexed for improved biosynthesis of a medically-relevant human milk oligosaccharide. Taken together, our findings suggest that combining transcriptional and translational control can access effects that are difficult to achieve with either mode independently. These combined tools for gene regulation will expand our abilities to precisely engineer bacteria for biotechnology and perform systematic genetic screens.

Highly-efficient in vivo production of lacto-N-fucopentaose V by a regio-specific α1,3/4-fucosyltransferase from Bacteroides fragilis NCTC 9343

Lacto-N-fucopentaose V (LNFP V) is a typical human milk pentasaccharide. Multi-enzymatic in vitro synthesis of LNFP V from lactose was reported, however, microbial cell factory approach to LNFP V production has not been reported yet. In this study, the biosynthetic pathway of LNFP V was examined in Escherichia coli. The previously constructed E. coli efficiently producing lacto-N-tetraose was used as the starting strain. GDP-fucose pathway module and a regio-specific glycosyltransferase with α1,3-fucosylation activity were introduced to realize the efficient synthesis of LNFP V. The α1,3/4-fucosyltransferase from Bacteroides fragilis was selected as the best enzyme for in vivo biosynthesis of LNFP V from nine candidates, with the highest titer and the lowest by-product accumulation. A beneficial variant K128D was obtained to further enhance LNFP V titer using computer-assisted site-directed mutagenesis. The final strain EW10 could produce 25.68 g/L LNFP V by fed-batch cultivation, with the productivity of 0.56 g/L·h.

Enzymatic biosynthesis of D-galactose derivatives: Advances and perspectives

D-Galactose derivatives, including galactosyl-conjugates and galactose-upgrading compounds, provide various physiological benefits and find applications in industries such as food, cosmetics, feed, pharmaceuticals. Many research on galactose derivatives focuses on identification, characterization, development, and mechanistic aspects of their physiological function, providing opportunities and challenges for the development of practical approaches for synthesizing galactose derivatives. This study focuses on recent advancements in enzymatic biosynthesis of galactose derivatives. Various strategies including isomerization, epimerization, transgalactosylation, and phosphorylation-dephosphorylation were extensively discussed under the perspectives of thermodynamic feasibility, theoretical yield, cost-effectiveness, and by-product elimination. Specifically, the enzymatic phosphorylation-dephosphorylation cascade is a promising enzymatic synthesis route for galactose derivatives because it can overcome the thermodynamic equilibrium of isomerization and utilize cost-effective raw materials. The study also elucidates the existing challenges and future trends in enzymatic biosynthesis of galactose derivatives. Collectively, this review provides a real-time summary aimed at promoting the practical biosynthesis of galactose derivatives through enzymatic catalysis.

Systematic Engineering of Escherichia coli for Efficient Production of Pseudouridine from Glucose and Uracil

In this study, we proposed a biological approach to efficiently produce pseudouridine (Ψ) from glucose and uracil in vivo using engineered Escherichia coli. By screening host strains and core enzymes, E. coli MG1655 overexpressing Ψ monophosphate (ΨMP) glycosidase and ΨMP phosphatase was obtained, which displayed the highest Ψ concentration. Then, optimization of the RBS sequences, enhancement of ribose 5-phosphate supply in the cells, and overexpression of the membrane transport protein UraA were investigated. Finally, fed-batch fermentation of Ψ in a 5 L fermentor can reach 27.5 g/L with a yield of 89.2 mol % toward uracil and 25.6 mol % toward glucose within 48 h, both of which are the highest to date. In addition, the Ψ product with a high purity of 99.8% can be purified from the fermentation broth after crystallization. This work provides an efficient and environmentally friendly protocol for allowing for the possibility of Ψ bioproduction on an industrial scale.

Efficient Biosynthesis of Lacto-N-Biose I, a Building Block of Type I Human Milk Oligosaccharides, by a Metabolically Engineered Escherichia coli

Lacto-N-biose I (LNB), termed a Type 1 disaccharide, is an important building block of human milk oligosaccharides. It shows promising prebiotic activity by stimulating the proliferation of many gut-associated bifidobacteria and thus displays good potential in infant foods or supplements. Enzymatic and microbial approaches to LNB synthesis have been studied, almost all of which involve glycosylation of LNB phosphorylase as the final step. Herein, we report a new and easier microbial LNB synthesis strategy through the route "lactose → lacto-N-triose II (LNTri II) → lacto-N-tetraose (LNT) → LNB". A previously constructed LNT-producing Escherichia coli BL21(DE3) strain was engineered for LNB biosynthesis by introducing Bifidobacterium bifidum LnbB. LNB was efficiently produced, accompanied by lactose regeneration. Genomic integration of key pathway genes related to LNTri II and LNT synthesis was performed to enhance LNB titers. The final engineered strain produced 3.54 and 26.88 g/L LNB by shake-flask and fed-batch cultivation, respectively.

Metabolic Engineering of Escherichia coli for High-Titer Biosynthesis of 3'-Sialyllactose

3'-Sialyllactose (3'-SL) is among the foremost and simplest sialylated breast milk oligosaccharides. In this study, an engineered Escherichia coli for high-titer 3'-SL biosynthesis was developed by introducing a multilevel metabolic engineering strategy, including (1) the introduction of precursor CMP-Neu5Ac synthesis pathway and high-performance α2,3-sialyltransferase (α2,3-SiaT) genes into strain BZ to achieve de novo synthesis of 3'-SL; (2) optimizing the expression of glmS-glmM-glmU involved in the UDP-GlcNAc and CMP-Neu5Ac synthesis pathways, and constructing a glutamine cycle system, balancing the precursor pools; (3) analysis of critical intermediates and inactivation of competitive pathway genes to redirect carbon flux to 3'-SL biosynthesis; and (4) enhanced catalytic performance of rate-limiting enzyme α2,3-SiaT by RBS screening, protein tag cloning. The final strain BZAPKA14 yielded 9.04 g/L 3'-SL in a shake flask. In a 3 L bioreactor, fed-batch fermentation generated 44.2 g/L 3'-SL, with an overall yield and lactose conversion of 0.53 g/(L h) and 0.55 mol 3'-SL/mol, respectively.

