The chemical synthesis of human milk oligosaccharides: Lacto-N-neotetraose (Galβ1→4GlcNAcβ1→3Galβ1→4Glc)

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.

Enzymatic modular synthesis of asymmetrically branched human milk oligosaccharides

Human milk oligosaccharides (HMOs) are intricate glycans that promote healthy growth of infants and have been incorporated into infant formula as food additives. Despite their importance, the limited availability of asymmetrically branched HMOs hinders the exploration of their structure and function relationships. Herein, we report an enzymatic modular strategy for the efficient synthesis of these HMOs. The key branching enzyme for the assembly of branched HMOs, human β1,6-N-acetylglucosaminyltransferase 2 (GCNT2), was successfully expressed in Pichia pastoris for the first time. Then, it was integrated with six other bacterial glycosyltransferases to establish seven glycosylation modules. Each module comprises a one-pot multi-enzyme (OPME) system for in-situ generation of costly sugar nucleotide donors, combined with a glycosyltransferase for specific glycosylation. This approach enabled the synthesis of 31 branched HMOs and 13 linear HMOs in a stepwise manner with well-programmed synthetic routes. The binding details of these HMOs with related glycan-binding proteins were subsequently elucidated using glycan microarray assays to provide insights into their biological functions. This comprehensive collection of synthetic HMOs not only serves as standards for HMOs structure identification in complex biological samples but also significantly enhances the fields of HMOs glycomics, opening new avenues for biomedical applications.

Protein Engineering of a Novel β-Galactosidase from Thermus scotoductus for Efficient Synthesis of Lacto-N-Neotetraose from Chitin Powder

A novel β-galactosidase (TsGal48) from Thermus scotoductus was cloned, and the enzyme was biochemically characterized. TsGal48 catalyzed the synthesis of lacto-N-neotetraose (LNnT) from lactose via the transglycosylation reaction with a maximal yield of 20%, which is the highest yield for the synthesis of LNnT so far. To further improve the yield of LNnT, TsGal48 was successfully engineered by directed evolution and site-saturation mutagenesis. A mutated β-galactosidase (mTsGal48) was selected and characterized. mTsGal48 produced LNnT with a yield of 27.7 g/L, which is 1.4-fold higher than that of TsGal48 (19.7 g/L). Then, a developed strategy for LNnT synthesis from chitin powder was provided in a 30 L bioreactor. The reaction process included chitin powder hydrolysis, lacto-N-triose II (LNT2) synthesis, and LNnT synthesis. The reaction time was reduced from 44 to 17 h in chitin powder hydrolysis and LNT2 synthesis. The content of LNnT was up to 25 g/L in the multienzyme system. The green and efficient route may be suitable for large-scale production of LNnT from chitin powder.

More than nutrition: Therapeutic potential and mechanism of human milk oligosaccharides against necrotizing enterocolitis

Human milk is the most valuable source of nutrition for infants. The structure and function of human milk oligosaccharides (HMOs), which are key components of human milk, have long been attracting particular research interest. Several recent studies have found HMOs to be efficacious in the prevention and treatment of necrotizing enterocolitis (NEC). Additionally, they could be developed in the future as non-invasive predictive markers for NEC. Based on previous findings and the well-defined functions of HMOs, we summarize potential protective mechanisms of HMOs against neonatal NEC, which include: modulating signal receptor function, promoting intestinal epithelial cell proliferation, reducing apoptosis, restoring intestinal blood perfusion, regulating microbial prosperity, and alleviating intestinal inflammation. HMOs supplementation has been demonstrated to be protective against NEC in both animal studies and clinical observations. This calls for mass production and use of HMOs in infant formula, necessitating more research into the safety of industrially produced HMOs and the appropriate dosage in infant formula.

