Recent advances in o-sialylation

Enzymatic synthesis of N-formylated sialosides via a five-enzyme cascade

Here we report an enzymatic approach to synthesize N-formylneuraminic acid (Neu5Fo) containing sialosides, through a five-enzyme cascade. This method stands as an alternative to traditional chemical syntheses, aiming for precision and efficiency in generating sialosides with a tailored N-formyl group generated directly from formic acid. The newly synthesized Neu5Fo was characterized using various NMR techniques revealing a conformational equilibrium at the amide bond of the formyl group in slow exchange on the NMR time scale with a trans : cis ratio of ∼2 : 1. This work not only suggests potential for exploring the biological roles of sialosides but also points to the possibility of developing novel therapeutic agents.

Many locks to one key: N-acetylneuraminic acid binding to proteins

Sialic acids play crucial roles in cell surface glycans of both eukaryotic and prokaryotic organisms, mediating various biological processes, including cell-cell interactions, development, immune response, oncogenesis and host-pathogen interactions. This review focuses on the β-anomeric form of N-acetylneuraminic acid (Neu5Ac), particularly its binding affinity towards various proteins, as elucidated by solved protein structures. Specifically, we delve into the binding mechanisms of Neu5Ac to proteins involved in sequestering and transporting Neu5Ac in Gram-negative bacteria, with implications for drug design targeting these proteins as antimicrobial agents. Unlike the initial assumptions, structural analyses revealed significant variability in the Neu5Ac binding pockets among proteins, indicating diverse evolutionary origins and binding modes. By comparing these findings with existing structures from other systems, we can effectively highlight the intricate relationship between protein structure and Neu5Ac recognition, emphasizing the need for tailored drug design strategies to inhibit Neu5Ac-binding proteins across bacterial species.

Synthesis and Thermodynamic Evaluation of Sialyl-Tn MUC1 Glycopeptides Binding to Macrophage Galactose-Type Lectin

Interactions between the tumor-associated carbohydrate antigens of Mucin 1 (MUC1) and the carbohydrate-binding proteins, lectins, often lead to the creation of a pro-tumor microenvironment favoring tumor initiation, progression, metastasis, and immune evasion. Macrophage galactose binding lectin (MGL) is a C-type lectin receptor found on antigen-presenting cells that facilitates the uptake of carbohydrate antigens for antigen presentation, modulating the immune response homeostasis, autoimmunity, and cancer. Considering the crucial role of tumor-associated forms of MUC1 and MGL in tumor immunology, a thorough understanding of their binding interaction is essential for it to be exploited for cancer vaccine strategies. The synthesis of MUC1 glycopeptide models carrying a single or multiple Tn and/or sialyl-Tn antigen(s) is described. A novel approach for the sialyl-Tn threonine building block suitable for the solid phase peptide synthesis was developed. The thermodynamic profile of the binding interaction between the human MGL and MUC1 glycopeptide models was analyzed using isothermal titration calorimetry. The measured dissociation constants for the sialyl-Tn-bearing peptide epitopes were consistently lower compared to the Tn antigen and ranged from 10 μM for mono- to 1 μM for triglycosylated MUC1 peptide, respectively. All studied interactions, regardless of the glycan's site of attachment or density, exhibited enthalpy-driven thermodynamics.

A Fluorinated Sialic Acid Vaccine Lead Against Meningitis B and C

Inspired by the specificity of α-(2,9)-sialyl epitopes in bacterial capsular polysaccharides (CPS), a doubly fluorinated disaccharide has been validated as a vaccine lead against Neisseria meningitidis serogroups C and/or B. Emulating the importance of fluorine in drug discovery, this molecular editing approach serves a multitude of purposes, which range from controlling α-selective chemical sialylation to mitigating competing elimination. Conjugation of the disialoside with two carrier proteins (CRM197 and PorA) enabled a semisynthetic vaccine to be generated; this was then investigated in six groups of six mice. The individual levels of antibodies formed were compared and classified as highly glycan-specific and protective. All glycoconjugates induced a stable and long-term IgG response and binding to the native CPS epitope was achieved. The generated antibodies were protective against MenC and/or MenB; this was validated in vitro by SBA and OPKA assays. By merging the fluorinated glycan epitope of MenC with an outer cell membrane protein of MenB, a bivalent vaccine against both serogroups was created. It is envisaged that validation of this synthetic, fluorinated disialoside bioisostere as a potent antigen will open new therapeutic avenues.

