Chemoenzymatic synthesis of C8-modified sialic acids and related α2-3- and α2-6-linked sialosides

Synthetic Sialosides Terminated with 8-N-Substituted Sialic Acid as Selective Substrates for Sialidases from Bacteria and Influenza Viruses

Sialosides containing C8-modified sialic acids are challenging synthetic targets but potentially useful probes for diagnostic substrate profiling of sialidases and elucidating the binding specificity of sialic acid-interacting proteins. Here, we demonstrate efficient chemoenzymatic methods for synthesizing para-nitrophenol-tagged α2-3- and α2-6-linked sialyl galactosides containing C8-acetamido, C8-azido, or C8-amino derivatized N-acetylneuraminic acid (Neu5Ac). High-throughput substrate specificity studies showed that the C8-modification of sialic acid significantly changes its recognition by sialidases from humans, various bacteria, and different influenza A and B viruses. Sialosides carrying Neu5Ac with a C8-azido modification were generally well tolerated by all the sialidases we tested, whereas sialosides containing C8-acetamido-modified Neu5Ac were only cleaved by selective bacterial sialidases. In contrast, sialosides with C8-amino-modified Neu5Ac were cleaved by a combination of selective bacterial and influenza A virus sialidases. These results indicate that sialosides terminated with a C8-amino or C8-acetamido-modified sialic acid can be used with other sialosides for diagnostic profiling of disease-causing sialidase-producing pathogens.

Chemoenzymatic Synthesis of GAA-7 Glycan Analogues and Evaluation of Their Neuritogenic Activities

Ganglioside GAA-7 exhibits higher neurite outgrowth than ganglioside GM1a and most echinodermatous gangliosides (EGs) when tested on neuron-like rat adrenal pheochromocytoma (PC12) cells in the presence of nerve growth factor (NGF). The unique structure of GAA-7 glycan, containing an uncommon sialic acid (8-O-methyl-N-glycolylneuraminic acid) and sialic acid-α-2,3-GalNAc linkage, makes it challenging to synthesize. We recently developed a streamlined method to chemoenzymatically synthesize GAA-7 glycan and employed this modular strategy to efficiently prepare a library of GAA-7 glycan analogues incorporating N-modified or 8-methoxyl sialic acids. Most of these synthetic glycans exhibited moderate efficacy in promoting neuronal differentiation of PC12 cells. Among them, the analogue containing common sialic acid shows greater potential than the GAA-7 glycan itself. This result reveals that methoxy modification is not essential for neurite outgrowth. Consequently, the readily available analogue presents a promising model for further biological investigations.

Enabling Chemoenzymatic Strategies and Enzymes for Synthesizing Sialyl Glycans and Sialyl Glycoconjugates

