Regioselective chemoenzymatic synthesis of ganglioside disialyl tetrasaccharide epitopes

Sugar Auxiliary Group Assisted Diversity-Oriented Enzymatic Modular Synthesis of 0-Series Ganglioside Glycans

Owing to the inaccessibility of β1-4-N-acetylgalactosaminyltransferase for direct glycan chain elongation, the enzymatic synthesis of 0-series gangliosides with extended backbones has not been explored. In this study, sialic acid was enzymatically introduced as an auxiliary group to overcome the limitation of substrate specificity of Campylobacter jejuni β1-4-N-acetylgalactosaminyltransferase (CjCgtA) to achieve the synthesis of desired extended 0-series ganglioside core structures, and the sialic acid auxiliary group could be removed by sialidase at appropriate stages. A bacterial α2-6-sialyltransferase from Photobacterium damselae (Pd2,6ST) exhibited unexpected acceptor substrate specificity for 0-series ganglioside core structures, providing ready access to complex gangliosides bearing the sialyl N-acetylgalactosamine unit. The 0-series ganglioside core structures as the key acceptor substrates were further diversified by sequential enzymatic modular assembly to generate a collection of 31 complex 0-series ganglioside glycans after removal of the sugar auxiliary group of sialic acid at the appropriate stage.

An Yb(OTf)₃-Mediated Indirect Activation Strategy for the Stereoselective Synthesis of α-Sialosides from 2-Fluorosialyl Donors

Most of chemical sialylation reactions are conducted at extremely low temperatures to achieve the formation of challenging sialic acid linkages with high stereoselectivities. Performing chemical sialylation at room temperature independent of enzymatic methods represents an effective approach, particularly significant in biological and biochemical research. Our study aims to develop a convenient method of providing α-sialyl glycosides. Herein, we carry out sialyation using Yb(OTf)3 as an activating promoter at room temperature in tetrahydrofurane and obtain excellent stereoselectivities when reacting the N-acetyl-5-N,4-O-carbonyl-2-fluorosialyl donor with galacto- or glucopyranosides. The advantages of this method include an over eight-month shelf life of the sialyl donors and minimal formation of the hydrolyzed or eliminated side-products. Sialylation of the C3 hydroxyl group in galactosides affords exclusive α-selectivities, and a one-pot synthesis of trisaccharide is accomplished by application of this method. Finally, we anticipate that this sialylation strategy can compensate for the limitations of the current enzymatic synthesis of complex glycans.

Comprehensive Modular Synthesis of Ganglioside Glycans and Evaluation of their Binding Affinities to Siglec-7 and Siglec-9

In the present work, bacterial glycosyltransferases are utilized to construct ganglioside glycans in a convergent approach via a sugar‒nucleotide regeneration system and one-pot multienzyme reactions. Starting from β-lactoside enables the diversification of both the glycan moieties and the linkages in the lower α-arm and upper β-arm. Overall, a comprehensive panel of 24 natural a-series (GM3, GM2, GM1a, GD1a, GT1a, and fucosyl-GM1), b-series (GD3, GD2, GD1b, GT1b, and GQ1b), c-series (GT3, GT2, GT1c, GQ1c, and GP1c), α-series (GM1α, GD1aα, and GT1aα), and o-series (GA2, GA1, GM1b, GalNAc-GM1b, and GD1c) ganglioside glycans are prepared, which are suitable for biological studies and further applications. Moreover, a microarray is constructed with these synthesized ganglioside glycans to investigate their binding specificity with recombinant Fc-fused Siglec-7 and Siglec-9, which are immune checkpoint-like glycan recognition proteins on natural killer cells. The microarray binding results reveal that GD3 and GT1aα are specific ligands for Siglec-7 and Siglec-9, respectively, and this discovery can lead to the identification of appropriate ligands for investigating the roles of these Siglecs in immunomodulation.

Chemoenzymatic Synthesis of Phosphosaccharides from Haemophilus parasuis Strains ER-6P (Serovar 15) and Nagasaki (Serovar 5)

Herein, we describe a chemoenzymatic and diversity-oriented approach for the first syntheses of octasaccharide repeating units of the capsular polysaccharides of Haemophilus parasuis serovar 15 and serovar 5. The synthetic method features efficient enzymatic assembly of sialyl galactose or N-acetyl-galactosamine building blocks, highly stereoselective chemical construction of α-type H-phosphonate, and the β-stereospecific 1,3-glycosylation reaction of a rare sugar donor.