Chemical Synthesis of Human Milk Oligosaccharides: para-Lacto-N-hexaose and para-Lacto-N-neohexaose

Human milk oligosaccharides (HMO) have emerged as a very active area of research in glycoscience and nutrition. HMO are involved in the early development of infants and may help to prevent certain diseases. The development of chemical methods for obtaining individual HMO aids the global effort dedicated to understanding the roles of these biomolecules. Reported herein is the chemical synthesis of two common core hexasaccharides found in human milk, i. e. para-lacto-N-hexaose (pLNH) and para-lacto-N-neohexaose (pLNnH). After screening multiple leaving groups and temporary protecting group combinations, a 3+3 convergent coupling strategy was found to work best for obtaining these linear glycans.

Human Milk Oligosaccharides: A Critical Review on Structure, Preparation, Their Potential as a Food Bioactive Component, and Future Perspectives

Human milk is the gold standard for infant feeding. Human milk oligosaccharides (HMOs) are a unique group of oligosaccharides in human milk. Great interest in HMOs has grown in recent years due to their positive effects on various aspects of infant health. HMOs provide various physiologic functions, including establishing a balanced infant's gut microbiota, strengthening the gastrointestinal barrier, preventing infections, and potential support to the immune system. However, the clinical application of HMOs is challenging due to their specificity to human milk and the difficulties and high costs associated with their isolation and synthesis. Here, the differences in oligosaccharides in human and other mammalian milk are compared, and the synthetic strategies to access HMOs are summarized. Additionally, the potential use and molecular mechanisms of HMOs as a new food bioactive component in different diseases, such as infection, necrotizing enterocolitis, diabetes, and allergy, are critically reviewed. Finally, the current challenges and prospects of HMOs in basic research and application are discussed.

N-acetyl-d-glucosamine-based oligosaccharides from chitin: Enzymatic production, characterization and biological activities

Chitin, the second most abundant biopolymer, possesses diverse applications in the food, agricultural, and pharmaceutical industries due to its functional properties. However, the potential applications of chitin are limited owing to its high crystallinity and low solubility. N-acetyl chitooligosaccharides and lacto-N-triose II, the two types of GlcNAc-based oligosaccharides, can be obtained from chitin by enzymatic methods. With their lower molecular weights and improved solubility, these two types of GlcNAc-based oligosaccharides display more various beneficial health effects when compared to chitin. Among their abilities, they have exhibited antioxidant, anti-inflammatory, anti-tumor, antimicrobial, and plant elicitor activities as well as immunomodulatory and prebiotic effects, which suggests they have the potential to be utilized as food additives, functional daily supplements, drug precursors, elicitors for plants, and prebiotics. This review comprehensively covers the enzymatic methods used for the two types of GlcNAc-based oligosaccharides production from chitin by chitinolytic enzymes. Moreover, current advances in the structural characterization and biological activities of these two types of GlcNAc-based oligosaccharides are summarized in the review. We also highlight current problems in the production of these oligosaccharides and trends in their development, aiming to offer some directions for producing functional oligosaccharides from chitin.

Formation of β-Configured Thioglycosides of d-Glucosamine and d-Galactosamine and Synthesis of Protected Human Milk Oligosaccharides

We report on the stereoselective multigram scale preparation of cyclohexyl- and phenyl thioglycosides of 2-azido-2-deoxy-β-d-gluco- and galactopyranosides from d-N-acetylglucosamine using a catalytic and solvent-free method. Two of the prepared building blocks were used as key intermediates for the synthesis of human milk oligosaccharides LNT and LNnT in their protected form.

High-Level Productivity of Lacto-N-neotetraose in Escherichia coli by Systematic Metabolic Engineering

Lacto-N-neotetraose (LNnT) is a critical component of human milk oligosaccharides. This study introduces a systems metabolic engineering method to produce LNnT in Escherichia coli. First, 12 target genes contributing to LNnT production were identified using a double-plasmid system. Subsequently, combinatorial optimization of the copy number was performed to tune the target gene expression strength. Next, the CRISPR/Cas9 system was used to block the UDP-Gal and UDP-GlcNAc competitive pathways, and the titer of LNnT reached 1.16 g/L (E27). Moreover, the lactoylglutathione lyase (GloA) was deleted to block the competing metabolite pathway from glycerol to lactate, and the titer of LNnT (1.46 g/L) was 26% higher than that of strain E27. Finally, the LNnT productivity was increased to 0.34 g/L/h in a 3 L bioreactor, which was 36% higher than the recently reported LNnT productivity. This research work opens an innovative framework for the effective production of LNnT.