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.

Enabling pathway design by multiplex experimentation and machine learning

The remarkable metabolic diversity observed in nature has provided a foundation for sustainable production of a wide array of valuable molecules. However, transferring the biosynthetic pathway to the desired host often runs into inherent failures that arise from intermediate accumulation and reduced flux resulting from competing pathways within the host cell. Moreover, the conventional trial and error methods utilized in pathway optimization struggle to fully grasp the intricacies of installed pathways, leading to time-consuming and labor-intensive experiments, ultimately resulting in suboptimal yields. Considering these obstacles, there is a pressing need to explore the enzyme expression landscape and identify the optimal pathway configuration for enhanced production of molecules. This review delves into recent advancements in pathway engineering, with a focus on multiplex experimentation and machine learning techniques. These approaches play a pivotal role in overcoming the limitations of traditional methods, enabling exploration of a broader design space and increasing the likelihood of discovering optimal pathway configurations for enhanced production of molecules. We discuss several tools and strategies for pathway design, construction, and optimization for sustainable and cost-effective microbial production of molecules ranging from bulk to fine chemicals. We also highlight major successes in academia and industry through compelling case studies.

Enhancing Glycosylation of Flavonoids by Engineering the Uridine Diphosphate Glucose Supply in Escherichia coli

Glycosylation can enhance the solubility and stability of flavonoids. The main limitation of the glycosylation process is low intracellular uridine diphosphate glucose (UDPG) availability. This study aimed to create a glycosylation platform strain in Escherichia coli BL21(DE3) by multiple metabolic engineering of the UDPG supply. Glycosyltransferase TcCGT1 was introduced to synthesize vitexin and orientin from apigenin and luteolin, respectively. To further expand this glycosylation platform strain, not only were UDP rhamnose and UDP galactose synthesis pathways constructed, but rhamnosyltransferase (GtfC) and galactosyltransferase (PhUGT) were also introduced, respectively. In a 5 L bioreactor with apigenin, luteolin, kaempferol, and quercetin as glycosyl acceptors, vitexin, orientin, afzelin, quercitrin, hyperoside, and trifolin glycosylation products reached 17.2, 36.5, 5.2, 14.1, 6.4, and 11.4 g/L, respectively, the highest titers reported to date for all. The platform strain has great potential for large-scale production of glycosylated flavonoids.

High-Level Production of Lacto-N-neotetraose in Escherichia coli by Stepwise Optimization of the Biosynthetic Pathway

Lacto-N-neotetraose (LNnT), an abundant human milk oligosaccharide (HMO), has been approved as a novel functional additive for infant formulas. Therefore, LNnT biosynthesis has attracted extensive attention. Here, a high LNnT-producing, low lacto-N-triose II (LNT II)-residue Escherichia coli strain was constructed. First, an initial LNnT-producing chassis strain was constructed by blocking lactose, UDP-N-acetylglucosamine, and UDP-galactose competitive consumption pathways and introducing β-1,3-N-acetylglucosaminyltransferase LgtA and β-1,4-galactosyltransferase LgtB. Subsequently, the supply of LNnT precursors was increased by enhancing UDP-N-acetylglucosamine and UDP-galactose synthesis, inactivating LNT II extracellular transporter SetA, and improving UTP synthesis. Then, modular engineering strategy was used to optimize LNnT biosynthetic pathway fluxes. Moreover, pathway fluxes were fine-tuned by modulating translation initiation strength of essential genes lgtB, prs, and lacY. Finally, LNnT production reached 6.70 g/L in a shake flask and 19.40 g/L in a 3 L bioreactor with 0.47 g/(L h) productivity, with 1.79 g/L LNT II residue, highest productivity level, and lowest LNT II residue thus far.

Permeabilized whole cells containing co-expressed cyclomaltodextrinase and maltooligosyltrehalose synthase facilitate the synthesis of nonreducing maltoheptaose (N-G7) from β-cyclodextrin

BACKGROUND

Maltodextrin is an important bulk ingredient in food and other industries; however, drawbacks such as uneven polymerization and high reducibility limit its utilization. Nonreducing maltoheptaose (N-G7) is a good substitute for maltodextrin owing to its single degree of polymerization and its nonreducing properties. In this study, in vitro cell factory biotransformation of β-cyclodextrin (β-CD) to N-G7 is demonstrated using coexpressed cyclomaltodextrinase (CDase, EC 3.2.1.54) and maltooligosyltrehalose synthase (MTSase, EC 5.4.99.15). However, the cell membrane prevents β-CD from entering the cell owing to its large diameter.

RESULTS

The amylase-deficient permeabilized host ΔycjM-ΔmalS-ΔlpxM is utilized for the coexpression of recombinant CDase and MTSase. Deletion of lpxM effectively allows the entry of β-cyclodextrin into the cell, despite its large diameter, without requiring any relevant cell membrane permeability-promoting reagent. This results in a 28.44% increase in the efficiency of β-CD entry into the cell, thus enabling intracellular N-G7 synthesis without the extracellular secretion of recombinant CDase and MTSase. After reacting for 5.5 h, the highest purity of N-G7 (65.50%) is obtained. However, hydrolysis decreases the purity of N-G7 to 49.30%, thus resulting in a conversion rate of 40.16% for N-G7 when the reaction lasts 6 h. Precise control of reaction time is crucial for obtaining high-purity N-G7.