Mild and General Protocol for Selective Deacetylation of Acetyl/Benzoyl-Protected Carbohydrates

Herein, we report a mild and general protocol for chemoselective deacetylation of mixed acetyl- and benzoyl-protected carbohydrates under mild acidic conditions. The protocol allows quick access to partially protected carbohydrates, which serve as versatile synthetic intermediates during the total synthesis of various mono- and oligosaccharide targets. The applicability of the developed protocol was successfully demonstrated on a range of carbohydrate substrates of various configurations and substitution patterns featuring functionalized aliphatic and aromatic aglycones. The protocol has shown excellent compatibility with the widely used O-anomeric protecting groups, prespacer aglycones, and thioglycoside glycosyl donors.

Integrated chemoenzymatic synthesis of a comprehensive sulfated ganglioside glycan library to decipher functional sulfoglycomics and sialoglycomics

Ganglioside glycans are ubiquitous and complex biomolecules that are involved in a wide range of biological functions and disease processes. Variations in sialylation and sulfation render the structural complexity and diversity of ganglioside glycans, and influence protein-carbohydrate interactions. Structural and functional insights into the biological roles of these glycans are impeded due to the limited accessibility of well-defined structures. Here we report an integrated chemoenzymatic strategy for expeditious and systematic synthesis of a comprehensive 65-membered ganglioside glycan library covering all possible patterns of sulfation and sialylation. This strategy relies on the streamlined modular assembly of three common sialylated precursors by highly stereoselective iterative sialylation, modular site-specific sulfation through flexible orthogonal protecting-group manipulations and enzymatic-catalysed diversification using three sialyltransferase modules and a galactosidase module. These diverse ganglioside glycans enable exploration into their structure-function relationships using high-throughput glycan microarray technology, which reveals that different patterns of sulfation and sialylation on these glycans mediate their unique binding specificities.

Recent advances in enzymatic and chemoenzymatic synthesis of N- and O-glycans

Glycosylation is one of the most common post-translational modifications of proteins, which plays essential roles in regulating the biological functions of proteins. Efficient and versatile methods for the synthesis of homogeneous and well-defined N- and O-glycans remain an urgent need for biological studies and biomedical applications. Despite their structural complexity, tremendous progress has been made in the synthesis of N- and O-glycans in recent years. This review discusses some recent advances in the enzymatic and chemoenzymatic synthesis of N- and O-glycans.

Chemical Evolution of Enzyme-Catalyzed Glycosylation

ConspectusThe limited availability of structurally well-defined diverse glycans remains a major obstacle for deciphering biological functions as well as biomedical applications of carbohydrates. Despite tremendous progress that has been made in past decades, the synthesis of structurally well-defined complex glycans still represents one of the most challenging topics in synthetic chemistry. Chemical synthesis of glycans is a time-consuming and labor-intensive process that requires elaborate planning and skilled personnel. In contrast, glycosyltransferase-catalyzed enzymatic synthesis provides a more efficient, convenient, low-cost, and sustainable alternative to affording diverse and complex glycans. However, the existing methods are still insufficient to fulfill the increasing demand for specific synthetic glycan libraries necessary for functional glycomics research. This is mainly attributed to the inherent character of the glycan biosynthetic pathway. In nature, there are too many glycosyltransferases involved in the in vivo glycan synthesis, but only a small number of them are available for in vitro enzymatic synthesis. For instance, humans have over 200 glycosyltransferases, but only a few of them could be produced from the conventional bacterial expression system, and most of these membrane-associated enzymes could be overexpressed only in eukaryotic cells. Moreover, the glycan biosynthetic pathway is a nontemplate-driven process, which eventually ends up with heterogeneous glycan product mixtures. Therefore, it is not a practical solution for the in vitro enzymatic synthesis of complex glycans by simply copying the glycan biosynthetic pathway.In the past decade, we have tried to develop a simplified and transformable approach to the enzymatic modular assembly of a human glycan library. Despite the structural complexity of human glycans, the glycoinformatic analysis based on the known glycan structure database and the human glycosyltransferase database indicates that there are approximately 56 disaccharide patterns present in the human glycome and only 16 disaccharide linkages are required to account for over 80% of the total disaccharide fragments, while 35 disaccharide linkages are sufficient to cover over 95% of all disaccharide fragments of human glycome. Regardless of the substrate specificity, if one glycosyltransferase could be used for the synthesis of all of the same glycosidic linkages in human glycome, it will require only a few dozen glycosyltransferases for the assembly of entire human glycans. According to the glycobioinformatics analysis results, we rationally designed about two dozen enzyme modules for the synthesis of over 20 common glycosidic linkages in human glycome, in which each enzyme module contains a glycosyltransferase and a group of enzymes for the in situ generation of a nucleotide-activated sugar donor. By sequential glycosylation using orchestrated enzyme modules, we have completed the synthesis of over 200 structurally well-defined complex human glycans including blood group antigens, O-mannosyl glycans, human milk oligosaccharides, and others. To overcome the product microheterogeneity problem of enzymatic synthesis in the nontemplate-driven glycan biosynthetic pathway, we developed several substrate engineering strategies to control or manipulate the outcome of glycosyltransferase-catalyzed reactions for the precise synthesis of structurally well-defined isomeric complex glycans.

