Biosynthesis of the human milk oligosaccharide 3-fucosyllactose in metabolically engineered Escherichia coli via the salvage pathway through increasing GTP synthesis and β-galactosidase modification

Recent advances of 3-fucosyllactose in health effects and production

Human milk oligosaccharides (HMOs) have been recognized as gold standard for infant development. 3-Fucosyllactose (3-FL), being one of the Generally Recognized as Safe HMOs, represents a core trisaccharide within the realm of HMOs; however, it has received comparatively less attention in contrast to extensively studied 2'-fucosyllactose. The objective of this review is to comprehensively summarize the health effects of 3-FL, including its impact on gut microbiota proliferation, antimicrobial effects, immune regulation, antiviral protection, and brain maturation. Additionally, the discussion also covers the commercial application and regulatory approval status of 3-FL. Lastly, an organized presentation of large-scale production methods for 3-FL aims to provide a comprehensive guide that highlights current strategies and challenges in optimization.

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.

Enhancing the soluble expression of α-1,2-fucosyltransferase in E. coli using high-throughput flow cytometry screening coupled with a split-GFP

2'-Fucosyllactose (2'-FL), one of the major human milk oligosaccharides, was produced in several engineered microorganisms. However, the low solubility of α-1,2-fucosyltransferase (α1,2-FucT) often becomes a bottleneck to produce maximum amount of 2'-FL in the microorganisms. To overcome this solubility issue, the following studies were conducted to improve the soluble expression of α1,2-FucT. Initially, hydrophobic amino acids in the hydrophilic region of the 6 α-helices were mutated, adhering to the α-helix rule. Subsequently, gfp11 was fused to the C-terminal of futC gene encoding α1,2-FucT (FutC), enabling selection of high-fluorescence mutants through split-GFP. Each mutant library was screened via fluorescence activated cell sorting (FACS) to separate soluble mutants for high-throughput screening. As a result, L80C single mutant and A121D/P124A/L125R triple mutant were found, and a combined quadruple mutant was created. Furthermore, we combined mutations of conserved sequences (Q150H/C151R/Q239S) of FutC, which showed positive effects in the previous studies from our lab, with the above quadruple mutants (L80C/A121D/P124A/L125R). The resulting strain produced approximately 3.4-fold higher 2'-FL titer than that of the wild-type, suggesting that the conserved sequence mutations are an independent subset of the mutations that further improve the solubility of the target protein acquired by random mutagenesis using split-GFP.

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).

Rational Design of an α-1,3-Fucosyltransferase for the Biosynthesis of 3-Fucosyllactose in Bacillus subtilis ATCC 6051a via De Novo GDP-l-Fucose Pathway

3-Fucosyllactose (3-FL) is an important oligosaccharide and nutrient in breast milk that can be synthesized in microbial cells by α-1,3-fucosyltransferase (α-1,3-FucT) using guanosine 5'-diphosphate (GDP)-l-fucose and lactose as substrates. However, the catalytic efficiency of known α-1,3-FucTs from various sources was limited due to their low solubility. To enhance the microbial production of 3-FL, the efficiencies of α-1,3-FucTs were evaluated and in Bacillus subtilis (B. subtilis) chassis cells that had been endowed with a heterologous synthetic pathway for GDP-l-fucose, revealing that the activity of FucTa from Helicobacter pylori (H. pylori) was higher than that of any of other reported homologues. To further improve the catalytic performance of FucTa, a rational design approach was employed, involving intracellular evaluation of the mutational sites of M32 obtained through directed evolution, analysis of the ligand binding site diversity, and protein structure simulation. Among the obtained variants, the FucTa-Y218 K variant exhibited the highest 3-FL yield, reaching 7.55 g/L in the shake flask growth experiment, which was 3.48-fold higher than that achieved by the wild-type enzyme. Subsequent fermentation optimization in a 5 L bioreactor resulted in a remarkable 3-FL production of 36.98 g/L, highlighting the great prospects of the designed enzyme and the strains for industrial applications.