Facile synthesis of C5-azido derivatives of thiosialosides and 2,3-dehydro-5-N-acetylneuraminic acid (DANA)

Neuraminic acid (Neu5Ac, also known as sialic acid) is an important monosaccharide found in glycoproteins and glycolipids which plays a vital role in regulation of physiological functions and pathological conditions. The study of sialoglycans has benefitted from the development of glycomimetic probes and inhibitors for proteins and enzymes that interact with and modify neuraminic acid in glycan chains. Methods to access sialoside intermediates with high yield are needed to facilitate the design of new targets. Here, we report the synthesis of C5-azido thiosialosides using a mild method to deprotect the C5-acetamido functional group followed by the use of a diazo-transfer reagent. We examined two diazo-transfer strategies and compared their yields and tolerance of acetate protecting groups. The same methods and comparisons were also performed for the 2,3-dehydro-5-N-acetylneuraminic acid (DANA) scaffold which is commonly used to generate inhibitors of neuraminidase (sialidase) enzymes. We found that C5-azido derivatives of both thiosialosides and DANA could be produced in five or six steps with yields up to 76 % and 83 %, respectively. Diazo-transfer reagents compared in this study were trifluoromethanesulfonyl azide (TfN3) and imidazole-1-sulfonyl azide (ImzSO2N3). We found that both reagents were compatible with this method and showed comparable yields. Finally, we show that C5-azido derivatives can help to avoid O, N-acyl protecting group migration which was observed in C5-NHAc analogs.

Synthesis of sialyl halides with various acyl protective groups

Glycosyl halides are historically one of the first glycosyl donors used in glycosylation reactions, and interest in glycosylation reactions involving this class of glycosyl donors is currently increasing. New methods for their activation have been proposed and effective syntheses of oligosaccharides with their participation have been developed. At the same time, the possibilities of using these approaches to the synthesis of sialosides are restricted by the limited diversity of known sialyl halides (previously, mainly sialyl chlorides, less often sialyl bromides and sialyl fluorides, with acetyl (Ac) groups at the oxygen atoms and AcNH, Ac2N and N3 groups at C-5 were used). This work describes the synthesis of six new N-acetyl- and N-trifluoroacetyl-sialyl chlorides and bromides with O-chloroacetyl and O-trifluoroacetyl protective groups. Preparation of N,O-trifluoroacetyl protected derivatives was made possible due to development of the synthesis of sialic acid methyl ester pentaol with N-trifluoroacetyl group.

Stereocontrolled Synthesis of α-3-Deoxy-d-manno-oct-2-ulosonic Acid (α-Kdo) Glycosides Using C3-p-Tolylthio-Substituted Kdo Donors: Access to Highly Branched Kdo Oligosaccharides

3-Deoxy-d-manno-oct-2-ulosonic acid (Kdo) is an eight-carbon monosaccharide found widely in bacterial lipopolysaccharides (LPSs) and capsule polysaccharides (CPSs). We developed an indirect method for the stereoselective synthesis of α-Kdo glycosides with a C3-p-tolylthio-substituted Kdo phosphite donor. The presence of the p-tolylthio group enhanced the reactivity, suppressed the formation of elimination by-products (2,3-enes), and provided complete α-stereocontrol. A variety of Kdo α-glycosides were synthesized by our method in excellent yields (up to 98 %). After glycosylation, the p-tolylthio group can be efficiently removed by free-radical reduction. Subsequently, the orthogonality of the phosphite donor and thioglycoside donor was demonstrated by the one-pot synthesis of a trisaccharide in Helicobacter pylori and Neisseria meningitidis LPS. Moreover, an efficient total synthesis route to the challenging 4,5-branched Kdo trisaccharide in LPSs from several A. baumannii strains was highlighted. To demonstrate the high reactivity of our approach further, the highly crowded 4,5,7,8-branched Kdo pentasaccharide was synthesized as a model molecule for the first time. Additionally, the reaction mechanism was investigated by DFT calculations.