Phosphates as Assisting Groups in Glycan Synthesis

In nature, phosphates are added to and cleaved from molecules to direct biological pathways. The concept was adapted to overcome limitations in the chemical synthesis of complex oligosaccharides. Phosphates were chemically placed on synthetic glycans to ensure site-specific enzymatic elongation by sialylation. In addition, the deliberate placement of phosphates helped to solubilize and isolate aggregating glycans. Upon traceless removal of the phosphates by enzymatic treatment with alkaline phosphatase, the native glycan structure was revealed, and the assembly of glycan nanostructures was triggered.

Identification of tagged glycans with a protein nanopore

Structural complexity of glycans derived from the diversities in composition, linage, configuration, and branching considerably complicates structural analysis. Nanopore-based single-molecule sensing offers the potential to elucidate glycan structure and even sequence glycan. However, the small molecular size and low charge density of glycans have restricted direct nanopore detection of glycan. Here we show that glycan sensing can be achieved using a wild-type aerolysin nanopore by introducing a facile glycan derivatization strategy. The glycan molecule can induce impressive current blockages when moving through the nanopore after being connected with an aromatic group-containing tag (plus a carrier group for the neutral glycan). The obtained nanopore data permit the identification of glycan regio- and stereoisomers, glycans with variable monosaccharide numbers, and distinct branched glycans, either independently or with the use of machine learning methods. The presented nanopore sensing strategy for glycans paves the way towards nanopore glycan profiling and potentially sequencing.

Different Biophysical Properties of Cell Surface α2,3- and α2,6-Sialoglycans Revealed by Electron Paramagnetic Resonance Spectroscopic Studies

Sialoglycans on HeLa cells were labeled with a nitroxide spin radical through enzymatic glycoengineering (EGE)-mediated installation of azide-modified sialic acid (Neu5Ac9N₃) and then click reaction-based attachment of a nitroxide spin radical. α2,6-Sialyltransferase (ST) Pd2,6ST and α2,3-ST CSTII were used for EGE to install α2,6- and α2,3-linked Neu5Ac9N₃, respectively. The spin-labeled cells were analyzed by X-band continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy to gain insights into the dynamics and organizations of cell surface α2,6- and α2,3-sialoglycans. Simulations of the EPR spectra revealed average fast- and intermediate-motion components for the spin radicals in both sialoglycans. However, α2,6- and α2,3-sialoglycans in HeLa cells possess different distributions of the two components, e.g., a higher average population of the intermediate-motion component for α2,6-sialoglycans (78%) than that for α2,3-sialoglycans (53%). Thus, the average mobility of spin radicals in α2,3-sialoglycans was higher than that in α2,6-sialoglycans. Given the fact that a spin-labeled sialic acid residue attached to the 6-O-position of galactose/N-acetyl-galactosamine would experience less steric hindrance and show more flexibility than that attached to the 3-O-position, these results may reflect the differences in local crowding/packing that restrict the spin-label and sialic acid motion for α2,6-linked sialoglycans. The studies further suggest that Pd2,6ST and CSTII may have different preferences for glycan substrates in the complex environment of the extracellular matrix. The discoveries of this work are biologically important as they are useful for interpreting the different functions of α2,6- and α2,3-sialoglycans and indicate the possibility of using Pd2,6ST and CSTII to target different glycoconjugates on cells.