Sialic acids are fascinating negatively charged nine-carbon monosaccharides. Sialic acid-containing glycans and glycoconjugates are structurally diverse, functionally important, and synthetically challenging molecules. We have developed highly efficient chemoenzymatic strategies that combine the power of chemical synthesis and enzyme catalysis to make sialic acids, sialyl glycans, sialyl glycoconjugates, and their derivatives more accessible, enabling the efforts to explore their functions and applications. The Account starts with a brief description of the structural diversity and the functional importance of naturally occurring sialic acids and sialosides. The development of one-pot multienzyme (OPME) chemoenzymatic sialylation strategies is then introduced, highlighting its advantages in synthesizing structurally diverse sialosides with a sialyltransferase donor substrate engineering tactic. With the strategy, systematic access to sialosides containing different sialic acid forms with modifications at C3/4/5/7/8/9, various internal glycans, and diverse sialyl linkages is now possible. Also briefly described is the combination of the OPME sialylation strategy with bacterial sialidases for synthesizing sialidase inhibitors. With the goal of simplifying the product purification process for enzymatic glycosylation reactions, glycosphingolipids that contain a naturally existing hydrophobic tag are attractive targets for chemoenzymatic total synthesis. A user-friendly highly efficient chemoenzymatic strategy is developed which involves three main processes, including chemical synthesis of lactosyl sphingosine as a water-soluble hydrophobic tag-containing intermediate, OPME enzymatic extension of its glycan component with a single C18-cartridge purification of the product, followed by a facile chemical acylation reaction. The strategy allows the introduction of different sialic acid forms and diverse fatty acyl chains into the products. Gram-scale synthesis has been demonstrated. OPME sialylation has also been demonstrated for the chemoenzymatic synthesis of sialyl glycopeptides and in vitro enzymatic N-glycan processing for the formation of glycoproteins with disialylated biantennary complex-type N-glycans. For synthesizing human milk oligosaccharides (HMOs) which are glycans with a free reducing end, acceptor substrate engineering and process engineering strategies are developed, which involve the design of a hydrophobic tag that can be easily installed into the acceptor substrate to allow facile purification of the product from enzymatic reactions and can be conveniently removed in the final step to produce target molecules. The process engineering involves heat-inactivation of enzymes in the intermediate steps in multistep OPME reactions for the production of long-chain sialoside targets in a single reaction pot and with a single C18-cartridge purification process. In addition, a chemoenzymatic synthon strategy has been developed. It involves the design of a derivative of the sialyltransferase donor substrate precursor, which is tolerated by enzymes in OPME reactions, introduced to enzymatic products, and then chemically converted to the desired target structures in the final step. The chemoenzymatic synthon approach has been used together with the acceptor substrate engineering method in the synthesis of complex bacterial glycans containing sialic acids, legionaminic acids, and derivatives. The biocatalysts characterized and their engineered mutants developed by the Chen group are described, with highlights on synthetically useful enzymes. We anticipate further development of chemoenzymatic strategies and biocatalysts to enable exploration of the sialic acid space.

Growing impact of sialic acid-containing glycans in future drug discovery

In nature, almost all cells are covered with a complex array of glycan chain namely sialic acids or nuraminic acids, a negatively charged nine carbon sugars which is considered for their great therapeutic importance since long back. Owing to its presence at the terminal end of lipid bilayer (commonly known as terminal sugars), the well-defined sialosides or sialoconjugates have served pivotal role on the cell surfaces and thus, the sialic acid-containing glycans can modulate and mediate a number of imperative cellular interactions. Understanding of the sialo-protein interaction and their roles in vertebrates in regard of normal physiology, pathological variance, and evolution has indeed a noteworthy journey in medicine. In this tutorial review, we present a concise overview about the structure, linkages in chemical diversity, biological significance followed by chemical and enzymatic modification/synthesis of sialic acid containing glycans. A more focus is attempted about the recent advances, opportunity, and more over growing impact of sialosides and sialoconjugates in future drug discovery and development.

A concise synthesis of l-gulose and its C-6 derivatives

A convenient route for the preparation of l-gulose and its C-6 derivatives starting from commercially available 2,3:5,6-diisopropylidene-d-mannofuranose via C-5 epimerization as the key step was developed. 1-O-Benzylation followed by regioselective hydrolysis of the 5,6-isopropylidene group furnished benzyl 2,3-isopropylidene-α-d-mannofuranoside, which was subjected upon regioselective one-pot 6-O-benzoylation and 5-O-mesylation, providing the corresponding 5-OMs-6-OBz derivative in excellent selectivity. Treatment of this mesylate compound with potassium t-butoxide to remove the benzoyl group followed by intramolecular S_N2 inversion led to benzyl 5,6-anhydro-2,3-isopropylidene-β-l-gulofuranoside, which could undergo not only nucleophilic substitutions to open the epoxide ring to give various C-6 derivatives, but also acidic hydrolysis to yield 1,6-anhydro-β-l-gulopyranose for further transformation into l-gulopyranosyl pentaacetate.

