Synthesis as an Expanding Resource in Human Milk Science

Glycosyltransferases: glycoengineers in human milk oligosaccharide synthesis and manufacturing

Human milk oligosaccharides (HMOs) are a diverse group of complex carbohydrates that play crucial roles in infant health, promoting a beneficial gut microbiota, modulating immune responses, and protecting against pathogens. Central to the synthesis of HMOs are glycosyltransferases, a specialized class of enzymes that catalyse the transfer of sugar moieties to form the complex glycan structures characteristic of HMOs. This review provides an in-depth analysis of glycosyltransferases, beginning with their classification based on structural and functional characteristics. The catalytic activity of these enzymes is explored, highlighting the mechanisms by which they facilitate the precise addition of monosaccharides in HMO biosynthesis. Structural insights into glycosyltransferases are also discussed, shedding light on how their conformational features enable specific glycosidic bond formations. This review maps out the key biosynthetic pathways involved in HMO production, including the synthesis of lactose, and subsequent fucosylation and sialylation processes, all of which are intricately regulated by glycosyltransferases. Industrial methods for HMO synthesis, including chemical, enzymatic, and microbial approaches, are examined, emphasizing the role of glycosyltransferases in these processes. Finally, the review discusses future directions in glycosyltransferase research, particularly in enhancing the efficiency of HMO synthesis and developing advanced analytical techniques to better understand the structural complexity and biological functions of HMOs.

Chemical Synthesis of Two Fucosylated Human Milk Oligosaccharides: 3-Fucosyllactose and Lacto-N-fucopentaose V

Glycans present in human milk have been the focus of many studies due to challenges associated with their synthesis. The development of new methods for obtaining individual glycans found in human milk has been a vibrant area of research in glycosciences. This study reports the synthesis of two fucosylated glycans found in human milk, 3-fucosyllactose, which was previously synthesized by both chemical and enzymatic methods, and the first chemical synthesis of lacto-N-fucopentaose V. Screening different protecting and leaving groups, as well as optimizing the glycosylation reaction conditions helped us to achieve efficient assembly and deprotection of these two fucosylated human milk oligosaccharides in good yields.

EASyMap-Guided Stepwise One-Pot Multienzyme (StOPMe) Synthesis and Multiplex Assays Identify Functional Tetraose-Core-Human Milk Oligosaccharides

Carbohydrates are biologically and medicinally important molecules that are attracting growing attention to their synthesis and applications. Unlike the biosynthetic processes for nucleic acids and proteins, carbohydrate biosynthesis is not template-driven, more challenging, and often leads to product variations. In lieu of templates for carbohydrate biosynthesis, we describe herein a new concept of designing enzyme assembly synthetic maps (EASyMaps) as blueprints to guide glycosyltransferase-dependent stepwise one-pot multienzyme (StOPMe) synthesis to systematically access structurally diverse carbohydrates in a target-oriented manner. The strategy is demonstrated for the construction of a comprehensive library of tetraose-core-containing human milk oligosaccharides (HMOs) presenting diverse functional important glycan epitopes shared by more complex HMOs. The tetraose-core-HMOs are attractive candidates for large-scale production and for the development of HMO-based nutraceuticals. To achieve the preparative-scale synthesis of targets containing a Neu5Acα2-6GlcNAc component, a human α2-6-sialyltransferase hST6GALNAC5 is successfully expressed in E. coli. Neoglycoproteins with controlled glycan valencies are prepared and immobilized on fluorescent magnetic beads. Multiplex bead assays reveal ligands of glycan-binding proteins from plants, influenza viruses, human, and bacteria, identifying promising HMO targets for functional applications. The concept of designing EASyMaps as blueprints to guide StOPMe synthesis in a systematic target-oriented manner is broadly applicable beyond the synthesis of HMOs. The efficient StOPMe process is suitable for the large-scale production of complex carbohydrates and can be potentially adapted for automation.