CONCLUSION

Whole-cell catalysis avoids cell fragmentation and facilitates the creation of an eco-friendly, energy-efficient biotransformation system; thus, it is a promising approach for N-G7 synthesis. © 2023 Society of Chemical Industry.

Bacterial glycobiotechnology: A biosynthetic route for the production of biopharmaceutical glycans

The recent advancement in the human glycome and progress in the development of an inclusive network of glycosylation pathways allow the incorporation of suitable machinery for protein modification in non-natural hosts and explore novel opportunities for constructing next-generation tailored glycans and glycoconjugates. Fortunately, the emerging field of bacterial metabolic engineering has enabled the production of tailored biopolymers by harnessing living microbial factories (prokaryotes) as whole-cell biocatalysts. Microbial catalysts offer sophisticated means to develop a variety of valuable polysaccharides in bulk quantities for practical clinical applications. Glycans production through this technique is highly efficient and cost-effective, as it does not involve expensive initial materials. Metabolic glycoengineering primarily focuses on utilizing small metabolite molecules to alter biosynthetic pathways, optimization of cellular processes for glycan and glycoconjugate production, characteristic to a specific organism to produce interest tailored glycans in microbes, using preferably cheap and simple substrate. However, metabolic engineering faces one of the unique challenges, such as the need for an enzyme to catalyze desired substrate conversion when natural native substrates are already present. So, in metabolic engineering, such challenges are evaluated, and different strategies have been developed to overcome them. The generation of glycans and glycoconjugates via metabolic intermediate pathways can still be supported by glycol modeling achieved through metabolic engineering. It is evident that modern glycans engineering requires adoption of improved strain engineering strategies for creating competent glycoprotein expression platforms in bacterial hosts, in the future. These strategies include logically designing and introducing orthogonal glycosylation pathways, identifying metabolic engineering targets at the genome level, and strategically improving pathway performance (for example, through genetic modification of pathway enzymes). Here, we highlight current strategies, applications, and recent progress in metabolic engineering for producing high-value tailored glycans and their applications in biotherapeutics and diagnostics.

Engineering Escherichia coli for high-level production of lacto-N-fucopentaose I by stepwise de novo pathway construction

Lacto-N-fucopentaose I (LNFP I) is an abundant and important fucosylated human milk oligosaccharide (HMO). Here, an efficient LNFP I-producing strain without by-product 2'-fucosyllactose (2'-FL) was developed by advisable stepwise de novo pathway construction in Escherichia coli. Specifically, the genetically stable lacto-N-triose II (LNTri II)-producing strains were constructed by the multicopy integration of β1,3-N-acetylglucosaminyltransferase. LNTri II can be further converted to lacto-N-tetraose (LNT) by LNT-producing β1,3-galactosyltransferase. The de novo and salvage pathways of GDP-fucose were introduced into highly efficient LNT-producing chassis. Specific α1,2-fucosyltransferase was verified to eliminate by-product 2'-FL, and binding free energy of the complex was analyzed to explain the product distribution. Subsequently, further attempts aiming to improve α1,2-fucosyltransferase activity and the supply of GDP-fucose were carried out. Our engineering strategies enabled the stepwise de novo construction of strains that produced up to 30.47 g/L of extracellular LNFP I, without accumulation of 2'-FL, and with only minor intermediates residue.

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

Lacto-N-tetraose (LNT) is an important neutral human milk oligosaccharide (HMO) and acts as a significant core structure for complex HMO biosynthesis. We previously achieved high-yield LNT biosynthesis (57.5 g/L) using fed-batch fermentation; however, residual lacto-N-triose II (LNTri II) was also found (21.58 g/L). Here, we re-engineered an efficient LNT-producing Escherichia coli with low LNTri II accumulation using genetically stable LNTri II-producing strains with a genomic insertion of lgtA (encoding β1,3-N-acetylglucosaminyltransferase). Comparable and low titers of LNT (3.73-4.61 g/L) and LNTri II (0.33-0.63 g/L), respectively, were obtained by introducing β1,3-galactosyltransferase. To reduce residual LNTri II, the E. coli transporter gene setA was disrupted, obviously reducing the accumulation of LNTri II and LNT. Next, the gene encoding β-N-acetylhexosaminidase (BbhI) was introduced into LNT-producing strains or E. coli BL21(DE3) for single- or mixed-strain cultivation, respectively. Finally, LNT was obtained (30.13 g/L) in a cocultivation system of mixed engineered strains without undesired LNTri II.

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.

Metabolic Engineering of Escherichia coli for High-Level Production of Lacto-N-neotetraose and Lacto-N-tetraose

Lacto-N-neotetraose (LNnT) and lacto-N-tetraose (LNT) are important oligosaccharides found in breast milk and are commonly used as nutritional supplements in infant formula. We used metabolic engineering techniques to optimize the modified Escherichia coli BL21 star (DE3) strain for efficient synthesis of LNnT and LNT using β-1,4-galactosyltransferase (HpgalT) from Helicobacter pylori and β-1,3-galactosyltransferase (SewbdO) from Salmonella enterica subsp. salamae serovar, respectively. Further, we optimized the expression of three key genes, lgtA, galE, and HpgalT (SewbdO), to synthesize LNnT or LNT and deleted several genes (ugd, ushA, agp, wcaJ, otsA, and wcaC) to block competition in the UDP-galactose synthesis pathway. The optimized strain produced LNnT or LNT with a titer of 22.07 or 48.41 g/L, respectively, in a supplemented batch culture, producing 0.41 or 0.73 g/L/h, respectively. The strategies used in this study contribute to the development of cell factories for high-level LNnT and LNT and their derivatives.