Goat mammary gland metabolism: An integrated Omics analysis to unravel seasonal weight loss tolerance

Seasonal weight loss (SWL), is a major limitation to animal production. In the Canary Islands, there are two dairy goat breeds with different levels of tolerance to SWL: Majorera (tolerant) and Palmera (susceptible). Our team has studied the response of these breeds to SWL using different Omics tools. The objective of this study was to integrate such results in a data driven approach and using dedicated tools, namely the DIABLO method. The outputs of our analysis mainly separate unrestricted from restricted goats. Metabolites behave as "hub" molecules, grouping interactions with several genes and proteins. Unrestricted goats upregulated protein synthesis, along with arginine catabolism and adipogenesis pathways, which are related with higher anabolic rates and a larger proportion of secretory tissue, in agreement with their higher milk production. Contrarily, restricted goats seemingly increased the synthesis of acetyl-CoA through serine and acetate conversion into pyruvate. This may have occurred to increase fatty acid synthesis and/or to use them as an energy source in detriment to glucose, which was more available in the diet of unrestricted goats. Lastly, restricted Palmera upregulated the expression of PEBP4 and GPD1 genes compared to all other groups, which might support their use as putative biomarkers for SWL susceptibility. SIGNIFICANCE: Seasonal weight loss (SWL) is a major issue influencing animal production in the tropics and Mediterranean. By studying its impact on the mammary gland of tolerant and susceptible dairy goat breeds, using Omics, we aim at surveying the tissue for possible biomarkers that reflect these traits. In this study, data integration of three Omics (transcriptomics, proteomics and metabolomics) was performed using bioinformatic tools, to relate putative biomarkers and evaluate all three levels of information; in a novel approach. This information can enhance selection programs, lowering the impact of SWL on food production systems.

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.

Recent progress in fucosylated derivatives of lacto-N-tetraose and lacto-N-neotetraose

Human milk oligosaccharides (HMOs) have attracted considerable attention owing to their unique physiological functions. Two important tetrasaccharides, lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT), are core structures of HMOs. Their safety has been evaluated and they can be added to infant formula as functional ingredients. The fucosylated derivatives of LNT and LNnT, mainly lacto-N-fucopentaose (LNFP) I, LNFP II, LNFP III, and lacto-N-difucohexaose I, exhibit prominent physiological characteristics, including modificating the intestinal microbiota, immunomodulation, anti-bacterial activities, and antiviral infection. However, they have received lesser attention than 2'-fucosyllactose. As precursors, LNT and LNnT are connected to one or two fucosyl units through α1,2/3/4 glycosidic bonds, forming a series of compounds with complex structures. These complex fucosylated oligosaccharides can be biologically synthesized using enzymatic and cell factory approaches. This review summarizes the occurrence, physiological effects, and biosynthesis of fucosylated LNT and LNnT derivatives and their future development.

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.

Rational design of GDP‑D‑mannose mannosyl hydrolase for microbial L‑fucose production

BACKGROUND

L‑Fucose is a rare sugar that has beneficial biological activities, and its industrial production is mainly achieved with brown algae through acidic/enzymatic fucoidan hydrolysis and a cumbersome purification process. Fucoidan is synthesized through the condensation of a key substance, guanosine 5'‑diphosphate (GDP)‑L‑fucose. Therefore, a more direct approach for biomanufacturing L‑fucose could be the enzymatic degradation of GDP‑L‑fucose. However, no native enzyme is known to efficiently catalyze this reaction. Therefore, it would be a feasible solution to engineering an enzyme with similar function to hydrolyze GDP‑L‑fucose.

RESULTS

Herein, we constructed a de novo L‑fucose synthetic route in Bacillus subtilis by introducing heterologous GDP‑L‑fucose synthesis pathway and engineering GDP‑mannose mannosyl hydrolase (WcaH). WcaH displays a high binding affinity but low catalytic activity for GDP‑L‑fucose, therefore, a substrate simulation‑based structural analysis of the catalytic center was employed for the rational design and mutagenesis of selected positions on WcaH to enhance its GDP‑L‑fucose‑splitting efficiency. Enzyme mutants were evaluated in vivo by inserting them into an artificial metabolic pathway that enabled B. subtilis to yield L‑fucose. WcaH^R36Y/N38R was found to produce 1.6 g/L L‑fucose during shake‑flask growth, which was 67.3% higher than that achieved by wild‑type WcaH. The accumulated L‑fucose concentration in a 5 L bioreactor reached 6.4 g/L.

CONCLUSIONS

In this study, we established a novel microbial engineering platform for the fermentation production of L‑fucose. Additionally, we found an efficient GDP‑mannose mannosyl hydrolase mutant for L‑fucose biosynthesis that directly hydrolyzes GDP‑L‑fucose. The engineered strain system established in this study is expected to provide new solutions for L‑fucose or its high value‑added derivatives production.