Systematic synthesis of bisected N-glycans and unique recognitions by glycan-binding proteins

Bisected N-glycans represent a unique class of protein N-glycans that play critical roles in many biological processes. Herein, we describe the systematic synthesis of these structures. A bisected N-glycan hexasaccharide was chemically assembled with two orthogonal protecting groups attached at the C2 of the branching mannose residues, followed by sequential installation of GlcNAc and LacNAc building blocks to afford two asymmetric bisecting "cores". Subsequent enzymatic modular extension of the "cores" yielded a comprehensive library of biantennary N-glycans containing the bisecting GlcNAc and presenting 6 common glycan determinants in a combinatorial fashion. These bisected N-glycans and their non-bisected counterparts were used to construct a distinctive glycan microarray to study their recognition by a wide variety of glycan-binding proteins (GBPs), including plant lectins, animal lectins, and influenza A virus hemagglutinins. Significantly, the bisecting GlcNAc could bestow (PHA-L, rDCIR2), enhance (PHA-E), or abolish (ConA, GNL, anti-CD15s antibody, etc.) N-glycan recognition of specific GBPs, and is tolerated by many others. In summary, synthesized compounds and the unique glycan microarray provide ideal standards and tools for glycoanalysis and functional glycomic studies. The microarray data provide new information regarding the fine details of N-glycan recognition by GBPs, and in turn improve their applications.

Chemoenzymatic synthesis and biological evaluation of ganglioside GM3 and lyso-GM3 as potential agents for cancer therapy

A highly efficient chemoenzymatic method for synthesizing ganglioside GM3 and lyso-GM3 was reported here. Enzymatic extension of the chemically synthesized lactosyl sphingosine using efficient one-pot multienzyme (OPME) reaction allowed glycosylation to be carried out in aqueous solutions realizing the greening of reactions. Ganglioside GM3 was synthesized through 10 steps with a total yield of 22%. Lyso-GM3 was very useful for kinds of derivatization. The anti-proliferation activity studies demonstrated that these compounds 14-16 with sphingosine exhibited more potency than the corresponding lyso-GM3 with ceramide. All ganglioside GM3 and lyso-GM3 can effectively inhibit the migration of melanoma B16-F10 cells. These chemoenzymaticlly synthesized GM3 and lyso-GM3 exhibited antitumor activities, which can provide valuable sights to search new antitumor agents for cancer therapy.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Chemoenzymatic modular assembly of O-GalNAc glycans for functional glycomics

O-GalNAc glycans (or mucin O-glycans) play pivotal roles in diverse biological and pathological processes, including tumor growth and progression. Structurally defined O-GalNAc glycans are essential for functional studies but synthetic challenges and their inherent structural diversity and complexity have limited access to these compounds. Herein, we report an efficient and robust chemoenzymatic modular assembly (CEMA) strategy to construct structurally diverse O-GalNAc glycans. The key to this strategy is the convergent assembly of O-GalNAc cores 1-4 and 6 from three chemical building blocks, followed by enzymatic diversification of the cores by 13 well-tailored enzyme modules. A total of 83 O-GalNAc glycans presenting various natural glycan epitopes are obtained and used to generate a unique synthetic mucin O-glycan microarray. Binding specificities of glycan-binding proteins (GBPs) including plant lectins and selected anti-glycan antibodies towards these O-GalNAc glycans are revealed by this microarray, promoting their applicability in functional O-glycomics. Serum samples from colorectal cancer patients and healthy controls are assayed using the array reveal higher bindings towards less common cores 3, 4, and 6 than abundant cores 1 and 2, providing insights into O-GalNAc glycan structure-activity relationships.

Acceptor-mediated regioselective enzyme catalyzed sialylation: chemoenzymatic synthesis of GAA-7 ganglioside glycan

Herein, we applied PmST1 (a sialyltransferase) to achieve acceptor-mediated regioselective sialylation (AMRS) on the nonreducing end GalNH₂ or GalAz (2-azido-2-deoxy galactose). Thus, C5 and C8-modified sialic acid was efficiently assembled on GalNH₂ (or GalAz) to achieve the synthesis of the GAA-7 (one of the echinodermatous gangliosides with higher neuritogenic activity) glycan moiety.

A Modular Chemoenzymatic Synthesis of Disialosyl Globopentaosylceramide (DSGb5Cer) Glycan

The total synthesis of the oligosaccharide moiety of disialosyl globopentaosylceramide (DSGb5 Cer), a dominant ganglioside isolated from malignant renal cell carcinoma tissues, is reported. The synthetic strategy relies on a chemical α(2,6)-sialylation at the internal GalNAc unit of a Gb5 pentasaccharide backbone that furnishes a Neu5Acα(2,6)GalNAc-linked hexasaccharide, suitable for an enzymatic α(2,3)-sialylation of the terminal Gal residue to construct a heptasaccharide glycan. Convergent access to this key α(2,6)-sialylated hexasaccharide was also achieved through a [3+3] glycosylation building upon a Galβ(1,3)[Neu5Acα(2,6)]GalNAc-based trisaccharide donor and a Gb3 acceptor. The synthetic DSGb5 glycan bearing a 6-azidohexyl aglycon at the reducing end could undergo further regioselective functionalization. This approach represents a viable chemoenzymatic method for accessing complex ganglioside glycans and should be useful for the synthesis and biological investigation of DSGb5 derivatives.