Phase-Separated Synthetic Organelles Based on Intrinsically Disordered Protein Domain for Metabolic Pathway Assembly in Escherichia coli

Extensive research efforts have been focused on spatial organization of biocatalytic cascades or catalytic networks in confined cellular environments. Inspired by the natural metabolic systems that spatially regulate pathways via sequestration into subcellular compartments, formation of artificial membraneless organelles through expressing intrinsically disordered proteins in host strains has been proven to be a feasible strategy. Here we report the engineering of a synthetic membraneless organelle platform, which can be used to extend compartmentalization and spatially organize pathway sequential enzymes. We show that heterologous overexpression of the RGG domain derived from the disordered P granule protein LAF-1 in an Escherichia coli strain can form intracellular protein condensates via liquid-liquid phase separation. We further demonstrate that different clients can be recruited to the synthetic compartments via directly fusing with the RGG domain or cooperating with different protein interaction motifs. Using the 2'-fucosyllactose de novo biosynthesis pathway as a model system, we show that clustering sequential enzymes into synthetic compartments can effectively increase the titer and yield of the target product compared to strains with free-floating pathway enzymes. The synthetic membraneless organelle system constructed here gives a promising approach in the development of microbial cell factories, wherein it could be used for the compartmentalization of pathway enzymes to streamline metabolic flux.

Directed evolution of a β-N-acetylhexosaminidase from Haloferula sp. for lacto-N-triose II and lacto-N-neotetraose synthesis from chitin

In our previous study, a β-N-acetylhexosaminidase (HaHex74) from Haloferula sp. showing high human milk oligosaccharides (HMOs) synthesis ability was identified and characterized. In this study, HaHex74 was further engineered by directed evolution and site-saturation mutagenesis to improve its transglycosylation activity for HMOs synthesis. A mutant (mHaHex74) with improved transglycosylation activity (HaHex74-Asn401Ile/His394Leu) was obtained and characterized. mHaHex74 exhibited maximal activity at pH 5.5 and 35 °C, respectively, which were distinct from that of HaHex74 (pH 6.5 and 45 °C). Moreover, mHaHex74 showed the highest LNT2 conversion ratio of 28.2% from N,N'-diacetyl chitobiose (GlcNAc₂), which is 2.2 folds higher than that of HaHex74. A three-enzyme cascade reaction for the synthesis of LNT2 and LNnT from chitin was performed in a 5-L reactor, and the contents of LNT2 and LNnT reached up to 15.0 g L¹ and 4.9 g L¹, respectively. Therefore, mHaHex74 maybe a good candidate for enzymatic synthesis of HMOs.

Biosynthesis of Human Milk Oligosaccharides: Enzyme Cascade and Metabolic Engineering Approaches

Human milk oligosaccharides (HMOs) have unique beneficial effects for infants and are considered as the new gold standard for premium infant formula. They are a collection of unconjugated glycans, and more than 200 distinct structures have been identified. Generally, HMOs are enzymatically produced by elongation and/or modification from lactose via stepwise glycosylation. Each glycosylation requires a specific glycosyltransferase (GT) and the corresponding nucleotide sugar donor. In this review, the typical HMO-producing GTs and the one-pot multienzyme modules for generating various nucleotide sugar donors are introduced, the principles for designing the enzyme cascade routes for HMO synthesis are described, and the important metabolic engineering strategies for mass production of HMOs are also reviewed. In addition, the future research directions in biotechnological production of HMOs were prospected.

Recent progress in health effects and biosynthesis of lacto-N-tetraose, the most dominant core structure of human milk oligosaccharide

Human milk oligosaccharides (HMOs), which are a group of complex carbohydrates highly abundant in human milk, have been recognized as critical functional biomolecules for infant health. Lacto-N-tetraose (LNT) is one of the most abundant HMO members and the most dominant core structure of HMO. The promising physiological effects of LNT have been well documented, including prebiotic property, antiadhesive antimicrobial activity, and antiviral effect. Its safety has been evaluated and it has been commercially added to infant formula as a functional ingredient. Because of great commercial importance of LNT, increasing attention has been paid to its highly efficient biological production. In particular, microbial synthesis based on metabolic engineering displays obvious advantages in large-scale production of LNT. This review contains important information about the recent progress in physiological effects, safety evaluation, and biosynthesis of LNT.

Spatial organization of pathway enzymes via self-assembly to improve 2'-fucosyllactose biosynthesis in engineered Escherichia coli

As one of the most abundant components in human milk oligosaccharides, 2'-fucosyllactose (2'-FL) possesses versatile beneficial health effects. Although most studies focused on overexpressing or fine-tuning the expression of pathway enzymes and achieved a striking increase of 2'-FL production, directly facilitating the metabolic flux toward the key intermediate GDP-l-fucose seems to be ignored. Here, multienzyme complexes consisting of sequential pathway enzymes were constructed by using specific peptide interaction motifs in recombinant Escherichia coli to achieve a higher titer of 2'-FL. Specifically, we first fine-tuned the expression level of pathway enzymes and balanced the metabolic flux toward 2'-FL synthesis. Then, two key enzymes (GDP-mannose 4,6-dehydratase and GDP- l-fucose synthase) were self-assembled into enzyme complexes in vivo via a short peptide interaction pair RIAD-RIDD (RI anchoring disruptor-RI dimer D/D domains), resulting in noticeable improvement of 2'-FL production. Next, to further strengthen the metabolic flux toward 2'-FL, three pathway enzymes were further aggregated into multienzyme assemblies by using another orthogonal protein interaction motif (Spycatcher-SpyTag or PDZ-PDZlig). Intracellular multienzyme assemblies remarkably enlarged the flux toward 2'-FL biosynthesis and showed a 2.1-fold increase of 2'-FL production compared with a strain expressing free-floating and unassembled enzymes. The optimally engineered strain EZJ23 accumulated 4.8 g/L 2'-FL in shake flask fermentation and was capable of producing 25.1 g/L 2'-FL by fed-batch cultivation. This work provides novel approaches for further improvement and large-scale production of 2'-FL and demonstrates the effectiveness of spatial assembly of pathway enzymes to improve the production of valuable products in the engineered host strain.