Production of 2’-Fucosyllactose using ⍺1,2-fucosyltransferase from a GRAS bacterial strain

2'-Fucosyllactose (2'-FL) is a major oligosaccharide found in human breast milk. It is produced from GDP-L-fucose and D-lactose by ⍺1,2-fucosyltransferase (⍺1,2-fucT), but the enzyme has been identified mostly in pathogens. In this study, an ⍺1,2-fucT was isolated from a Generally Recognized as Safe (GRAS) Bacillus megaterium strain. The enzyme was successfully expressed in metabolically-engineered Escherichia coli. Furthermore, replacement of non-conserved amino acid residues with conserved ones in the protein led to an increase in the rate of 2'-FL production. As a result, fed-batch fermentation of E. coli produced 30 g/L of 2'-FL from glucose and lactose. Thus, the overproduction of 2'-FL using a novel enzyme from a GRAS bacteria strain was successfully demonstrated.

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.

Engineering Pheromone-Mediated Quorum Sensing with Enhanced Response Output Increases Fucosyllactose Production in Saccharomyces cerevisiae

Engineering dynamic control of gene expression is desirable because many engineered functions interfere with endogenous cellular processes that have evolved to facilitate growth and survival. Minimizing conflict between growth and production phases can therefore improve product titers in microbial cell factories. We developed an autoinduced gene expression system by rewiring the Saccharomyces cerevisiae pheromone response pathway. To ameliorate growth reduction due to the early onset response at low population densities, α-pheromone of Kluyveromyces lactis (Kα) instead of S. cerevisiae (Sα) was expressed in mating type "a" yeast. Kα-induced expression of pathway genes was further enhanced by the transcriptional activator Gal4p expressed under the control of the pheromone-responsive FUS1 promoter (P_fus1). As a demonstration, the engineered circuit combined with the deletion of the endogenous galactose metabolic pathway genes was applied to the production of human milk oligosaccharides, 2'-fucosyllactose (2'-FL) and 3-fucosllactose (3-FL). The engineered strains produced 3.37 g/L 2'-FL and 2.36 g/L 3-FL on glucose with a volumetric productivity of 0.14 and 0.03 g/L·h^-1 in batch flask cultivation, respectively. These represented 147 and 153% increases over the control strains on galactose wherein the respective pathway genes are expressed under GAL promoters only. Further fed-batch fermentation achieved titers of 32.05 and 20.91 g/L for 2' and 3-FL, respectively. The genetic program developed here thus represents a promising option for implementing dynamic regulation in yeast and could be used for the production of biochemicals that may place a heavy metabolic burden on cell growth.

Engineered Bacillus subtilis for the de novo production of 2'-fucosyllactose

BACKGROUND

The most abundant human milk oligosaccharide in breast milk, 2'-fucosyllactose (2'-FL), has been approved as an additive to infant formula due to its multifarious nutraceutical and pharmaceutical functions in promoting neonate health. However, the low efficiency of de novo synthesis limits the cost-efficient bioproduction of 2'-FL.

RESULTS

This study achieved 2'-FL de novo synthesis in a generally recognized as safe (GRAS) strain Bacillus subtilis. First, a de novo biosynthetic pathway for 2'-FL was introduced by expressing the manB, manC, gmd, wcaG, and futC genes from Escherichia coli and Helicobacter pylori in B. subtilis, resulting in 2'-FL production of 1.12 g/L. Subsequently, a 2'-FL titer of 2.57 g/L was obtained by reducing the competitive lactose consumption, increasing the regeneration of the cofactor guanosine-5'-triphosphate (GTP), and enhancing the supply of the precursor mannose-6-phosphate (M6P). By replacing the native promoter of endogenous manA gene (encoding M6P isomerase) with a constitutive promoter P7, the 2'-FL titer in shake flask reached 18.27 g/L. The finally engineered strain BS21 could produce 88.3 g/L 2'-FL with a yield of 0.61 g/g lactose in a 3-L bioreactor, without the addition of antibiotics and chemical inducers.

CONCLUSIONS

The efficient de novo synthesis of 2'-FL can be achieved by the engineered B. subtilis, paving the way for the large-scale bioproduction of 2'-FL titer in the future.

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.

Enhancing biosynthesis of 2'-Fucosyllactose in Escherichia coli through engineering lactose operon for lactose transport and α -1,2-Fucosyltransferase for solubility