Synthesis of 6'-galactosyllactose, a deviant human milk oligosaccharide, with the aid of Candida antarctica lipase-B

Research on human milk oligosaccharides (HMOs) has increased over the past decade showing great interest in their beneficial effects. Here we describe a method for the selective deacetylation using immobilised Candida antarctica lipase-B, Novozyme N435 (N435), of pyranose saccharides in organic media with the aim of simplifying and improving the pathways for the synthesis of HMOs. By first studying in depth the deacetylation reaction of peracetylated D-glucose two reaction conditions were found, which were used on different HMO building blocks, peracetylated saccharides and thioglycosides. D-Glucose based saccharides showed selectivity towards the fourth and the sixth position deacetylation. While α-anomer of peracetylated D-galactose remained unreactive and β-anomer favoured the first position deacetylation. Peracetylated L-fucose, on the other hand, had no selectivity as the main product was fully unprotected L-fucose. Taking the peracetylated D-glucose deacetylation reaction product and selectively protecting the primary hydroxyl group in the sixth position left only the fourth position open for the glycosylation. Meanwhile, the deacetylation product of D-galactose thioglycoside, with the sixth position deacetylated, had both acceptor and donor capabilities. Using the two aforementioned products derived from the N435 deacetylation reactions a deviant HMO, 6'-galactosyllactose (6'-GL) was synthesised.

Substrate and Process Engineering for Biocatalytic Synthesis and Facile Purification of Human Milk Oligosaccharides

Innovation in process development is essential for applying biocatalysis in industrial and laboratory production of organic compounds, including beneficial carbohydrates such as human milk oligosaccharides (HMOs). HMOs have attracted increasing attention for their potential application as key ingredients in products that can improve human health. To efficiently access HMOs through biocatalysis, a combined substrate and process engineering strategy is developed, namely multistep one-pot multienzyme (MSOPME) design. The strategy allows access to a pure tagged HMO in a single reactor with a single C18-cartridge purification process, despite the length of the target. Its efficiency is demonstrated in the high-yielding (71-91 %) one-pot synthesis of twenty tagged HMOs (83-155 mg), including long-chain oligosaccharides with or without fucosylation or sialylation up to nonaoses from a lactoside without the isolation of the intermediate oligosaccharides. Gram-scale synthesis of an important HMO derivative - tagged lacto-N-fucopentaose-I (LNFP-I) - proceeds in 84 % yield. Tag removal is carried out in high efficiency (94-97 %) without the need for column purification to produce the desired natural HMOs with a free reducing end. The method can be readily adapted for large-scale synthesis and automation to allow quick access to HMOs, other glycans, and glycoconjugates.

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.

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.

Synthesis of N-Acetyllactosamine and N-Acetyllactosamine-Based Bioactives

N-Acetyllactosamine (LacNAc) or more specifically β-d-galactopyranosyl-1,4-N-acetyl-d-glucosamine is a unique acyl-amino sugar and a key structural unit in human milk oligosaccharides, an antigen component of many glycoproteins, and an antiviral active component for the development of effective drugs against viruses. LacNAc is useful itself and as a basic building block for producing various bioactive oligosaccharides, notably because this synthesis may be used to add value to dairy lactose. Despite a significant amount of information in the literature on the benefits, structures, and types of different LacNAc-derived oligosaccharides, knowledge about their effective synthesis for large-scale production is still in its infancy. This work provides a comprehensive analysis of existing production strategies for LacNAc and important LacNAc-based structures, including sialylated LacNAc as well as poly- and oligo-LacNAc. We conclude that direct extraction from milk is too complex, while chemical synthesis is also impractical at an industrial scale. Microbial routes have application when multiple step reactions are needed, but the major route to large-scale biochemical production will likely lie with enzymatic routes, particularly those using β-galactosidases (for LacNAc synthesis), sialidases (for sialylated LacNAc synthesis), and β-N-acetylhexosaminidases (for oligo-LacNAc synthesis). Glycosyltransferases, especially for the biosynthesis of extended complex LacNAc structures, could also play a major role in the future. In these cases, immobilization of the enzyme can increase stability and reduce cost. Processing parameters, such as substrate concentration and purity, acceptor/donor ratio, water activity, and temperature, can affect product selectivity and yield. More work is needed to optimize these reaction parameters and in the development of robust, thermally stable enzymes to facilitate commercial production of these important bioactive substances.