Advancement of the Cleavage Methods of Carbohydrate-derived Isopropylidene and Cyclohexylidene Ketals

Abstract:

Carbohydrates, amino acids, and nucleosides, the fundamental building blocks of complex biomolecules in nature, are essential starting materials for the fabrication of natural and unnatural structural entities, which necessitate the masking and demasking of various functional groups with the utmost chemoselectivity, mildness, and efficiency to avoid unintended bond breaking and formation, as well as associated reactions. Ketals, benzylidene, methoxymethyl, p-methoxybenzyl, silyl ethers, trityl, tert-butyl carbamate, and other functional groups are widely used in modern organic synthesis. In carbohydrate chemistry, the commonly used protecting functionality of isopropylidene and cyclohexylidene ketals necessitates effective methods for selective cleavage. This review summarises different methods for deblocking isopropylidene and cyclohexylidene ketals using inorganic acids, Lewis acid, silica-supported inorganic acids, Amberlite-120 (H+) resin, phosphotungstic acid, Nafion-H, NaBArF4.2H2O, montmorillonite clay, Dowex 50W-X8, camphorsulphonic acid (CSA), ceric ammonium nitrate, molecular iodine, ionic liquids, zeolites and so on.

Synthesis of N-acetylmannosamine-6-phosphate derivatives to investigate the mechanism of N-acetylmannosamine-6-phosphate 2-epimerase

The synthesis of analogues of natural enzyme substrates can be used to help deduce enzymatic mechanisms. N-Acetylmannosamine-6-phosphate 2-epimerase is an enzyme in the bacterial sialic acid catabolic pathway. To investigate whether the mechanism of this enzyme involves a re-protonation mechanism by the same neighbouring lysine that performed the deprotonation or a unique substrate-assisted proton displacement mechanism involving the substrate C5 hydroxyl, the syntheses of two analogues of the natural substrate, N-acetylmannosamine-6-phosphate, are described. In these novel analogues, the C5 hydroxyl has been replaced with a proton and a methyl ether respectively. As recently reported, Staphylococcus aureus N-acetylmannosamine-6-phosphate 2-epimerase was co-crystallized with these two compounds. The 5-deoxy variant bound to the enzyme active site in a different orientation to the natural substrate, while the 5-methoxy variant did not bind, adding to the evidence that this enzyme uses a substrate-assisted proton displacement mechanism. This mechanistic information may help in the design of potential antibacterial drug candidates.

Engineering analysis of multienzyme cascade reactions for 3'-sialyllactose synthesis

Sialo‐oligosaccharides are important products of emerging biotechnology for complex carbohydrates as nutritional ingredients. Cascade bio‐catalysis is central to the development of sialo‐oligosaccharide production systems, based on isolated enzymes or whole cells. Multienzyme transformations have been established for sialo‐oligosaccharide synthesis from expedient substrates, but systematic engineering analysis for the optimization of such transformations is lacking. Here, we show a mathematical modeling‐guided approach to 3ʹ‐sialyllactose (3SL) synthesis from N‐acetyl‐ d‐neuraminic acid (Neu5Ac) and lactose in the presence of cytidine 5ʹ‐triphosphate, via the reactions of cytidine 5ʹ‐monophosphate‐Neu5Ac synthetase and α2,3‐sialyltransferase. The Neu5Ac was synthesized in situ from N‐acetyl‐ d‐mannosamine using the reversible reaction with pyruvate by Neu5Ac lyase or the effectively irreversible reaction with phosphoenolpyruvate by Neu5Ac synthase. We show through comprehensive time‐course study by experiment and modeling that, due to kinetic rather than thermodynamic advantages of the synthase reaction, the 3SL yield was increased (up to 75%; 10.4 g/L) and the initial productivity doubled (15 g/L/h), compared with synthesis based on the lyase reaction. We further show model‐based optimization to minimize the total loading of protein (saving: up to 43%) while maintaining a suitable ratio of the individual enzyme activities to achieve 3SL target yield (61%–75%; 7–10 g/L) and overall productivity (3–5 g/L/h). Collectively, our results reveal the principal factors of enzyme cascade efficiency for 3SL synthesis and highlight the important role of engineering analysis to make multienzyme‐catalyzed transformations fit for oligosaccharide production.

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.