Liquid-Phase and Ultrahigh-Frequency-Acoustofluidics-Based Solid-Phase Synthesis of Biotin-Tagged 6′/3′-Sialyl-N-Acetylglucosamine by Sequential One-Pot Multienzyme System

6′/3′-Sialylated N-acetyllactosamine (6′/3′-SLN) is important for discrimination of the source (human or avian) of influenza virus strains. Biotinylated oligosaccharides have been widely used for analysis and quick detection. The development of efficient strategies to synthesize biotin-tagged 6′/3′-SLN have become necessary. Effective mixing is essential for enzymatic solid-phase oligosaccharide synthesis (SPOS). In the current study, newly developed technology ultrahigh-frequency-acoustofluidics (UHFA), which can provide a powerful source for efficient microfluidic mixing, solid-phase oligosaccharide synthesis and one-pot multienzyme (OPME) system, were used to develop a new strategy for oligosaccharide synthesis. Firstly, biotinylated N-acetylglucosamine was designed and chemically synthesized through traditional approaches. Secondly, biotinylated 6′- and 3′-sialyl-N-acetylglucosamines were prepared in solution through two sequential OPME modules in with a yield of ~95%. Thirdly, 6′-SLN was also prepared through UHFA-based enzymatic solid-phase synthesis on magnetic beads with a yield of 64.4%. The current strategy would be potentially used for synthesis of functional oligosaccharides.

Efficient chemoenzymatic synthesis of fluorinated sialyl Thomsen-Friedenreich antigens and investigation of their characteristics

A set of fluorinated sialyl-T derivatives were efficiently synthesized using one-pot multi-enzyme (OPME) chemoenzymatic approach. The P. multocida α2-3-sialyltransferase (PmST1) involved in the synthesis showed extremely flexible donor and acceptor substrate specificities. These sialosides have been successfully investigated with stability towards Clostridium perfringens sialidase substrate specificity assay using ¹H NMR spectroscopy. Hydrolysis studies monitored by ¹H NMR clearly demonstrated that the fluorine substitution obviously reduced hydrolysis rate of Clostridium perfringens sialidase. To further investigate the fluorine influence, structure-dependent variation of sialoside-lectin binding was observed for MAL and different sialoside-immobilized surfaces. Subtle changes on the ligand of carbohydrate-binding protein were distinguished by SPR. These fluorinated sialyl-T derivatives obtained are valuable probes for further biological studies or antitumor drug design.

A simple bacterial expression system for human ppGalNAc-T and used for the synthesis of O-GalNAc glycosylated interleukin 2

Mucin-type O-glycosylation (hereafter referred to as O-GalNAc glycosylation) is one of the most abundant glycosylation on proteins. It is initiated by the members of polypeptide N-acetyl-α-galactosaminyltransferases (ppGalNAc-Ts) family. The ppGalNAc-Ts could be used as tool enzymes to modify target proteins including therapeutic glycoprotein drugs with O-GalNAc glycosylation at specific glycosylated sites in vitro. Obtaining a large amount of ppGalNAc-T can greatly increase the yield of therapeutic O-glycoprotein and reduce the culture costs. In this study, we reported a simple Escherichia coli (E. coli) expression system capable of producing human ppGalNAc-Ts. By co-expressing human PDI, we could simply obtain active ppGalNAc-Ts with high efficiency. Using the E. coli expressed ppGalNAc-T2, we site-specifically synthesized O-glycosylated IL-2 at the native glycosylated site Thr23 residue. These results reveal the E. coli system we constructed is suitable to produce active ppGalNAc-Ts and thus has the potential value for basic research and production of therapeutic O-glycoproteins in vitro.

Application of Enzymes in Regioselective and Stereoselective Organic Reactions

Nowadays, biocatalysts have received much more attention in chemistry regarding their potential to enable high efficiency, high yield, and eco-friendly processes for a myriad of applications. Nature’s vast repository of catalysts has inspired synthetic chemists. Furthermore, the revolutionary technologies in bioengineering have provided the fast discovery and evolution of enzymes that empower chemical synthesis. This article attempts to deliver a comprehensive overview of the last two decades of investigation into enzymatic reactions and highlights the effective performance progress of bio-enzymes exploited in organic synthesis. Based on the types of enzymatic reactions and enzyme commission (E.C.) numbers, the enzymes discussed in the article are classified into oxidoreductases, transferases, hydrolases, and lyases. These applications should provide us with some insight into enzyme design strategies and molecular mechanisms.