Metabolic Engineering of Escherichia coli BL21(DE3) for 2'-Fucosyllactose Synthesis in a Higher Productivity

2'-Fucosyllactose (2'-FL) is the most abundant human milk oligosaccharides (HMOs). 2'-FL exhibits great benefits for infant health, such as preventing infantile diarrhea and promoting the growth of intestinal probiotics. The microbial cell factory technique has shown promise for the massive production of 2'-FL. Here, we aimed to construct a recombinant E. coli BL21(DE3) strain for the hyperproduction of 2'-FL. Initially, multicopy genomic integration and expression of the lactose permease gene lacY reduced the formation of byproducts. Furthermore, a more efficient Shine-Dalgarno sequence was used to replace the wild-type sequence in the manC-manB and gmd-wcaG gene clusters, which significantly increased the 2'-FL titer. Based on these results, we overexpressed the sugar efflux transporter SetA and knocked out the pgi gene. This further improved 2'-FL synthesis when glycerol was used as the sole carbon source. Finally, a new α-1,2-fucosyltransferase was identified in Neisseria sp., which exhibited a higher capacity for 2'-FL production. Fed-batch fermentation produced 141.27 g/L 2'-FL in 45 h with a productivity of 3.14 g/L × h. This productivity rate achieved the highest recorded 2'-FL levels, indicating the potential of engineered E. coli BL21 (DE3) strains for use in the industrial production of 2'-FL.

Versatile Strategy for the Chemoenzymatic Synthesis of Branched Human Milk Oligosaccharides Containing the Lacto-N-Biose Motif

Human milk oligosaccharides (HMOs) exhibit prebiotic, antimicrobial, and immunomodulatory properties and confer significant benefits to infants. Branched HMOs are constructed through diverse glycosidic linkages and prominently feature the lacto-N-biose (LNB, Gal-β1,3-GlcNAc) motif with fucose and/or sialic acid modifications, displaying structural complexity that surpasses that of N- and O-glycans. However, synthesizing comprehensive libraries of branched HMO is challenging due to this complexity. Although a few systematic synthetic strategies have emerged, many of them rely on labor-intensive chemical methodologies or exploit the substrate specificity of human N-acetylglucosaminyltransferase 2 (hGCNT2). In this study, we capitalized on the substrate promiscuities of hGCNT2 and bacterial glycosyltransferases (GTs) to construct a universal tetrasaccharide core in a highly efficient manner. This core was systematically and flexibly extended to generate diverse branched HMOs utilizing the promiscuity of bacterial GTs coupled with N-trifluoroacetyl glucosamine (GlcNTFA), which facilitated sugar chain elongation. The GlcNTFA residues were subsequently converted into various N-modified glucosamines through straightforward chemical manipulations to modulate the activities of additional GTs during glycan extension. These masked amino groups were ultimately reverted to N-acetyl groups, facilitating the synthesis of a broad range of asymmetric and multiantennary HMOs featuring LNB moieties, including many previously inaccessible structures.

Efficient fermentative production of lactodifucotetraose by controlling sequential glycosyltransferase reactions in Escherichia coli

Lactodifucotetraose (LDFT) is a human milk oligosaccharide (HMO) that might reduce inflammation in infants. In this study, we established a useful production process of LDFT by engineering two key enzymes, α1,2-fucosyltransferase (α1,2-FucT) and α1,3-fucosyltransferase (α1,3-FucT). First, we verified which of 2'-fucosyllactose (2'-FL) or 3-fucosyllactose (3-FL) (mostly unverified) was more useful. We searched for FucTs that functioned efficiently in vivo against the raw material lactose or the two intermediates 2'-FL or 3-FL by external substrate addition to culture medium. We found that α1,2- FucT (HMFT) from Helicobacter mustelae and the N-terminal truncated form of α1,3-FucT from Bacteroides fragilis (BfFucTΔN10) had high potential. 3-FL was not efficiently converted to LDFT, which might be attributed to the low reactivity of HMFT to 3-FL as well as the low uptake efficiency of 3-FL by LacY, as revealed by a growth test with exogenously added FL as the sole carbon source and heterologously expressed intracellular fucosidase. Furthermore, because 3-FL accumulation had a negative impact on cell growth, we avoided the route passing through 3-FL. By adjusting the copy numbers of HMFT and BffucTΔN10, we produced LDFT from lactose predominantly via 2'-FL. Finally, 17.5 g/L of LDFT (with 6.8 g/L 2'-FL and no 3-FL or residual lactose) accumulated in a 3-L fed-batch culture after 77 h. This study reports the detailed analysis of multiple pathways and shows the control of glycosyltransferases can improve the production efficiency of complex HMOs.