Development of Diverse Range of Biologically Relevant Carbohydrate-Containing Molecules: Twenty Years of Our Journey*

There is an increasing demand for significant amount of carbohydrate-containing molecules owing to their complete chemical, biological, and pharmacological investigations to better understand their role in many important biological events. Clinical studies of a wide range of simple carbohydrates or their derivatives, glycohybrids, glycoconjugates, and neoglycoconjugates have been conducted worldwide for the successful treatment of various frontline diseases. Herein, a brief perspective of carbohydrate-based molecular scaffolding and my experience during the last 20 years in the area of synthetic carbohydrate chemistry, mainly for their impact in drug discovery & development, is presented.

A Chemoenzymatic Synthon Strategy for Synthesizing N-Acetyl Analogues of O-Acetylated N. meningitidis W Capsular Polysaccharide Oligosaccharides

O-Acetylated sialic acid has been found in the Neisseria meningitidis serogroup W (NmW) capsular polysaccharide (CPS) and is a required structural component of clinically used NmW CPS-based polysaccharide and polysaccharide-conjugate vaccines. The role of sialic acid O-acetylation in NmW CPS, however, is not clearly understood. This is partially due to the lack of a precise control of the percentage and the location of O-acetylation which is labile and susceptible to migration. We explore chemoenzymatic synthetic strategies for preparing N-acetylated analogues of O-acetylated NmW CPS oligosaccharides which can serve as structurally stable probe mimics. Substrate specificity studies of NmW CPS polymerase (NmSiaD_W) identified 4-azido-4-deoxy-N-acetylmannosamine (ManNAc4N₃) and 6-azido-6-deoxy-N-acetylmannosamine (ManNAc6N₃) as suitable chemoenzymatic synthons for synthesizing N-acetyl analogues of NmW CPS oligosaccharides containing 7-O-acetyl-N-acetylneuraminic acid (Neu5,7Ac₂) and/or 9-O-acetyl-N-acetylneuraminic acid (Neu5,9Ac₂). The synthesis was achieved by NmSiaD_W-dependent sequential one-pot multienzyme (OPME) strategy with in situ generation of the corresponding sugar nucleotides from simple monosaccharides or derivatives to form N₃-oligosaccharides which were converted to the desired NAc-oligosaccharides by an efficient one-step chemical transformation.

Modified Sialic Acids on Mucus and Erythrocytes Inhibit Influenza A Virus Hemagglutinin and Neuraminidase Functions

Sialic acids (Sia) are the primary receptors for influenza viruses and are widely displayed on cell surfaces and in secreted mucus. Sia may be present in variant forms that include O-acetyl modifications at C-4, C-7, C-8, and C-9 positions and N-acetyl or N-glycolyl at C-5. They can also vary in their linkages, including α2-3 or α2-6 linkages. Here, we analyze the distribution of modified Sia in cells and tissues of wild-type mice or in mice lacking CMP-N-acetylneuraminic acid hydroxylase (CMAH) enzyme, which synthesizes N-glycolyl (Neu5Gc) modifications. We also examined the variation of Sia forms on erythrocytes and in saliva from different animals. To determine the effect of Sia modifications on influenza A virus (IAV) infection, we tested for effects on hemagglutinin (HA) binding and neuraminidase (NA) cleavage. We confirmed that 9-O-acetyl, 7,9-O-acetyl, 4-O-acetyl, and Neu5Gc modifications are widely but variably expressed in mouse tissues, with the highest levels detected in the respiratory and gastrointestinal (GI) tracts. Secreted mucins in saliva and surface proteins of erythrocytes showed a high degree of variability in display of modified Sia between different species. IAV HAs from different virus strains showed consistently reduced binding to both Neu5Gc- and O-acetyl-modified Sia; however, while IAV NAs were inhibited by Neu5Gc and O-acetyl modifications, there was significant variability between NA types. The modifications of Sia in mucus may therefore have potent effects on the functions of IAV and may affect both pathogens and the normal flora of different mucosal sites.IMPORTANCE Sialic acids (Sia) are involved in numerous different cellular functions and are receptors for many pathogens. Sia come in chemically modified forms, but we lack a clear understanding of how they alter interactions with microbes. Here, we examine the expression of modified Sia in mouse tissues, on secreted mucus in saliva, and on erythrocytes, including those from IAV host species and animals used in IAV research. These Sia forms varied considerably among different animals, and their inhibitory effects on IAV NA and HA activities and on bacterial sialidases (neuraminidases) suggest a host-variable protective role in secreted mucus.