Synthesis of N-Glycolylneuraminic Acid (Neu5Gc) and Its Glycosides

Sialic acids constitute a family of negatively charged structurally diverse monosaccharides that are commonly presented on the termini of glycans in higher animals and some microorganisms. In addition to N-acetylneuraminic acid (Neu5Ac), N-glycolyl neuraminic acid (Neu5Gc) is among the most common sialic acid forms in nature. Nevertheless, unlike most animals, human cells loss the ability to synthesize Neu5Gc although Neu5Gc-containing glycoconjugates have been found on human cancer cells and in various human tissues due to dietary incorporation of Neu5Gc. Some pathogenic bacteria also produce Neu5Ac and the corresponding glycoconjugates but Neu5Gc-producing bacteria have yet to be found. In addition to Neu5Gc, more than 20 Neu5Gc derivatives have been found in non-human vertebrates. To explore the biological roles of Neu5Gc and its naturally occurring derivatives as well as the corresponding glycans and glycoconjugates, various chemical and enzymatic synthetic methods have been developed to obtain a vast array of glycosides containing Neu5Gc and/or its derivatives. Here we provide an overview on various synthetic methods that have been developed. Among these, the application of highly efficient one-pot multienzyme (OPME) sialylation systems in synthesizing compounds containing Neu5Gc and derivatives has been proven as a powerful strategy.

Chemoenzymatic Synthesis of DSGb5 and Sialylated Globo-series Glycans

Sialic-acid-binding, immunoglobulin-type lectin-7 (Siglec-7) is present on the surface of natural killer cells. Siglec-7 shows preference for disialylated glycans, including α(2,8)-α(2,3)-disialic acids or internally branched α(2,6)-NeuAc, such as disialosylglobopentaose (DSGb5). Herein, DSGb5 was synthesized by a one-pot multiple enzyme method from Gb5 by α2,3-sialylation (with PmST1) followed by α2,6-sialylation (with Psp2,6ST) in 23 % overall yield. DSGb5 was also chemoenzymatically synthesized. The protection of the nonreducing-end galactose of Gb5 as 3,4-O-acetonide, 3,4-O-benzylidene, and 4,6-O-benzylidene derivatives provided DSGb5 in overall yields of 26 %, 12 %, and 19 %, respectively. Gb3, Gb4, and Gb5 were enzymatically sialylated to afford a range of globo-glycans. Surprisingly, DSGb5 shows a low affinity for Siglec-7 in a glycan microarray binding affinity assay. Among the synthesized globo-series glycans, α6α3DSGb4 shows the highest binding affinity for Siglec-7.

Redox-Controlled Site-Specific α2-6-Sialylation

The first bacterial α2-6-sialyltransferase cloned from Photobacterium damselae (Pd2,6ST) has been widely applied for the synthesis of various α2-6-linked sialosides. However, the extreme substrate flexibility of Pd2,6ST makes it unsuitable for site-specific α2-6-sialylation of complex substrates containing multiple galactose and/or N-acetylgalactosamine units. To tackle this problem, a general redox-controlled site-specific sialylation strategy using Pd2,6ST is described. This approach features site-specific enzymatic oxidation of galactose units to mask the unwanted sialylation sites and precisely controlling the site-specific α2-6-sialylation at intact galactose or N-acetylgalactosamine units.

An automated platform for the enzyme-mediated assembly of complex oligosaccharides

An automated platform that can synthesize a wide range of complex carbohydrates will greatly increase their accessibility and should facilitate progress in glycoscience. Here we report a fully automated process for enzyme-mediated oligosaccharide synthesis that can give easy access to different classes of complex glycans including poly-N-acetyllactosamine derivatives, human milk oligosaccharides, gangliosides and N-glycans. Our automated platform uses a catch and release approach in which glycosyltransferase-catalysed reactions are performed in solution and product purification is accomplished by solid phase extraction. We developed a sulfonate tag that can easily be installed and enables highly efficient solid phase extraction and product release using a single set of washing conditions, regardless of the complexity of the glycan. Using this custom-built synthesizer, as many as 15 reaction cycles can be performed in an automated fashion without a need for lyophilization or buffer exchange steps.

Oligosaccharide Synthesis and Translational Innovation

The translation of biological glycosylation in humans to the clinical applications involves systematic studies using homogeneous samples of oligosaccharides and glycoconjugates, which could be accessed by chemical, enzymatic or other biological methods. However, the structural complexity and wide-range variations of glycans and their conjugates represent a major challenge in the synthesis of this class of biomolecules. To help navigate within many methods of oligosaccharide synthesis, this Perspective offers a critical assessment of the most promising synthetic strategies with an eye on the therapeutically relevant targets.