Enzymatic Synthesis of Disialyllacto-N-Tetraose (DSLNT) and Related Human Milk Oligosaccharides Reveals Broad Siglec Recognition of the Atypical Neu5Acα2-6GlcNAc Motif

Sialic acids (Sias) are ubiquitously expressed on all types of glycans, typically as terminating residues. They usually link to galactose, N-acetylgalactosamine, or other Sia residues, forming ligands of many glycan-binding proteins. An atypical linkage to the C6 of N-acetylglucosamine (GlcNAc) has been identified in human milk oligosaccharides (HMOs, e.g., DSLNT) and tumor-associated glycoconjugates. Herein, describe the systematic synthesis of these HMOs in an enzymatic modular manner. The synthetic strategy relies on a novel activity of ST6GalNAc6 for efficient construction of the Neu5Acα2-6GlcNAc linkage, and another 12 specific enzyme modules for sequential HMO assembly. The structures enabled comprehensive exploration of their structure-function relationships using glycan microarrays, revealing broad yet distinct recognition by Siglecs of the atypical Neu5Acα2-6GlcNAc motif. The work provides tools and new insight for the functional study and potential applications of Siglecs and HMOs.

Controllable Enzymatic Synthesis of Natural Asymmetric Human Milk Oligosaccharides

Among human milk oligosaccharides (HMOs), linear HMOs are synthesized through mature but varied routes. Although branched HMOs can be synthesized by chemical, enzymatic, or chemoenzymatic methods, these methods cannot be easily applied to the synthesis of asymmetric multiantennary oligosaccharides. Herein, we developed a controllable method to synthesize asymmetric biantennary HMOs. In our synthetic route, GlcNAcβ1,3(GlcN3β1,6)Glaβ1,4Glc was first chemically synthesized as the core tetrasaccharide, which contains β1,6GlcN3 as the "stop" sugar in transferase-catalyzed glycosylation. The desired sugars at the GlcNAcβ1-3Gal arm can be assembled using galactosyltransferase, N-acetylglucosaminyltransferase, and fucosyltransferase. Then, the Staudinger reduction and acetylation were used to transform GlcN3 to GlcNAc and assemble sugars by initiating the "go" process. By manipulating transferase-catalyzed glycosylations, 22 natural asymmetric biantennary oligosaccharides were synthesized.

Is it possible to obtain substitutes for human milk oligosaccharides from bovine milk, goat milk, or other mammal milks?

Considering the current level of chemical and biological synthesis technology, it was a sensible selection to obtain milk oligosaccharides (MOs) from other mammals as the potential substitute for human MOs (HMOs) that possessed various structural features in the infant formula. Through a comprehensive analysis of the content, structure, and function of MOs in six distinct varieties of mammal milk, it has been shown that goat milk was the most suitable material for the preparation as a human milk substitute. Goat MOs (GMOs) had a relatively high content and diverse structural features compared to those found in other mammalian milks. The concentration of GMOs in colostrum ranged from 60 to 350 mg/L, whereas in mature milk, it ranged from 200 to 24,00 mg/L. The acidic oligosaccharides in goat milk have attracted considerable attention due to their closeness in acidic content and structural diversity with HMOs. Simultaneously, it was discovered that some structures, like N-glycolylneuraminic acid, were found to have a certain content in GMOs and served essential functional properties. Moreover, studies focused on the extraction of MOs from goat milk indicated that the production of GMOs on an industrial scale was viable. Furthermore, it is imperative to do further study on GMOs to enhance the preparation process, discover of new MOs structures and bioactivity evaluation, which will contribute to the development of both the commercial production of MOs and the goat milk industry.

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.