Stereoselective Synthesis of Sialyl Lewisa Antigen and the Effective Anticancer Activity of Its Bacteriophage Qβ Conjugate as an Anticancer Vaccine

Sialyl Lewisa (sLea ), also known as cancer antigen 19-9 (CA19-9), is a tumor-associated carbohydrate antigen. The overexpression of sLea on the surface of a variety of cancer cells makes it an attractive target for anticancer immunotherapy. However, sLea -based anticancer vaccines have been under-explored. To develop a new vaccine, efficient stereoselective synthesis of sLea with an amine-bearing linker was achieved, which was subsequently conjugated with a powerful carrier bacteriophage, Qβ. Mouse immunization with the Qβ-sLea conjugate generated strong and long-lasting anti-sLea IgG antibody responses, which were superior to those induced by the corresponding conjugate of sLea with the benchmark carrier keyhole limpet hemocyanin. Antibodies elicited by Qβ-sLea were highly selective toward the sLea structure, could bind strongly with sLea -expressing cancer cells and human pancreatic cancer tissues, and kill tumor cells through complement-mediated cytotoxicity. Furthermore, vaccination with Qβ-sLea significantly reduced tumor development in a metastatic cancer model in mice, demonstrating tumor protection for the first time by a sLea -based vaccine, thus highlighting the significant potential of sLea as a promising cancer antigen.

Synthesis of mannan oligosaccharide-sialic acid conjugates and its inhibition on Aβ42 aggregation

In this work, a mannan-oligosaccharide conjugate with sialic acid capable of perturbing Aβ42 aggregation was designed and synthesized. Mannan oligosaccharides with degree polymerization of 3-13 were obtained by stepwise hydrolysis of locust bean gum using β-mannanase and α-galactosidase, named as LBOS. The activated LBOS was further chemically conjugated with sialic acid (Sia, N-acetylneuraminic acid) by fluoro-mercapto chemical coupling to synthesize a conjugate LBOS-Sia, and then phosphorylated to obtain pLBOS-Sia. The successful synthesis of pLBOS-Sia was confirmed by infrared1 chromatography, mass spectrometry, and ¹H NMR. The soluble protein analysis, microscopic observation, thioflavin T-labeling, and circular dichroism spectroscopy revealed that both LBOS-Sia and pLBOS-Sia can inhibit Aβ42 aggregation. MTT assay showed that LBOS-Sia and pLBOS-Sia had no cytotoxicity to BV-2 cells, and could substantially reduce the release of pro-inflammatory factor TNF-α induced by Aβ42 in BV-2 cells, and inhibit the occurrence of neuroinflammation. In future, this novel structure of mannan oligosaccharide-sialic acid conjugate can be potentially used to for the development of glycoconjugates against AD targeting Aβ.

Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

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.

Ferrier/Aza-Wacker/Epoxidation/Glycosylation (FAWEG) Sequence to Access 1,2-Trans 3-Amino-3-deoxyglycosides

3-Amino-3-deoxyglycosides constitute an essential class of nitrogen-containing sugars. Among them, many important 3-amino-3-deoxyglycosides possess a 1,2-trans relationship. In view of their numerous biological applications, the synthesis of 3-amino-3-deoxyglycosyl donors giving rise to a 1,2-trans glycosidic linkage is thus an important challenge. Even though glycals are highly polyvalent donors, the synthesis and reactivity of 3-amino-3-deoxyglycals have been little studied. In this work, we describe a new sequence, involving a Ferrier rearrangement and subsequent aza-Wacker cyclization that allows the rapid synthesis of orthogonally protected 3-amino-3-deoxyglycals. Finally a 3-amino-3-deoxygalactal derivative was submitted for the first time to an epoxidation/glycosylation with high yield and great diastereoselectivity, highlighting FAWEG (Ferrier/Aza-Wacker/Epoxidation/Glycosylation) as a new approach to access 1,2-trans 3-amino-3-deoxyglycosides.

Enabling Technologies in Carbohydrate Chemistry: Automated Glycan Assembly, Flow Chemistry and Data Science

The synthesis of defined oligosaccharides is a complex task. Several enabling technologies have been introduced in the last two decades to facilitate synthetic access to these valuable biomolecules. In this concept, we describe the technological solutions that have advanced glycochemistry using automated glycan assembly, flow chemistry and data science as examples. We highlight how the synergies between these different technologies can further advance the field, with progress toward the realization of a self-driving lab for glycan synthesis.