Catalytic Cycle of Neisseria meningitidis CMP-Sialic Acid Synthetase Illustrated by High-Resolution Protein Crystallography

Cytidine 5'-monophosphate (CMP)-sialic acid synthetase (CSS) is an essential enzyme involved in the biosynthesis of carbohydrates and glycoconjugates containing sialic acids, a class of α-keto acids that are generally terminal key recognition residues by many proteins that play important biological and pathological roles. The CSS from Neisseria meningitidis (NmCSS) has been commonly used with other enzymes such as sialic acid aldolase and/or sialyltransferase in synthesizing a diverse array of compounds containing sialic acid or its naturally occurring and non-natural derivatives. To better understand its catalytic mechanism and substrate promiscuity, four NmCSS crystal structures trapped at various stages of the catalytic cycle with bound substrates, substrate analogues, and products have been obtained and are presented here. These structures suggest a mechanism for an "open" and "closed" conformational transition that occurs as sialic acid binds to the NmCSS/cytidine-5'-triphosphate (CTP) complex. The closed conformation positions critical residues to help facilitate the nucleophilic attack of sialic acid C2-OH to the α-phosphate of CTP, which is also aided by two observed divalent cations. Product formation drives the active site opening, promoting the release of products.

Natural and Synthetic Sialylated Glycan Microarrays and Their Applications

This focused chapter serves as a short survey of glycan microarrays that are available with sialylated glycans, including both defined and shotgun arrays, their generation, and their utility in studying differential binding interactions to sialylated compounds, highlighting N-glycolyl (Gc) modified sialylated compounds. A brief discussion of binding interactions by lectins, antibodies, and viruses, and their relevance that have been observed with sialylated glycan microarrays is presented, as well as a discussion of cross-comparisons of array platforms and efforts to centralize and standardize the glycan microarray data.

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.

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.

Chemoenzymatic synthesis of Neu5Ac9NAc-containing α2-3- and α2-6-linked sialosides and their use for sialidase substrate specificity studies

O-Acetylation of sialic acid (Sia) modulates its recognition by sialic acid-binding proteins and plays an important role in biological and pathological processes. 9-O-Acetylation is the most common modification of sialic acid in human. However, study of O-acetylated sialoglycans is hampered due to the instability of O-acetyl group towards pH changes and sensitivity to esterases. Our previous studies demonstrated a chemical biology method to this problem by replacing the oxygen atom in the C9 ester group of sialic acid by a nitrogen to form an amide. Here, we synthesized a library of sixteen new 9-acetamido-9-deoxy-N-acetylneuraminic acid (Neu5Ac9NAc)-containing α2-3- and α2-6-linked sialosides with various underlying glycans using efficient one-pot three-enzyme (OP3E) sialylation systems. Neu5Ac9NAc-containing compounds with a para-nitrophenol aglycon have been used together with their 9-O-acetyl analogs in microtiter plate-based high-throughput substrate specificity studies of nine different sialidases including those from humans and bacteria. In general, similar to 9-O-acetylation, 9-N-acetyl modification of sialic acid in the substrates lowers sialic acid-cleavage activity of most sialidases. In most cases, Neu5Ac9NAc is a good analog of 9-O-acetyl sialic acid. However, exceptions do exist. For example, 9-N- and 9-O-acetyl modifications have different effects on the sialosides cleave efficiencies of a commercially available C. perfringens sialidase as well as recombinant Streptococcus pneumoniae sialidase SpNanC and Bifidobacterium infantis sialidase BiNanH2. The mechanism for the difference awaits further investigation.