2'-Fucosyllactose (2'-FL) is the most abundant oligosaccharide in human milk and one of the most actively studied human milk oligosaccharides (HMO). When 2'-FL is produced through biological production using a microorganism, like Escherichia coli, D-lactose is often externally fed as an acceptor substrate for fucosyltransferase (FT). When D-glucose is used as a carbon source for the cell growth and D-lactose is transported by lactose permease (LacY) in lac operon, D-lactose transport is under the control of catabolite repression (CR), limiting the supply of D-lactose for FT reaction in the cell, hence decreasing the production of 2'-FL. In this study, a remarkable increase of 2'-FL production was achieved by relieving the CR from the lac operon of the host E. coli BL21 and introducing adequate site-specific mutations into α-1,2-FT (FutC) for enhancement of catalytic activity and solubility. For the host engineering, the native lac promoter (P_lac ) was substituted for tac promoter (P_tac ), so that the lac operon could be turned on, but not subjected to CR by high D-glucose concentration. Next, for protein engineering of FutC, family multiple sequence analysis for conserved amino acid sequences and protein-ligand substrate docking analysis led us to find several mutation sites, which could increase the solubility of FutC and its activity. As a result, a combination of four mutation sites (F40S/Q150H/C151R/Q239S) was identified as the best candidate, and the quadruple mutant of FutC enhanced 2'-FL titer by 2.4-fold. When the above-mentioned E. coli mutant host transformed with the quadruple mutant of futC was subjected to fed-batch culture, 40 g l^-1 of 2'-FL titer was achieved with the productivity of 0.55 g l^-1 h^-1 and the specific 2'-FL yield of 1.0 g g^-1 DCW. This article is protected by copyright. All rights reserved.

Efficient Production of 2'-Fucosyllactose from l-Fucose via Self-Assembling Multienzyme Complexes in Engineered Escherichia coli

2'-Fucosyllactose (2'-FL) has been widely used as a nutritional additive in infant formula due to its multifarious nutraceutical and pharmaceutical functions in neonate health. As such, it is essential to develop an efficient and extensive microbial fermentation platform to cater to the needs of the 2'-FL market. In this study, a spatial synthetic biology strategy was employed to promote 2'-FL biosynthesis in recombinant Escherichia coli. First, the salvage pathway for 2'-FL production from l-fucose and lactose was constructed by introducing a bifunctional enzyme l-fucokinase/GDP-l-fucose pyrophosphorylase (Fkp) derived from Bacteroides fragilis and an α-1,2-fucosyltransferase (FutC) derived from Helicobacter pylori into engineered E. coli BL21(DE3). Next, the endogenous genes involved in the degradation and shunting of the substrate and key intermediate were inactivated to improve the availability of precursors for 2'-FL biosynthesis. Moreover, to further improve the yield and titer of 2'-FL, a short peptide pair (RIAD-RIDD) was used to form self-assembling multienzyme complexes in vivo. The spatial localization of peptides and stoichiometry of enzyme assemblies were subsequently optimized to further improve 2'-FL production. Finally, cofactor regeneration was also considered to alleviate the potential cofactor deficiency and redox flux imbalance in the biocatalysis process. Fed-batch fermentation of the final WLS20 strain accumulated 30.5 g/L extracellular 2'-FL with the yield and productivity of 0.661 mol/mol fucose and 0.48 g/L/h, respectively. This research has demonstrated that the application of spatial synthetic biology and metabolic engineering strategies can dramatically enlarge the titer and yield of 2'-FL biosynthesis in engineered E. coli.

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.

Engineered Microbial Routes for Human Milk Oligosaccharides Synthesis

Human milk oligosaccharides (HMOs) are one of the important ingredients in human milk, which have attracted great interest due to their beneficial effect on the health of newborns. The large-scale production of HMOs has been researched using engineered microbial routes due to the availability, safety, and low cost of host strains. In addition, the development of molecular biology technology and metabolic engineering has promoted the effectiveness of HMOs production. According to current reports, 2'-fucosyllactose (2'-FL), 3-fucosyllactose (3-FL), lacto-N-tetraose (LNT), lacto-N-neotetraose (LNnT), 3'-sialyllactose (3'-SL), 6'-sialyllactose (6'-SL), and some fucosylated HMOs with complex structures have been produced via the engineered microbial route, with 2'-FL having been produced the most. However, due to the uncertainty of metabolic patterns, the selection of host strains has certain limitations. Aside from that, the expression of appropriate glycosyltransferase in microbes is key to the synthesis of different HMOs. Therefore, finding a safe and efficient glycosyltransferase has to be addressed when using engineered microbial pathways. In this review, the latest research on the production of HMOs using engineered microbial routes is reported. The selection of host strains and adapting different metabolic pathways helped researchers designing engineered microbial routes that are more conducive to HMOs production.