Guide RNA structure design enables combinatorial CRISPRa programs for biosynthetic profiling

Engineering metabolism to efficiently produce chemicals from multi-step pathways requires optimizing multi-gene expression programs to achieve enzyme balance. CRISPR-Cas transcriptional control systems are emerging as important tools for programming multi-gene expression, but poor predictability of guide RNA folding can disrupt expression control. Here, we correlate efficacy of modified guide RNAs (scRNAs) for CRISPR activation (CRISPRa) in E. coli with a computational kinetic parameter describing scRNA folding rate into the active structure (rS = 0.8). This parameter also enables forward design of scRNAs, allowing us to design a system of three synthetic CRISPRa promoters that can orthogonally activate (>35-fold) expression of chosen outputs. Through combinatorial activation tuning, we profile a three-dimensional design space expressing two different biosynthetic pathways, demonstrating variable production of pteridine and human milk oligosaccharide products. This RNA design approach aids combinatorial optimization of metabolic pathways and may accelerate routine design of effective multi-gene regulation programs in bacterial hosts.

Recent Advances in the Microbial Production of Human Milk Oligosaccharides

Human milk oligosaccharides (HMOs) are naturally occurring, non-digestible sugars found in human milk. They have recently become a popular target for industrial synthesis due to their positive effects on the developing gut microbiome and immune system of infants. Microbial synthesis has shown great promise in driving down the cost of these sugars and making them more available for consumers and researchers. The application of common metabolic engineering techniques such as gene knockouts, gene overexpression, and expression of exogenous genes has enabled the rational design of whole-cell biocatalysts which can produce increasingly complex HMOs. Herein, we discuss how these strategies have been applied to produce a variety of sugars from sialylated to complex fucosylated HMOs. With increased availability of HMOs, more research can be done to understand their beneficial effects.

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.

Semisynthesis of homogeneous spike RBD glycoforms from SARS-CoV-2 for profiling the correlations between glycan composition and function

Vaccines have been the primary remedy in the global fight against coronavirus disease 2019 (COVID-19). The receptor-binding domain (RBD) of the spike protein, a critical viral immunogen, is affected by the heterogeneity of its glycan structures and relatively low immunogenicity. Here, we describe a scalable synthetic platform that enables the precise synthesis of homogeneously glycosylated RBD, facilitating the elucidation of carbohydrate structure-function relationships. Five homogeneously glycosylated RBDs bearing biantennary glycans were prepared, three of which were conjugated to T-helper epitope (Tpep) from tetanus toxoid to improve their weak immune response. Relative to natural HEK293-derived RBD, synthetic RBDs with biantennary N-glycan elicited a higher level of neutralising antibodies against SARS-CoV-2 in mice. Furthermore, RBDs containing Tpep elicited significant immune responses in transgenic mice expressing human angiotensin-converting enzyme 2. Our collective data suggest that trimming the N-glycans and Tpep conjugation of RBD could potentially serve as an effective strategy for developing subunit vaccines providing efficient protection.

Glycosyltransferases as versatile tools to study the biology of glycans

All cells are decorated with complex carbohydrate structures called glycans that serve as ligands for glycan-binding proteins (GBPs) to mediate a wide range of biological processes. Understanding the specific functions of glycans is key to advancing an understanding of human health and disease. However, the lack of convenient and accessible tools to study glycan-based interactions has been a defining challenge in glycobiology. Thus, the development of chemical and biochemical strategies to address these limitations has been a rapidly growing area of research. In this review, we describe the use of glycosyltransferases (GTs) as versatile tools to facilitate a greater understanding of the biological roles of glycans. We highlight key examples of how GTs have streamlined the preparation of well-defined complex glycan structures through chemoenzymatic synthesis, with an emphasis on synthetic strategies allowing for site- and branch-specific display of glyco-epitopes. We also describe how GTs have facilitated expansion of glyco-engineering strategies, on both glycoproteins and cell surfaces. Coupled with advancements in bioorthogonal chemistry, GTs have enabled selective glyco-epitope editing of glycoproteins and cells, selective glycan subclass labeling, and the introduction of novel biomolecule functionalities onto cells, including defined oligosaccharides, antibodies, and other proteins. Collectively, these approaches have contributed great insight into the fundamental biological roles of glycans and are enabling their application in drug development and cellular therapies, leaving the field poised for rapid expansion.

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.