Sequential One-Pot Multienzyme Chemoenzymatic Synthesis of Glycosphingolipid Glycans

Glycosphingolipids are a diverse family of biologically important glycolipids. In addition to variations on the lipid component, more than 300 glycosphingolipid glycans have been characterized. These glycans are directly involved in various molecular recognition events. Several naturally occurring sialic acid forms have been found in sialic acid-containing glycosphingolipids, namely gangliosides. However, ganglioside glycans containing less common sialic acid forms are currently not available. Herein, highly effective one-pot multienzyme (OPME) systems are used in sequential for high-yield and cost-effective production of glycosphingolipid glycans, including those containing different sialic acid forms such as N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc), 2-keto-3-deoxy-d-glycero-d-galacto-nononic acid (Kdn), and 8-O-methyl-N-acetylneuraminic acid (Neu5Ac8OMe). A library of 64 structurally distinct glycosphingolipid glycans belonging to ganglio-series, lacto-/neolacto-series, and globo-/isoglobo-series glycosphingolipid glycans is constructed. These glycans are essential standards and invaluable probes for bioassays and biomedical studies.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

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.

Michael F, Vinogradov EV, Stupak J, Zheng B, Ohnishi M, Unemo M, Lewis LA, Taylor RE, Landig CS, Diaz S, Reed GW, Varki A, Rice PA, Ram S. Utilizing CMP-Sialic Acid Analogs to Unravel Neisseria gonorrhoeae Lipooligosaccharide-Mediated Complement Resistance and Design Novel Therapeutics

Neisseria gonorrhoeae deploys a novel immune evasion strategy wherein the lacto-N-neotetraose (LNnT) structure of lipooligosaccharide (LOS) is capped by the bacterial sialyltransferase, using host cytidine-5’-monophosphate (CMP)-activated forms of the nine-carbon nonulosonate (NulO) sugar N-acetyl-neuraminic acid (Neu5Ac), a sialic acid (Sia) abundant in humans. This allows evasion of complement-mediated killing by recruiting factor H (FH), an inhibitor of the alternative complement pathway, and by limiting classical pathway activation (“serum-resistance”). We utilized CMP salts of six additional natural or synthetic NulOs, Neu5Gc, Neu5Gc8Me, Neu5Ac9Ac, Neu5Ac9Az, legionaminic acid (Leg5Ac7Ac) and pseudaminic acid (Pse5Ac7Ac), to define structural requirements of Sia-mediated serum-resistance. While all NulOs except Pse5Ac7Ac were incorporated into the LNnT-LOS, only Neu5Gc incorporation yielded high-level serum-resistance and FH binding that was comparable to Neu5Ac, whereas Neu5Ac9Az and Leg5Ac7Ac incorporation left bacteria fully serum-sensitive and did not enhance FH binding. Neu5Ac9Ac and Neu5Gc8Me rendered bacteria resistant only to low serum concentrations. While serum-resistance mediated by Neu5Ac was associated with classical pathway inhibition (decreased IgG binding and C4 deposition), Leg5Ac7Ac and Neu5Ac9Az incorporation did not inhibit the classical pathway. Remarkably, CMP-Neu5Ac9Az and CMP-Leg5Ac7Ac each prevented serum-resistance despite a 100-fold molar excess of CMP-Neu5Ac in growth media. The concomitant presence of Leg5Ac7Ac and Neu5Ac on LOS resulted in uninhibited classical pathway activation. Surprisingly, despite near-maximal FH binding in this instance, the alternative pathway was not regulated and factor Bb remained associated with bacteria. Intravaginal administration of CMP-Leg5Ac7Ac to BALB/c mice infected with gonorrhea (including a multidrug-resistant isolate) reduced clearance times and infection burden. Bacteria recovered from CMP-Leg5Ac7Ac-treated mice were sensitive to human complement ex vivo, simulating in vitro findings. These data reveal critical roles for the Sia exocyclic side-chain in gonococcal serum-resistance. Such CMP-NulO analogs may provide a novel therapeutic strategy against the global threat of multidrug-resistant gonorrhea.