Multi-level metabolic engineering of Escherichia coli for high-titer biosynthesis of 2'-fucosyllactose and 3-fucosyllactose

Fucosyllactoses (FL), including 2'-fucosyllactose (2'-FL) and 3-fucosyllactose (3-FL), have garnered considerable interest for their value in newborn formula and pharmaceuticals. In this study, an engineered Escherichia coli was developed for high-titer FL biosynthesis by introducing multi-level metabolic engineering strategies, including (1) individual construction of the 2'/3-FL-producing strains through gene combination optimization of the GDP-L-fucose module; (2) screening of rate-limiting enzymes (α-1,2-fucosyltransferase and α-1,3-fucosyltransferase); (3) analysis of critical intermediates and inactivation of competing pathways to redirect carbon fluxes to FL biosynthesis; (4) enhancement of the catalytic performance of rate-limiting enzymes by the RBS screening, fusion peptides and multi-copy gene cloning. The final strains EC49 and EM47 produced 9.36 g/L for 2'-FL and 6.28 g/L for 3-FL in shake flasks with a modified-M9CA medium. Fed-batch cultivations of the two strains generated 64.62 g/L of 2'-FL and 40.68 g/L of 3-FL in the 3-L bioreactors, with yields of 0.65 mol 2'-FL/mol lactose and 0.67 mol 3-FL/mol lactose, respectively. This research provides a viable platform for other high-value-added compounds production in microbial cell factories.

Integration of Multiple Phage Attachment Sites System to Create the Chromosomal T7 System for Protein Production in Escherichia coli Nissle 1917

Escherichia coli Nissle 1917 (EcN) is a probiotic used to treat gastrointestinal diseases. The probiotic and endotoxin-free characteristics of EcN support its potential to be developed into a microbial expression system. With this aim, in this study, the powerful T7 expression system was constructed in the cryptic plasmid-free EcN (EcNP) to generate the T7 expression host ENL6P. The concept of multiple copies of gene expression cassettes regulated by the chromosomal T7 promoter was promoted due to plasmid instability issues with protein production in ENL6P. The integration of multiple phage attachment sites (IMPACT) system, which combined Cre-lox72, CRIM, and lambda red recombinase systems, was designed to simplify the manipulation and achieve the multiple φ80 bacterial attachment sites (attB) in ENL6P to generate the new strain ENL6PP4 with four φ80 attB sites. The strain can simultaneously integrate four copies of gene expression cassettes in the chromosome to produce recombinant proteins. The IMPACT systems incorporated several tools in gene editing to rapidly achieve more robust and stable microbial strains for research and various industrial applications.

Designing a Highly Efficient Biosynthetic Route for Lacto-N-Neotetraose Production in Escherichia coli

Recently, the biosynthesis of human milk oligosaccharides (HMOs) has been attracting increasing attention. Lacto-N-neotetraose (LNnT) is one of the most important neutral-core HMOs with promising health effects for infants. It has received Generally Recognized as Safe (GRAS) status and is the second HMO commercially added in infant formula after 2'-fucosyllactose. In previous studies, a series of engineered Escherichia coli strains have been constructed and optimized to produce high titers of precursor lacto-N-triose II. On the basis of these strains, LNnT-producing strains were constructed by overexpressing the β1,4-galactosyltransferase-encoding gene from Aggregatibacter actinomycetemcomitans NUM4039 (Aa-β1,4-GalT). Interestingly, an appreciable LNnT titer was obtained by weakening the metabolic flux of the UDP-GlcNAc pathway and simply overexpressing the essential genes lgtA, galE, and Aa-β1,4-GalT in lacZ-, wecB-, and nagB-deleted E. coli. Subsequently, LNnT synthesis was optimized through balancing the expression of these three biosynthetic enzymes. The optimized strain produced LNnT with an extracellular titer of 12.1 g/L in fed-batch cultivation, with the productivity and specific yield of 0.25 g/L·h and 0.27 g/g dry cell weight, respectively.

Engineering Escherichia coli for the High-Titer Biosynthesis of Lacto-N-tetraose

Lacto-N-tetraose (LNT), a member of the human milk oligosaccharides family, has received widespread attention because of its importance in infant health. We constructed a whole-cell biotransformation method in Escherichia coli BL21(DE3) for high-titer LNT synthesis. The approach was performed by using a systematic design and metabolic engineering based on the metabolic pathway of LNT. The lgtA (encoding β-1,3-N-acetylglucosaminyltransferase) and wbgO (encoding β-1,3-galactosyltransferase) genes were introduced into the engineered E. coli BL21(DE3) to construct an LNT-producing starting strain B1 (0.22 g/L). Then, the genes related to the LNT metabolic pathway were screened in two vectors to evaluate LNT synthesis. The lgtA-wbgO and galE-galT-galK genes were overexpressed through the two-plasmid system in E. coli BL21(DE3). The titer of LNT (3.42 g/L) had a gain of 14.55 times compared with that of B1. Furthermore, the ugd gene, which was associated with the UDP-Gal bypass pathway, was inactivated to further improve LNT production in shake-flask cultivation (4.14 g/L). The final fed-batch cultivation of the engineered strain produced 31.56 g/L of LNT. This study provided a strategy for the effective production of LNT in E. coli.