High-Yield Synthesis of 2'-Fucosyllactose from Glycerol and Glucose in Engineered Escherichia coli

2'-Fucosyllactose (2'-FL) is vital for the growth and development of newborns. In this study, we developed a synthesis pathway for 2'-FL in Escherichia coli BL21 (DE3). Then, we optimized the solubility of α-1,2-fucosyltransferase, thereby enhancing the production yield of 2'-FL. Based on this finding, we further enhanced the expression of guanosine inosine kinase Gsk and knocked out the isocitrate lyase regulator gene iclR. This strategy reduced the formation of byproduct acetate during the metabolic process and alleviated carbon source overflow effects in the strain, resulting in further improvement of the yield of 2'-FL. In a 3 L bioreactor, employing fed-batch fermentation with glycerol and glucose as substrates, the engineered strain BWLAI-RSZL exhibited impressive 2'-FL titers of 121.9 and 111.56 g/L, along with productivity levels of 1.57 and 1.31 g/L/h, respectively. The reported 2'-FL titers reached a groundbreaking level, irrespective of the carbon source employed (glycerol or glucose), highlighting the significant potential for large-scale industrial synthesis of 2'-FL.

Application of Redox-Active Ester Catalysis to the Synthesis of Pyranose Alkyl C-Glycosides

The direct coupling of shelf-stable, tetrachloro-N-hydroxyphthalimide ester (TCNHPI) glycosyl donors with a variety of alkylzinc reagents under redox catalysis is described. Alkyl C-glycosides are formed directly by a decarboxylative, Negishi-type process in 31-73% yields without the need for photocatalytic activation or additional reductants. Extension of this approach to the coupling of TCNHPI donors with stereodefined α-alkoxy furan-containing alkylzinc halides enabled de novo synthesis of methylene-linked exo-C-disaccharides via an Achmatowicz rearrangement.

Microbial Production of Human Milk Oligosaccharides

Human milk oligosaccharides (HMOs) are complex nonnutritive sugars present in human milk. These sugars possess prebiotic, immunomodulatory, and antagonistic properties towards pathogens and therefore are important for the health and well-being of newborn babies. Lower prevalence of breastfeeding around the globe, rising popularity of nutraceuticals, and low availability of HMOs have inspired efforts to develop economically feasible and efficient industrial-scale production platforms for HMOs. Recent progress in synthetic biology and metabolic engineering tools has enabled microbial systems to be a production system of HMOs. In this regard, the model organism Escherichia coli has emerged as the preferred production platform. Herein, we summarize the remarkable progress in the microbial production of HMOs and discuss the challenges and future opportunities in unraveling the scope of production of complex HMOs. We focus on the microbial production of five HMOs that have been approved for their commercialization.

Enhancing lactose recognition of a key enzyme in 2'-fucosyllactose synthesis: α-1,2-fucosyltransferase

BACKGROUND

2'-Fucosyllactose, a representative oligosaccharide in human milk, is an emerging and promising food and pharmaceutical ingredient due to its powerful health benefits, such as participating in immune regulation, regulation of intestinal flora, etc. To enable economically viable production of 2'-fucosyllactose, different biosynthesis strategies using precursors and pathway enzymes have been developed. The α-1,2-fucosyltransferases are an essential part involved in these strategies, but their strict substrate selectivity and unsatisfactory substrate tolerance are one of the key roadblocks limiting biosynthesis.

RESULTS

To tackle this issue, a semi-rational manipulation combining computer-aided designing and screening with biochemical experiments were adopted. The mutant had a 100-fold increase in catalytic efficiency compared to the wild-type. The highest 2'-fucosyllactose yield was up to 0.65 mol mol^-1 lactose with a productivity of 2.56 g mL^-1  h^-1 performed by enzymatic catalysis in vitro. Further analysis revealed that the interactions between the mutant and substrates were reduced. The crucial contributions of wild-type and mutant to substrate recognition ability were closely related to their distinct phylotypes in terms of amino acid preference.

CONCLUSION

It is envisioned that the engineered α-1,2-fucosyltransferase could be harnessed to relieve constraints imposed on the bioproduction of 2'-fucosyllactose and lay a theoretical foundation for elucidating the substrate recognition mechanisms of fucosyltransferases. © 2022 Society of Chemical Industry.