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection gonorrhea, has developed widespread resistance to almost every conventional antibiotic currently in clinical use. Novel therapeutics are urgently needed against this pathogen. Gonococci have the capacity to scavenge CMP-N-acetyl-neuraminic acid (CMP-Neu5Ac, a CMP-activated 9-carbon sugar that is a member of the ‘sialic acid family’) from the host to ‘cap’ its lipooligosaccharide with Neu5Ac, which renders gonococci resistant to complement, a key arm of innate immune defenses. Here, we show that gonococci also utilize derivatives (or analogs) of CMP-Neu5Ac, which not only fail to render the bacteria resistant to complement, but also prevent complement inhibition mediated by the ‘physiologic’ human sialic acid donor, CMP-Neu5Ac. When administered intravaginally to mice, a representative analog significantly shortened the duration and burden of gonococcal infection. Thus, CMP-sialic acid analogs may represent promising preventive or therapeutic agents against multidrug-resistant gonorrhea that poses a global threat to public health.

Chemoenzymatic synthesis of sialosides containing C7-modified sialic acids and their application in sialidase substrate specificity studies

Modifications at the glycerol side chain of sialic acid in sialosides modulate their recognition by sialic acid-binding proteins and sialidases. However, limited work has been focused on the synthesis and functional studies of sialosides with C7-modified sialic acids. Here we report chemical synthesis of C4-modified ManNAc and mannose and their application as sialic acid precursors in a highly efficient one-pot three-enzyme system for chemoenzymatic synthesis of α2-3- and α2-6-linked sialyl para-nitrophenyl galactosides in which the C7-hydroxyl group in sialic acid (N-acetylneuraminic acid, Neu5Ac, or 2-keto-3-deoxynonulosonic acid, Kdn) was systematically substituted by -F, -OMe, -H, and -N3 groups. Substrate specificity study of bacterial and human sialidases using the obtained sialoside library containing C7-modified sialic acids showed that sialosides containing C7-deoxy Neu5Ac were selective substrates for all bacterial sialidases tested but not for human NEU2. The information obtained from sialidase substrate specificity can be used to guide the design of new inhibitors that are selective against bacterial sialidases.

Glycans in immune recognition and response

Glycans at the forefront of cells facilitate immune recognition processes. Cancer cells commonly present altered cell surface glycosylation, especially manifested in the expression of sialic acid at the termini of glycolipids and glycoproteins. Although tumor-associated carbohydrate antigens (TACAs) result in expression of altered-self, most such carbohydrates do not elicit strong humoral responses. Various strategies had been devised to elicit increased immunogenicity of such TACA aiming for potent immunotherapeutic antibodies or cancer vaccines. However some carbohydrates are immunogenic in humans and hold potential for novel glycotherapies. N-Glycolylneuraminic acid (Neu5Gc) is a foreign immunogenic sugar in humans originating from the diet (e.g., red meat) and subsequently expressed on the cell surface, especially accumulating on carcinoma. Consequently, the human immune system detects this non-self carbohydrate generating a broad anti-Neu5Gc antibody response. The co-existence of Neu5Gc/anti-Neu5Gc within humans spurs chronic inflammation mediated disease, including cancer. Concurrently, anti-Neu5Gc antibodies hold potential for novel targeted therapy. αGal is another foreign immunogenic carbohydrate antigen in humans and all humans have circulating anti-Gal antibodies. This review aims to describe the immunogenicity of Neu5Gc and its implications for human diseases, highlighting differences and similarities with αGal and its potential for novel targeted theranostics.

Structural basis for substrate specificity and mechanism of N-acetyl-D-neuraminic acid lyase from Pasteurella multocida