Enhanced bioproduction of fucosylated oligosaccharide 3-fucosyllactose in engineered Escherichia coli with an improved de novo pathway

3-fucosyllactose (3-FL) and 2'-fucosyllactose (2'-FL), are two important fucosylated oligosaccharides in human milk. Extensive studies on 2'-FL enabled its official approval for use in infant formula. However, development of 3-FL has been somewhat sluggish due to its low content in human milk and poor yield in enlarged production. Here, an α-1,3-fucosyltransferase mutant was introduced into an engineered Escherichia coli (E. coli) capable of producing GDP-L-fucose, leading to a promising 3-FL titer in a 5.0-L bioreactor. To increase the availability of cofactors (NADPH and GTP) for optimized 3-FL production, zwf, pntAB, and gsk genes were successively overexpressed, finally resulting in a higher 3-FL level with a titer of 35.72 g/L and a yield of 0.82 mol 3-FL/mol lactose. Unexpectedly, the deletion of pfkA gene led to a much lower performance of 3-FL production than the control strain. Still, our strategy achieved the highest 3-FL level in E. coli to date.

Microbial production of human milk oligosaccharide lactodifucotetraose

Human milk oligosaccharides (HMOs) are potent bioactive compounds that modulate neonatal health and are of interest for development as potential drug treatments for adult diseases. The potential of these molecules, their limited access from natural sources, and difficulty in large-scale isolation of individual HMOs for studies and applications have motivated the development of chemical syntheses and in vitro enzymatic catalysis strategies. Whole cell biocatalysts are emerging as alternative self-regulating production platforms that have the potential to reduce the cost for enzymatic synthesis of HMOs. Whole cell biocatalysts for the production of short-chained, linear and small monofucosylated HMOs have been reported but those for fucosylated structures with higher complexity have not been explored. In this study, we established a strategy for producing a difucosylated HMO, lactodifucotetraose (LDFT), from lactose and L-fucose in Escherichia coli. We used two bacterial fucosyltransferases with narrow acceptor selectivity to drive the sequential fucosylation of lactose and intermediate 2'-fucosyllactose (2'-FL) to produce LDFT. Deletion of substrate degradation pathways that decoupled cellular growth from LDFT production, enhanced expression of native substrate transporters and modular induction of the genes in the LDFT biosynthetic pathway allowed complete conversion of lactose into LDFT and minor quantities of the side product 3-fucosyllactose (3-FL). Overall, 5.1 g/L of LDFT was produced from 3 g/L lactose and 3 g/L L-fucose in 24 h. Our results demonstrate promising applications of engineered microbial biosystems for the production of multi-fucosylated HMOs for biochemical studies.

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

Lacto-N-triose II (LNT II), a core structural unit of human milk oligosaccharides (HMOs), has attracted substantial attention for its nutraceutical potentials and applications in the production of complex HMOs. In this study, Escherichia coli was metabolically engineered to efficiently produce LNT II using glycerol as a carbon source and lactose as a substrate. The UDP-N-acetylglucosamine (UDP-GlcNAc) biosynthesis pathway was strengthened, and β-1,3-N-acetylglucosaminyltransferase (LgtA) was introduced to construct an LNT II-producing base strain. To increase the titer and yield of LNT II, combinatorial optimization of the copy number and the ribosomal binding site sequence was performed to tune the gene expression strength and translation rates of the pathway enzymes. Next, multipoint mutations were introduced to glucosamine-6-phosphatesynthase (GlmS) to relieve the feedback inhibition. Then, a series of engineered strains were constructed by blocking the futile pathways by the deletion of the relevant genes. Finally, the culture conditions were optimized. LNT II titer was improved step-by-step from 0.53 to 5.52 g/L in shake-flask cultivations. Fed-batch culture of the final engineered strain produced 46.2 g/L of LNT II, with an LNT II productivity and content of 0.77 g/(L·h) and 0.95 g/g dry cell weight, respectively.

Biological strategies for oligo/polysaccharide synthesis: biocatalyst and microbial cell factory

Oligosaccharides and polysaccharides constitute the principal components of carbohydrates, which are important biomacromolecules that demonstrate considerable bioactivities. However, the variety and structural complexity of oligo/polysaccharides represent a major challenge for biological and structural explorations. To access structurally defined oligo/polysaccharides, biological strategies using glycoenzyme biocatalysts have shown remarkable synthetic potential attributed to their regioselectivity and stereoselectivity that allow mild, structurally controlled reaction without addition of protecting groups necessary in chemical strategies. This review summarizes recent biotechnological approaches of oligo/polysaccharide synthesis, which mainly includes in vitro enzymatic synthesis and cell factory synthesis. We have discussed the important factors involved in the production of nucleotide sugars. Furthermore, the strategies established in the cell factory and enzymatic syntheses are summarized, and we have highlighted concepts like metabolic flux rebuilding and regulation, enzyme engineering, and route design as important strategies. The research challenges and prospects are also outlined and discussed.