Enzyme cascades for the synthesis of nucleotide sugars: Updates to recent production strategies

Nucleotide sugars play an elementary role in nature as building blocks of glycans, polysaccharides, and glycoconjugates used in the pharmaceutical, cosmetics, and food industries. As substrates of Leloir-glycosyltransferases, nucleotide sugars are essential for chemoenzymatic in vitro syntheses. However, high costs and the limited availability of nucleotide sugars prevent applications of biocatalytic cascades on a large industrial scale. Therefore, the focus is increasingly on nucleotide sugar synthesis strategies to make significant application processes feasible. The chemical synthesis of nucleotide sugars and their derivatives is well established, but the yields of these processes are usually low. Enzyme catalysis offers a suitable alternative here, and in the last 30 years, many synthesis routes for nucleotide sugars have been discovered and used for production. However, many of the published procedures shy away from assessing the practicability of their processes. With this review, we give an insight into the development of the (chemo)enzymatic nucleotide sugar synthesis pathways of the last years and present an assessment of critical process parameters such as total turnover number (TTN), space-time yield (STY), and enzyme loading.

Biosynthesis of human milk oligosaccharides via metabolic engineering approaches: current advances and challenges

Human milk oligosaccharides (HMOs) are structurally complex unconjugated glycans that are the third largest solid component in human milk. HMOs have drawn increasing attention because of their beneficial effects to infant health. Of the more than 200 HMOs, only less than 10 have been used in medical or food industries. Although HMO research has been becoming increasingly intensive and booming, the limited availability of HMOs still cannot meet the demand in health effect research and large-scale application. Therefore, efficient synthetic approaches and strategies for HMO production are urgently needed. The goal of this review is to highlight recent advances in microbial cell factory development for HMO biosynthesis. Key challenges in representative HMO production are also highlighted. The further perspectives in general HMO biosynthesis are discussed.

High-Level De Novo Biosynthesis of 2'-Fucosyllactose by Metabolically Engineered Escherichia coli

2'-Fucosyllactose (2'-FL) is the most abundant oligosaccharide in human milk. In this study, a highly efficient biosynthetic route for 2'-FL production was designed via the de novo pathway of GDP-l-fucose using engineered Escherichia coli BL21(DE3). Specifically, plasmid-based strains with previously deleted lacZ and wcaJ were further reconstructed by introducing de novo pathway genes and α1,2-fucosyltransferase-encoding wbgL to realize 2'-FL synthesis. The 2'-FL titer was enhanced to 3.92 g/L by further introducing rcsA and rcsB. Subsequently, the additional wbgL expression cassette was chromosomally integrated into recA locus to strengthen fucosylation reaction and a strong constitutive promoter (P_J23119) was used to replace the original promoters of manC-manB and gmd-wcaG to improve 2'-FL synthesis. The maximal 2'-FL titer reached 9.06 and 79.23 g/L in shake-flask and fed-batch cultivation, respectively. The 2'-FL productivity reached 1.45 g/L/h, showing remarkable production potential in large-scale industrial application.

Current strategies for characterization of mucin-domain glycoproteins

Glycosylation, and especially O-linked glycosylation, remains a critical blind spot in the understanding of post-translational modifications. Due to their nature as proteins defined by a large density and abundance of O-glycosylation, mucins present extra challenges in the analysis of their structure and function. However, recent breakthroughs in multiple areas of research have rendered mucin-domain glycoproteins more accessible to current characterization techniques. In particular, the adaptation of mucinases to glycoproteomic workflows, the manipulation of cellular glycosylation pathways, and the advances in synthetic methods to more closely mimic mucin domains have introduced new and exciting avenues to study mucin glycoproteins. Here, we summarize recent developments in understanding the structure and biological function of mucin domains and their associated glycans, from glycoproteomic tools and visualization methods to synthetic glycopeptide mimetics.

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.