A Comparison of Two Structurally Related Human Milk Oligosaccharide Conjugates in a Model of Diet-Induced Obesity

Obesity is the largest risk factor for the development of chronic diseases in industrialized countries. Excessive fat accumulation triggers a state of chronic low-grade inflammation to the detriment of numerous organs. To address this problem, our lab has been examining the anti-inflammatory mechanisms of two human milk oligosaccharides (HMOs), lacto-N-fucopentaose III (LNFPIII) and lacto-N-neotetraose (LNnT). LNFPIII and LNnT are HMOs that differ in structure via presence/absence of an α1,3-linked fucose. We utilize LNFPIII and LNnT in conjugate form, where 10-12 molecules of LNFPIII or LNnT are conjugated to a 40 kDa dextran carrier (P3DEX/NTDEX). Previous studies from our lab have shown that LNFPIII conjugates are anti-inflammatory, act on multiple cell types, and are therapeutic in a wide range of murine inflammatory disease models. The α1,3-linked fucose residue on LNFPIII makes it difficult and more expensive to synthesize. Therefore, we asked if LNnT conjugates induced similar therapeutic effects to LNFPIII. Herein, we compare the therapeutic effects of P3DEX and NTDEX in a model of diet-induced obesity (DIO). Male C57BL/6 mice were placed on a high-fat diet for six weeks and then injected twice per week for eight weeks with 25µg of 40 kDa dextran (DEX; vehicle control), P3DEX, or NTDEX. We found that treatment with P3DEX, but not NTDEX, led to reductions in body weight, adipose tissue (AT) weights, and fasting blood glucose levels. Mice treated with P3DEX also demonstrated improvements in glucose homeostasis and insulin tolerance. Treatment with P3DEX or NTDEX also induced different profiles of serum chemokines, cytokines, adipokines, and incretin hormones, with P3DEX notably reducing circulating levels of leptin and resistin. P3DEX also reduced WAT inflammation and hepatic lipid accumulation, whereas NTDEX seemed to worsen these parameters. These results suggest that the small structural difference between P3DEX and NTDEX has significant effects on the conjugates' therapeutic abilities. Future work will focus on identifying the receptors for these conjugates and delineating the mechanisms by which P3DEX and NTDEX exert their effects.

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.

Novel and emerging prebiotics: Advances and opportunities

Consumers are conscientiously changing their eating preferences toward healthier options, such as functional foods enriched with pre- and probiotics. Prebiotics are attractive bioactive compounds with multidimensional beneficial action on both human and animal health, namely on the gastrointestinal tract, cardiometabolism, bones or mental health. Conventionally, prebiotics are non-digestible carbohydrates which generally present favorable organoleptic properties, temperature and acidic stability, and are considered interesting food ingredients. However, according to the current definition of prebiotics, application categories other than food are accepted, as well as non-carbohydrate substrates and bioactivity at extra-intestinal sites. Regulatory issues are considered a major concern for prebiotics since a clear understanding and application of these compounds among the consumers, regulators, scientists, suppliers or manufacturers, health-care providers and standards or recommendation-setting organizations are of utmost importance. Prebiotics can be divided in several categories according to their development and regulatory status. Inulin, galactooligosaccharides, fructooligosaccharides and lactulose are generally classified as well established prebiotics. Xylooligosaccharides, isomaltooligosaccharides, chitooligosaccharides and lactosucrose are classified as "emerging" prebiotics, while raffinose, neoagaro-oligosaccharides and epilactose are "under development." Other substances, such as human milk oligosaccharides, polyphenols, polyunsaturated fatty acids, proteins, protein hydrolysates and peptides are considered "new candidates." This chapter will encompass actual information about the non-established prebiotics, mainly their physicochemical properties, market, legislation, biological activity and possible applications. Generally, there is a lack of clear demonstrations about the effective health benefits associated with all the non-established prebiotics. Overcoming this limitation will undoubtedly increase the demand for these compounds and their market size will follow the consumer's trend.