Efficient Production of a Functional Human Milk Oligosaccharide 3'-Sialyllactose in Genetically Engineered Escherichia coli

3'-Sialyllactose (3'-SL) is one of the most important and simplest sialylated human milk oligosaccharides. In this study, a plasmid-based pathway optimization along with chromosomal integration strategies was applied for 3'-SL production. Specifically, the precursor CMP-Neu5Ac synthesis pathway genes and α2,3-sialyltransferase-encoding gene were introduced into Escherichia coli BL21(DE3)ΔlacZ to realize 3'-SL synthesis. Genes nanA and nanK involved in Neu5Ac catabolism were further deleted to reduce the metabolic flux of competitive pathway. Several α2,3-sialyltransferases from different species were selected to evaluate the sialylation effect. The precursor pools were balanced and improved by optimizing key enzyme expression involved in the UDP-GlcNAc and CMP-Neu5Ac synthesis pathway. Finally, an additional α2,3-sialyltransferase expression cassette was integrated into chromosome to maximize 3'-SL synthesis, and 4.5 g/L extracellular 3'-SL was produced at a shake-flask level. The extracellular 3'-SL concentration was raised to 23.1 g/L in a 5 L bioreactor fermentation, which represents the highest extracellular value ever reported.

Pathway Optimization and Uridine 5'-Triphosphate Regeneration for Enhancing Lacto-N-Tetraose Biosynthesis in Engineered Escherichia coli

Recently, human milk oligosaccharides (HMOs) have attracted increasing attention and display great commercial importance, especially for the infant formula industry. Lacto-N-tetraose (LNT) is an important neutral HMO commercially added in infant formula and a core structure for synthesizing complex HMOs. Previously, a novel LNT-generating β-1,3-galactosyltransferase from Pseudogulbenkiania ferrooxidans was identified and used for construction of an LNT-producing engineered Escherichia coli. In this work, LNT biosynthesis was further enhanced by pathway optimization and uridine 5'-triphosphate (UTP) regeneration. The main strategies included genomic integration of UDP-glucose 4-epimerase-encoding gene, fine-tuning of the LNT pathway-related genes, blocking of competitive pathways related to UDP-galactose, and overexpression of UTP supply related genes. The maximal LNT titer reached 6.16 and 57.5 g/L by shake-flask and fed-batch fermentation, respectively.

Designing of an Efficient Whole-Cell Biocatalyst System for Converting L-Lysine Into Cis-3-Hydroxypipecolic Acid

Cis-3-hydroxypipecolic acid (cis-3-HyPip), a key structural component of tetrapeptide antibiotic GE81112, which has attracted substantial attention for its broad antimicrobial properties and unique ability to inhibit bacterial translation initiation. In this study, a combined strategy to increase the productivity of cis-3-HyPip was investigated. First, combinatorial optimization of the ribosomal binding site (RBS) sequence was performed to tune the gene expression translation rates of the pathway enzymes. Next, in order to reduce the addition of the co-substrate α-ketoglutarate (2-OG), the major engineering strategy was to reconstitute the tricarboxylic acid (TCA) cycle of Escherichia coli to force the metabolic flux to go through GetF catalyzed reaction for 2-OG to succinate conversion, a series of engineered strains were constructed by the deletion of the relevant genes. In addition, the metabolic flux (gltA and icd) was improved and glucose concentrations were optimized to enhance the supply and catalytic efficiency of continuous 2-OG supply powered by glucose. Finally, under optimal conditions, the cis-3-HyPip titer of the best strain catalysis reached 33 mM, which was remarkably higher than previously reported.

Transporter Engineering Enables the Efficient Production of Lacto-N-triose II and Lacto-N-tetraose in Escherichia coli

Lacto-N-triose (LNT II) and lacto-N-tetraose (LNT) are human milk oligosaccharides (HMOs) with various potential functions for infants. HMO production by Escherichia coli fermentation has attracted attention in recent years. However, little is known about the cellular export of HMOs. In this study, we identified four endogenous E. coli transporter genes (setA, setB, ydeA, and mdfA), overexpression of which significantly increased the efficiency of LNT II production. The setA-enhanced strain accumulated 34.2 g/L LNT II in a 3 L bioreactor. In the production of LNT, which uses LNT II as an intermediate, disruption of setA remarkably decreased the LNT II accumulation and enhanced the titer of LNT. Furthermore, by heterologous expression of extracellular β-1,3-N-acetylglucosaminidase from Bifidobacterium bifidum, which degrades LNT II, we eliminated LNT II completely. This study shows that regulation of sugar efflux transporters in E. coli can increase the production of HMOs and decrease the amounts of undesired byproducts.

Recent Advances on Lacto-N-neotetraose, a Commercially Added Human Milk Oligosaccharide in Infant Formula

Human milk oligosaccharides (HMOs) act as the important prebiotics and display many unique health effects for infants. Lacto-N-neotetraose (LNnT), an abundant HMO, attracts increasing attention because of its unique beneficial effects to infants and great commercial importance. It occurs in all groups of human milk, but the concentration generally decreases gradually with the lactation period. It has superior prebiotic property for infants, and its other health effects have also been verified, including being immunomodulatory, anti-inflammatory, preventing necrotizing enterocolitis, antiadhesive antimicrobials, antiviral activity, and promoting maturation of intestinal epithelial cells. Safety evaluation and clinical trial studies suggest that LNnT is safe and well-tolerant for infants. It has been commercially added as a functional ingredient in infant formula. LNnT can be synthesized via chemical, enzymatic, or cell factory approachs, among which the metabolic engineering-based cell factory synthesis is considered to be the most practical and effective. In this article, the occurrence and physiological effects of LNnT were reviewed in detail, the safety evaluation and regulation status of LNnT were described, various approaches to LNnT synthesis were comprehensively summarized and compared, and the future perspectives of LNnT-related studies were provided.