N-Acetylneuraminate lyases (NALs) or sialic acid aldolases catalyze the reversible aldol cleavage of N-acetylneuraminic acid (Neu5Ac, the most common form of sialic acid) to form pyruvate and N-acetyl-d-mannosamine. Although equilibrium favors sialic acid cleavage, these enzymes can be used for high-yield chemoenzymatic synthesis of structurally diverse sialic acids in the presence of excess pyruvate. Engineering these enzymes to synthesize structurally modified natural sialic acids and their non-natural derivatives holds promise in creating novel therapeutic agents. Atomic-resolution structures of these enzymes will greatly assist in guiding mutagenic and modeling studies to engineer enzymes with altered substrate specificity. We report here the crystal structures of wild-type Pasteurella multocida N-acetylneuraminate lyase and its K164A mutant. Like other bacterial lyases, it assembles into a homotetramer with each monomer folding into a classic (β/α)₈ TIM barrel. Two wild-type structures were determined, in the absence of substrates, and trapped in a Schiff base intermediate between Lys164 and pyruvate, respectively. Three structures of the K164A variant were determined: one in the absence of substrates and two binary complexes with N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). Both sialic acids bind to the active site in the open-chain ketone form of the monosaccharide. The structures reveal that every hydroxyl group of the linear sugars makes hydrogen bond interactions with the enzyme, and the residues that determine specificity were identified. Additionally, the structures provide some clues for explaining the natural discrimination of sialic acid substrates between the P. multocida and Escherichia coli NALs.

Exploration of sialic acid diversity and biology using sialoglycan microarrays

Sialic acids (Sias) are a group of α-keto acids with a nine-carbon backbone, which display many types of modifications in nature. The diversity of natural Sia presentations is magnified by a variety of glycosidic linkages to underlying glycans, the sequences and classes of such glycans, as well as the spatial organization of Sias with their surroundings. This diversity is closely linked to the numerous and varied biological functions of Sias. Relatively large libraries of natural and unnatural Sias have recently been chemically/chemoenzymatically synthesized and/or isolated from natural sources. The resulting sialoglycan microarrays have proved to be valuable tools for the exploration of diversity and biology of Sias. Here we provide an overview of Sia diversity in nature, the approaches used to generate sialoglycan microarrays, and the achievements and challenges arising.

Cross-comparison of protein recognition of sialic acid diversity on two novel sialoglycan microarrays

DNA and protein arrays are commonly accepted as powerful exploratory tools in research. This has mainly been achieved by the establishment of proper guidelines for quality control, allowing cross-comparison between different array platforms. As a natural extension, glycan microarrays were subsequently developed, and recent advances using such arrays have greatly enhanced our understanding of protein-glycan recognition in nature. However, although it is assumed that biologically significant protein-glycan binding is robustly detected by glycan microarrays, there are wide variations in the methods used to produce, present, couple, and detect glycans, and systematic cross-comparisons are lacking. We address these issues by comparing two arrays that together represent the marked diversity of sialic acid modifications, linkages, and underlying glycans in nature, including some identical motifs. We compare and contrast binding interactions with various known and novel plant, vertebrate, and viral sialic acid-recognizing proteins and present a technical advance for assessing specificity using mild periodate oxidation of the sialic acid chain. These data demonstrate both the diversity of sialic acids and the analytical power of glycan arrays, showing that different presentations in different formats provide useful and complementary interpretations of glycan-binding protein specificity. They also highlight important challenges and questions for the future of glycan array technology and suggest that glycan arrays with similar glycan structures cannot be simply assumed to give similar results.

Sialic acid metabolism and sialyltransferases: natural functions and applications

Sialic acids are a family of negatively charged monosaccharides which are commonly presented as the terminal residues in glycans of the glycoconjugates on eukaryotic cell surface or as components of capsular polysaccharides or lipooligosaccharides of some pathogenic bacteria. Due to their important biological and pathological functions, the biosynthesis, activation, transfer, breaking down, and recycle of sialic acids are attracting increasing attention. The understanding of the sialic acid metabolism in eukaryotes and bacteria leads to the development of metabolic engineering approaches for elucidating the important functions of sialic acid in mammalian systems and for large-scale production of sialosides using engineered bacterial cells. As the key enzymes in biosynthesis of sialylated structures, sialyltransferases have been continuously identified from various sources and characterized. Protein crystal structures of seven sialyltransferases have been reported. Wild-type sialyltransferases and their mutants have been applied with or without other sialoside biosynthetic enzymes for producing complex sialic acid-containing oligosaccharides and glycoconjugates. This mini-review focuses on current understanding and applications of sialic acid metabolism and sialyltransferases.