Chemical synthesis of human milk oligosaccharides: lacto-N-neohexaose (Galβ1 → 4GlcNAcβ1→)2 3,6Galβ1 → 4Glc

The first chemical synthesis of lacto-N-neohexaose (LNnH) has been completed using a convergent synthetic strategy. The reaction conditions and donor-acceptor combinations have been carefully refined to minimize side reactions and achieve high yields in all glycosylation steps. Lacto-N-neotetraose, another common human milk oligosaccharide, was also synthesized en route to the target LNnH.

Picoloyl protecting group in synthesis: focus on a highly chemoselective catalytic removal

The picoloyl ester (Pico) has proven to be a versatile protecting group in carbohydrate chemistry. It can be used for the purpose of stereocontrolling glycosylations via an H-bond-mediated Aglycone Delivery (HAD) method. It can also be used as a temporary protecting group that can be efficiently introduced and chemoselectively cleaved in the presence of practically all other common protecting groups used in synthesis. Herein, we will describe a new method for rapid, catalytic, and highly chemoselective removal of the picoloyl group using inexpensive copper(ii) or iron(iii) salts.

Like mother, like microbe: human milk oligosaccharide mediated microbiome symbiosis

Starting shortly after parturition, and continuing throughout our lifetime, the gut microbiota coevolves with our metabolic and neurological programming. This symbiosis is regulated by a complex interplay between the host and environmental factors, including diet and lifestyle. Not surprisingly, the development of this microbial community is of critical importance to health and wellness. In this targeted review, we examine the gut microbiome from birth to 2 years of age to characterize the role human milk oligosaccharides play in early formation of microbial flora.

CRISPRi-Guided Multiplexed Fine-Tuning of Metabolic Flux for Enhanced Lacto-N-neotetraose Production in Bacillus subtilis

Lacto-N-neotetraose (LNnT), one of the oligosaccharides in human milk, has many beneficial effects on infant health. In a recent work, we have constructed a recombinant Bacillus subtilis strain for the production of LNnT. Here, we further improved LNnT production with a xylose-induced clustered regularly interspaced short palindromic repeats interference system. In particular, the expressions of pfkA and pyk genes in the Embden-Meyerhof-Parnas pathway module, zwf gene in the pentose phosphate pathway module, and mnaA gene in the teichoic acid synthesis module were downregulated. The LNnT titer was increased from 1.32 to 1.55 g/L. Furthermore, to improve the conversion efficiency of lacto-N-triose II to LNnT, we knocked out tuaD gene in branch pathway and improved the expression of lgtB gene, resulting in the further increase of LNnT titer to 2.01 g/L. Finally, the addition time and amount of inducer xylose were optimized, and LNnT titer reached 2.30 g/L in shake flask and 5.41 g/L in 3 L bioreactor.

Chemical Synthesis of Human Milk Oligosaccharides: Lacto-N-hexaose Galβ1→3GlcNAcβ1→3 [Galβ1→4GlcNAcβ1→6] Galβ1→4Glc

The first synthesis of lacto-N-hexaose (LNH) has been completed using a convergent strategy. The donor-acceptor protecting-leaving group combinations were found to be of paramount significance for achieving successful glycosylations and minimizing side reactions. Lacto-N-tetraose, another common human milk oligosaccharide, was also obtained en route to the target LNH.

The chemical synthesis of human milk oligosaccharides: Lacto-N-tetraose (Galβ1→3GlcNAcβ1→3Galβ1→4Glc)

The total chemical synthesis of lacto-N-tetraose (LNT) has been completed using both convergent and linear strategies. Similarly to that of our previous HMO syntheses, the donor-acceptor protecting-leaving group combinations were found to be of paramount significance to achieving successful glycosylations and minimizing side reactions.