Chemoenzymatic Synthesis of Asymmetrically Branched Human Milk Oligosaccharide Lacto-N-Hexaose

We herein reported the first chemoenzymatic synthesis of lacto-N-hexaose (LNH) by combining chemical carbohydrate synthesis with a selectively enzymatic glycosylation strategy. A tetrasaccharide core structure GlcNH₂β1→3 (GlcNAcβ1→6) Galβ1→4Glc, a key precursor for subsequent enzymatic glycan extension toward asymmetrically branched human milk oligosaccharides, was synthesized in this work. When the order of galactosyltransferase-catalyzed reactions was appropriately arranged, the β1,4-galactosyl and β1,3-galactosyl moieties could be sequentially assembled on the C6-arm and C3-arm of the tetrasaccharide, respectively, to achieve an efficient LNH synthesis. Lacto-N-neotetraose (LNnH), another common human milk oligosaccharide, was also synthesized en route to the target LNH.

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.

Synthesis of Type-I and Type-II LacNAc-Repeating Oligosaccharides as the Backbones of Tumor-Associated Lewis Antigens

Type-I and Type-II LacNAc are Gal-GlcNAc disaccharides bearing a β1,3- or β1,4-linkage respectively. They exist as the backbones of Lewis antigens that are highly expressed in several cancers. Owing to the promise of developing carbohydrate-based anti-cancer vaccines, glycan synthesis at a large scale is indeed an important task. Synthesis of Type-I and Type-II tandem repeat oligomers has been hampered by the presence of GlcNAc residues. Particularly, N-protecting group plays a determining role in affecting glycosyl donor’s reactivity and acceptor’s nucleophilicity. This review discusses several representative studies that assembled desirable glycans in an efficient manner, such as chemoselective one-pot synthesis and chemoenzymatic methods. Additionally, we also highlight solutions that have been offered to tackle long-lasting problems, e.g., prevention of the oxazoline formation and change of donor/acceptor reactivity. In retrospect of scientific achievements, we present the current restrictions and remaining challenges in this less explored frontier.

Combinatorial metabolic engineering of Escherichia coli for de novo production of 2'-fucosyllactose

2'-Fucosyllactose (2'-FL) is a kind of fucosylated lactose of human milk oligosaccharides (HMOs). In this work, Escherichia coli MG1655 was metabolically engineered to increase 2'-FL production. The 2'-FL titer was raised from 0.0118 to 0.8062 g/L by increasing three gene copies of α1,2-fucosyltransferase (FutC) and by deleting the lon, wcaJ, lacZ, and lacI genes in the competitive pathways. Additionally, the 2'-FL titer was raised to 2.9 g/L by fusing a TrxA tag at the N-terminus of FutC, and then to 3.4 g/L by deleting glutathione reductase (Gor). Finally, the 2'-FL reached 3.3 g/L in the final strain E. coli MG27 through the de novo pathway in shake flask, and reached 10.3 g/L in a 3-L fermentor.

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.

Transdermal Photothermal-Pharmacotherapy to Remodel Adipose Tissue for Obesity and Metabolic Disorders

Despite the increasing prevalence of obesity, the current medications, which act indirectly on the central nervous system to suppress appetite or on the gastrointestinal tract to inhibit fat absorption, suffer from poor effectiveness and side effects. Here, we developed a transdermal mild photothermal therapy directly acting on the root of evil (subcutaneous white adipose depot) to induce its ameliorating remodeling (browning, lipolysis, and apoptosis), based on the injectable thermoresponsive hydrogel encapsulated with copper sulfide nanodots. Further, combining pharmaceutical therapy with codelivery of mirabegron leads to a strong therapeutic synergy. This method not only ensures high effectiveness and low side effects due to localized and targeted application but also remotely creates significant improvements in systemic metabolism. Specifically, as compared to the untreated group, it totally inhibits obesity development in high-fat-diet fed mice (15% less in body weight) with decreased masses of both subcutaneous (40%) and visceral fats (54%), reduced serum levels of cholesterol (54%)/triglyceride (18%)/insulin (74%)/glucose (45%), and improved insulin sensitivity (65% less in insulin resistance index). This self-administrable method is amenable for long-term home-based treatment. Finally, multiple interconnected signaling pathways are revealed, providing mechanistic insights to develop effective strategies to combat obesity and associated metabolic disorders.