Occurrence, functional properties, and preparation of 3-fucosyllactose, one of the smallest human milk oligosaccharides

Human milk oligosaccharides (HMOs) are receiving wide interest and high attention due to their health benefits, especially for newborns. The HMOs-fortified products are expected to mimic human milk not only in the kinds of added oligosaccharides components but also the appropriate proportion between these components, and further provide the nutrition and physiological effects of human milk to newborns as closely as possible. In comparison to intensively studied 2'-fucosyllactose (2'-FL), 3-fucosyllactose (3-FL) has less attention in almost all respects. Nerveless, 3-FL naturally occurs in breast milk and increases roughly over the course of lactation with a nonnegligible content, and plays an irreplaceable role in human milk and delivers functional properties to newborns. According to the safety evaluation, 3-FL shows no acute oral toxicity, genetic toxicity, and subchronic toxicity. It has been approved as generally recognized as safe (GRAS). Biological production of 3-FL can be realized by enzymatic and cell factory approaches. The α1,3- or α1,3/4-fucosyltransferase is the key enzyme for 3-FL biosynthesis. Various metabolic engineering strategies have been applied to enhance 3-FL yield using cell factory approach. In conclusion, this review gives an overview of the recent scientific literatures regarding occurrence, bioactive properties, safety evaluation, and biotechnological preparation of 3-FL.

Glycosyltransferase from Bacteroides gallinaceum Is a Novel α-1,3-Fucosyltransferase that Can Be Used for 3-Fucosyllactose Production In Vivo by Metabolically Engineered Escherichia coli

As one of the attractive fucosylated human milk oligosaccharides, the biological production of 3-fucosyllactose (3-FL) has received great attention, as it exhibits many excellent physiological functions for infants. In this work, a novel 3-FL-producing α-1,3-fucosyltransferase (α1,3-FucT) named FutM2 from Bacteroides gallinaceum was first selected from nine potential candidates in the NCBI database. Then, a highly 3-FL-producing engineered Escherichia coli strain was constructed by modular pathway enhancement including the GDL-l-fucose precursor supply by overexpressing manC, manB, gmd, and wcaG (CBGW), and the 3-FL synthesis pathway by introducing B. gallinaceum FutM2. Finally, a titer of 20.3 g L^-1 and productivity of 0.40 g L^-1 h^-1 of 3-FL were achieved in the 3-L bioreactor by engineered E. coli (ΔlacZΔwcaJ) harboring pCDF-CBGW and pET-futM2. Our study provided a novel α1,3-FucT from B. gallinaceum that could be used for 3-FL production, presenting an efficient microbial cell factory platform to de novo synthesize 3-FL from glycerol.

Cell-Free Biosynthesis System: Methodology and Perspective of in Vitro Efficient Platform for Pyruvate Biosynthesis and Transformation

The modification of intracellular metabolic pathways by metabolic engineering has generated many engineered strains with relatively high yields of various target products in the past few decades. However, the unpredictable accumulation of toxic products, the cell membrane barrier, and competition between the carbon flux of cell growth and product synthesis have severely retarded progress toward the industrial-scale production of many essential chemicals. On the basis of an in-depth understanding of intracellular metabolic pathways, scientists intend to explore more sustainable methods and construct a cell-free biosynthesis system in vitro. In this review, the synthesis and application of pyruvate as a platform compound is used as an example to introduce cell-free biosynthesis systems. We systematically summarize a proposed methodology workflow of cell-free biosynthesis systems, including pathway design, enzyme mining, enzyme modification, multienzyme assembly, and pathway optimization. Some new methods, such as machine learning, are also mentioned in this review.

Engineering Escherichia coli for highly efficient production of lacto-N-triose II from N-acetylglucosamine, the monomer of chitin

BACKGROUND

Lacto-N-triose II (LNT II), an important backbone for the synthesis of different human milk oligosaccharides, such as lacto-N-neotetraose and lacto-N-tetraose, has recently received significant attention. The production of LNT II from renewable carbon sources has attracted worldwide attention from the perspective of sustainable development and green environmental protection.

RESULTS

In this study, we first constructed an engineered E. coli cell factory for producing LNT II from N-acetylglucosamine (GlcNAc) feedstock, a monomer of chitin, by introducing heterologous β-1,3-acetylglucosaminyltransferase, resulting in a LNT II titer of 0.12 g L^-1. Then, lacZ (lactose hydrolysis) and nanE (GlcNAc-6-P epimerization to ManNAc-6-P) were inactivated to further strengthen the synthesis of LNT II, and the titer of LNT II was increased to 0.41 g L^-1. To increase the supply of UDP-GlcNAc, a precursor of LNT II, related pathway enzymes including GlcNAc-6-P deacetylase, glucosamine synthase, and UDP-N-acetylglucosamine pyrophosphorylase, were overexpressed in combination, optimized, and modulated. Finally, a maximum titer of 15.8 g L^-1 of LNT II was obtained in a 3-L bioreactor with optimal enzyme expression levels and β-lactose and GlcNAc feeding strategy.

CONCLUSIONS

Metabolic engineering of E. coli is an effective strategy for LNT II production from GlcNAc feedstock. The titer of LNT II could be significantly increased by modulating the gene expression strength and blocking the bypass pathway, providing a new utilization for GlcNAc to produce high value-added products.