Strategies for chemoenzymatic synthesis of carbohydrates

Carbohydrates are structurally complex but functionally important biomolecules. Therefore, they have been challenging but attractive synthetic targets. While substantial progress has been made on advancing chemical glycosylation methods, incorporating enzymes into carbohydrate synthetic schemes has become increasingly practical as more carbohydrate biosynthetic and metabolic enzymes as well as their mutants with synthetic application are identified and expressed for preparative and large-scale synthesis. Chemoenzymatic strategies that integrate the flexibility of chemical derivatization with enzyme-catalyzed reactions have been extremely powerful. Briefly summarized here are our experiences on developing one-pot multienzyme (OPME) systems and representative chemoenzymatic strategies from others using glycosyltransferase-catalyzed reactions for synthesizing diverse structures of oligosaccharides, polysaccharides, and glycoconjugates. These strategies allow the synthesis of complex carbohydrates including those containing naturally occurring carbohydrate postglycosylational modifications (PGMs) and non-natural functional groups. By combining these srategies with facile purification schemes, synthetic access to the diverse space of carbohydrate structures can be automated and will not be limited to specialists.

Highly efficient chemoenzymatic synthesis and facile purification of α-Gal pentasaccharyl ceramide Galα3nLc4βCer

A highly efficient chemoenzymatic method for synthesizing glycosphingolipids using α-Gal pentasaccharyl ceramide as an example is reported here. Enzymatic extension of the chemically synthesized lactosyl sphingosine using efficient sequential one-pot multienzyme (OPME) reactions allowed glycosylation to be carried out in aqueous solutions. Facile C18 cartridge-based quick (<30 minutes) purification protocols were established using minimal amounts of green solvents (CH₃CN and H₂O). Simple acylation in the last step led to the formation of the target glycosyl ceramide in 4 steps with an overall yield of 57%.

Enzymatic and Chemoenzymatic Syntheses of Disialyl Glycans and Their Necrotizing Enterocolitis Preventing Effects

Necrotizing enterocolitis (NEC) is one of the most common and devastating intestinal disorders in preterm infants. Therapies to meet the clinical needs for this special and highly vulnerable population are extremely limited. A specific human milk oligosaccharide (HMO), disialyllacto-N-tetraose (DSLNT), was shown to contribute to the beneficial effects of breastfeeding as it prevented NEC in a neonatal rat model and was associated with lower NEC risk in a human clinical cohort study. Herein, gram-scale synthesis of two DSLNT analogs previously shown to have NEC preventing effect is described. In addition, four novel disialyl glycans have been designed and synthesized by enzymatic or chemoenzymatic methods. Noticeably, two disialyl tetraoses have been produced by enzymatic sialylation of chemically synthesized thioethyl β-disaccharides followed by removal of the thioethyl aglycon. Dose-dependent and single-dose comparison studies showed varying NEC-preventing effects of the disialyl glycans in neonatal rats. This study helps to refine the structure requirement of the NEC-preventing effect of disialyl glycans and provides important dose-dependent information for using DSLNT analogs as potential therapeutics for NEC prevention in preterm infants.

Converting Pasteurella multocidaα2-3-sialyltransferase 1 (PmST1) to a regioselective α2-6-sialyltransferase by saturation mutagenesis and regioselective screening

A microtiter plate-based screening assay capable of determining the activity and regioselectivity of sialyltransferases was developed. This assay was used to screen two single-site saturation libraries of Pasteurella multocidaα2-3-sialyltransferase 1 (PmST1) for α2-6-sialyltransferase activity and total sialyltransferase activity. PmST1 double mutant P34H/M144L was found to be the most effective α2-6-sialyltransferase and displayed 50% reduced donor hydrolysis and 50-fold reduced sialidase activity compared to the wild-type PmST1. It retained the donor substrate promiscuity of the wild-type enzyme and was used in an efficient one-pot multienzyme (OPME) system to selectively catalyze the sialylation of the terminal galactose residue in a multigalactose-containing tetrasaccharide lacto-N-neotetraoside.