Metabolic Engineering of Escherichia coli for Efficient Biosynthesis of Lacto-N-tetraose Using a Novel β-1,3-Galactosyltransferase from Pseudogulbenkiania ferrooxidans

Human milk oligosaccharides (HMOs) attract considerable interest in recent years because of their particular role in infant health. Lacto-N-tetraose (LNT), one of the most abundant HMOs, has been commercially added in the infant formula as a functional fortifier. In this study, a novel LNT-producing β-1,3-galactosyltransferase (β-1,3-GalT) from Pseudogulbenkiania ferrooxidans was screened from 14 putative candidates, and a highly LNT-producing metabolically engineered Escherichia coli strain was constructed based on a previously constructed lacto-N-triose II (LNT II)-producing strain, by strengthening UDP-galactose synthesis and introduction of P. ferrooxidans β-1,3-GalT. The engineered strain produced 3.11 and 25.49 g/L LNT in shake-flask and fed-batch cultivation, with the molar conversion ratio of LNT II to LNT of 88.15 and 85.09%, respectively. The productivity and specific yield of LNT in fed-batch cultivation were measured to be 0.61 g/L·h and 0.76 g/g dry cell weight, respectively. To the best of our knowledge, it is the highest LNT yield ever reported.

Efficient biosynthesis of lacto-N-neotetraose by a novel β-1,4-galactosyltransferase from Aggregatibacter actinomycetemcomitans NUM4039

Lacto-N-neotetraose (LNnT) is a unique tetrasaccharide naturally occurring in human milk, as an important member of human milk oligosaccharides. Because of promising beneficial effects, it has been commercially added as a functional fortifier in infant formula. β-1,4-Galactosyltransferase (β-1,4-GalT) catalyzes LNnT biosynthesis from uridine 5'-diphospho-galactose (UDP-Gal) to lacto-N-triose II (LNT II). There have been only two LNnT-producing bacterial β-1,4-GalTs, including the ones from Neisseria meningitidis and Histophilus somni. In this study, a novel LNnT-producing β-1,4-GalT was identified from Aggregatibacter actinomycetemcomitans. The enzyme was easily overexpressed in E. coli in soluble form. It displayed much higher transglycosylation versus hydrolysis activity, indicating its great potential in LNnT biosynthesis. The enzyme produced 13 mM LNnT from 20 mM LNT II and 60 mM UDP-Gal, with the yield of 65 % on LNT II and very low level of UDP-Gal hydrolysis. Therefore, it could be considered as a good candidate for the practical LNnT production.

Synthesis as an Expanding Resource in Human Milk Science

Few classes of natural products rival the structural audacity of oligosaccharides. Their complexity, however, has stood as an immense roadblock to translational research, as access to homogeneous material from nature is challenging. Thus, while carbohydrates are critical to the myriad functional and structural aspects of the biological sciences, their behavior is largely descriptive. This challenge presents an attractive opportunity for synthetic chemistry, particularly in the area of human milk science. First, there is an inordinate need for synthesizing homogeneous human milk oligosaccharides (HMOs). Superimposed on this goal is the mission of conducting syntheses at scale to enable animal studies. Herein, we present a personalized rumination of our involvement, and that of our colleagues, which has led to the synthesis and characterization of HMOs and mechanistic probes. Along the way, we highlight chemical, chemoenzymatic, and synthetic biology based approaches. We close with a discussion on emergent challenges and opportunities for synthesis, broadly defined, in human milk science.

Physiological effects, biosynthesis, and derivatization of key human milk tetrasaccharides, lacto-N-tetraose, and lacto-N-neotetraose

Human milk oligosaccharides (HMOs) have recently attracted ever-increasing interest because of their versatile physiological functions. In HMOs, two tetrasaccharides, lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT), constitute the essential components, each accounting 6% (w/w) of total HMOs. Also, they serve as core structures for fucosylation and sialylation, generating functional derivatives and elongation generating longer chains of core structures. LNT, LNnT, and their fucosylated and/or sialylated derivatives account for more than 30% (w/w) of total HMOs. For derivatization, LNT and LNnT can be modified into a series of complex fucosylated and/or sialylated HMOs by transferring fucose residues at α1,2-, α1,3-, and α1,3/4-linkage and/or sialic acid residues at α2,3- and α2,6-linkage. Such structural diversity allows these HMOs to possess great commercial value and an application potential in the food and pharmaceutical industries. In this review, we first elaborate the physiological functions of these tetrasaccharides and derivatives. Next, we extensively review recent developments in the biosynthesis of LNT, LNnT, and their derivatives in vitro and in vivo by employing advanced enzymatic reaction systems and metabolic engineering strategies. Finally, future perspectives in the synthesis of these HMOs using enzymatic and metabolic engineering approaches are presented.

A Novel β-1,4-Galactosyltransferase from Histophilus somni Enables Efficient Biosynthesis of Lacto-N-Neotetraose via Both Enzymatic and Cell Factory Approaches

Human milk oligosaccharides (HMOs) attract particular attention because of their health benefits for infants. Lacto-N-neotetraose (LNnT) is one of the most abundant neutral core structures of HMOs. Bacterial β-1,4-galactosyltransferase (β-1,4-GalT) displays an irreplaceable role in the practical application of LNnT biosynthesis. In this study, a novel β-1,4-GalT from Histophilus somni was identified to efficiently synthesize LNnT from UDP-Gal and lacto-N-triose II (LNT II). The optimum pH and temperature were determined to be pH 6.0 and 30 °C, respectively. The enzyme showed both transgalactosylation and hydrolysis activity, with a specific activity of 3.7 and 6.6 U/mg, respectively. LNnT was synthesized using H. somni β-1,4-GalT via both enzymatic and cell factory approaches, and both approaches provided an LNnT ratio with the remaining LNT II at approximately 1:2 when reactions attained a balance. These findings indicated that H. somni β-1,4-GalT has a potential in biosynthesis of LNnT and derivatives in future.