High-throughput assays of leloir-glycosyltransferase reactions: The applications of rYND1 in glycotechnology

Glycosyltransferases (GTs) play a critical role in the enzymatic and chemoenzymatic synthesis of oligosaccharides and glycoconjugates. However, the development of these synthetic approaches has been limited by a lack of sensitive screening methods for the isolation of novel natural GTs or their active variants. Herein, we describe the results of our investigation towards the soluble expression and potential application of the Saccharomyces cerevisiae apyrase YND1. By replacing the hydrophobic transmembrane domain of YND1 with three glycine-serine repeats, this protein was successfully expressed in a soluble form in Escherichia coli. This new protein was then used to develop a two-step nucleoside diphosphate (NDP)-based Leloir-GT high-throughput assay. Purified rYND1 was initially added to a GT reaction to hydrolyze NDP to nucleoside phosphate plus inorganic phosphate, which was determined using a phosphorus molybdenum blue chromogenic reaction. Purified rYND1 was shown to have a positive effect on saccharide synthesis by eliminating the potential by-product inhibition from NDP. Most of the mono-sugar donors used for Leloir-GTs are activated by uridine diphosphate and guanosine diphosphate, which can be catalyzed by rYND1. The rYND1 is amenable to screening methods and could be applied to a wide range of Leloir-GT-catalyzed reactions, therefore representing a remarkable step forward in glycotechnology.

One-pot multienzyme (OPME) systems for chemoenzymatic synthesis of carbohydrates

Glycosyltransferase-catalyzed enzymatic and chemoenzymatic syntheses are powerful approaches for the production of oligosaccharides, polysaccharides, glycoconjugates, and their derivatives. Enzymes involved in the biosynthesis of sugar nucleotide donors can be combined with glycosyltransferases in one pot for efficient production of the target glycans from simple monosaccharides and acceptors. The identification of enzymes involved in the salvage pathway of sugar nucleotide generation has greatly facilitated the development of simplified and efficient one-pot multienzyme (OPME) systems for synthesizing major glycan epitopes in mammalian glycomes. The applications of OPME methods are steadily gaining popularity mainly due to the increasing availability of wild-type and engineered enzymes. Substrate promiscuity of these enzymes and their mutants allows OPME synthesis of carbohydrates with naturally occurring post-glycosylational modifications (PGMs) and their non-natural derivatives using modified monosaccharides as precursors. The OPME systems can be applied in sequence for synthesizing complex carbohydrates. The sequence of the sequential OPME processes, the glycosyltransferase used, and the substrate specificities of the glycosyltransferases define the structures of the products. The OPME and sequential OPME strategies can be extended to diverse glycans in other glycomes when suitable enzymes with substrate promiscuity become available. This Perspective summarizes the work of the authors and collaborators on the development of glycosyltransferase-based OPME systems for carbohydrate synthesis. Future directions are also discussed.

The chemical synthesis of aryltetralin glycosides

Led by etoposide and teniposide, the synthesis of aryltetralin glycosides has been experiencing flourishing development in the past five decades. Herein, a review focusing on the total synthesis of aryltetralin glycosides is provided. The main body of this review is composed of two parts, one is the enantioselective synthesis of aryltetralin derivatives and the other one is the construction of key glycosidic linkages. In each part the contents are organised based on the different strategies or protocols applied in the original documents. The total synthesis of aryltetralin glycosides represents the developing direction of this field, and sooner or later will replace the currently applied semi-total synthesis method, using the aglycon residue acquired directly from natural sources. This account provides a comprehensive and deep insight into the field of aryltetralin glycoside synthesis for chemists who have the intention of committing themselves to the development of aryltetralin glycoside medicine.

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

Efficient chemoenzymatic synthesis of novel galacto-N-biose derivatives and their sialylated forms

Galacto-N-biose (GNB) derivatives were efficiently synthesized from galactose derivatives via a one-pot two-enzyme system containing two promiscuous enzymes from Bifidobacterium infantis: a galactokinase (BiGalK) and a d-galactosyl-β1-3-N-acetyl-d-hexosamine phosphorylase (BiGalHexNAcP). Mono-sialyl and di-sialyl galacto-N-biose derivatives were then prepared using a one-pot two-enzyme system containing a CMP-sialic acid synthetase and an α2-3-sialyltransferase or an α2-6-sialyltransferase.

Synthesis and labeling of α-(2,9)-trisialic acid with cyanine dyes for imaging of glycan-binding receptors on living cells

The cyanine tagged oligosialic acid was utilized as an efficient fluorescent probe to image the glycan-binding receptors on PC-12 cells.

Diversity-oriented synthesis of fused thioglycosyl benzo[e][1,4]oxathiepin-5-ones and benzo[f][1,4]thiazepin-5(2H)-ones by a sequence of palladium-catalyzed glycosyl thiol arylation and deprotection–lactonization reactions

An efficient synthesis of thioglycosylated benzo[e][1,4]-oxathiepin-5-one and benzothiazepinone derivatives has been reported.