Phase-Transfer Catalyzed Microfluidic Glycosylation: A Small Change in Concentration Results in a Dramatic Increase in Stereoselectivity

Phase-transfer catalysis (PTC) is widely used in glycochemistry for the preparation of aryl glycosides by the glycosylation reaction. While investigating the possibility of synthesis of 4-(3-chloropropoxy)phenyl sialoside (Neu5Ac-OCPP) from N-acetylsialyl chloride with O-acetyl groups (1), we have recently discovered a strong dependence of the PTC glycosylation outcome on the mixing mode: under batch conditions, only α-anomer of Neu5Ac-OCPP was obtained, albeit in low yield (13%), while under microfluidic conditions the yield of Neu5Ac-OCPP increased to 36%, although stereoselectivity decreased (α/β ≤ 6.2). Here, we report that the outcome of this reaction, performed under microfluidic conditions using a Comet X-01 micromixer (at 2 μL/min flow rate), non-linearly depends on the concentration of N-acetylsialyl chloride 1 (5–200 mmol/L). The target Neu5Ac-OCPP was obtained in a noticeably higher yield (up to 66%) accompanied by enhanced stereoselectivity (α/β = 17:1–32:1) in the high concentration range (C > 50 mmol/L), whereas the yield (10–36%) and especially, stereoselectivity (α/β = 0.9:1–6.2:1) were lower in the low concentration range (C ≤ 50 mmol/L). This dramatic stepwise increase in stereoselectivity above critical concentration (50 mmol/L) is apparently related to the changes in the presentation of molecules on the surface of supramers of glycosyl donor, which exist in different concentration ranges.

Challenges and Strategies for a Thorough Characterization of Antibody Acidic Charge Variants

Heterogeneity of therapeutic Monoclonal antibody (mAb) drugs are due to protein variants generated during the manufacturing process. These protein variants can be critical quality attributes (CQAs) depending on their potential impact on drug safety and/or efficacy. To identify CQAs and ensure the drug product qualities, a thorough characterization is required but challenging due to the complex structure of biotherapeutics. Past characterization studies for basic and acidic variants revealed that full characterizations were limited to the basic charge variants, while the quantitative measurements of acidic variants left gaps. Consequently, the characterization and quantitation of acidic variants are more challenging. A case study of a therapeutic mAb1 accounted for two-thirds of the enriched acidic variants in the initial characterization study. This led to additional investigations, closing the quantification gaps of mAb1 acidic variants. This work demonstrates that a well-designed study with the right choices of analytical methods can play a key role in characterization studies. Thus, the updated strategies for more complete antibody charge variant characterization are recommended.

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.

Advanced Chemical Methods for Stereoselective Sialylation and Their Applications in Sialoglycan Syntheses

Sialic acid is an important component of cell surface glycans, which are responsible for many vital body functions and should therefore be thoroughly studied to understand their biological roles and association with disorders. The difficulty of isolating large quantities of homogenous-state sialoglycans from natural sources has inspired the development of the corresponding chemical synthesis methods affording acceptable purities, yields, and amounts. However, the related syntheses are challenging because of the difficulties in α-glycosylation of sialic acid, which arises from its certain structural features such as the absence of a stereodirecting group at the C3 position and presence of carboxyl group at the anomeric position. Moreover, the structural complexities of sialoglycans with diverse numbers and locations of sialic acid on the glycan chains pose additional barriers. Thus, efficient α-stereoselective routes to sialosides remain highly sought after, although various types of sialyl donors/acceptors have been developed for the straightforward synthesis of α-sialosides. Herein, we review the latest progress in the α-stereoselective synthesis of sialosides and their applications in the preparation of gangliosides and other sialoglycans.

Synthesis of an STnThr analogue, structurally based on a TnThr antigen mimetic

The monosaccharide Tn and the disaccharide STn are tumor antigens with similar structures and common biosynthetic pathways. Both are always over-expressed simultaneously on tumor cell surfaces. We report herein the efficient synthesis of the STnThr antigen analogue 2, featuring the immunogenic TnThr mimetic 1 aglycon. Analogously to the native STn, 2 is recognized by the influenza N1 neuraminidase. A model of the N1·2 complex showed the sialyl moiety of 2 well nested in the active site pocket, with docking unaffected by the rigid aglycon. The analogue 2 is, therefore, in association with mimetic 1, a good determinant for the design of new multiantigen cancer vaccines.

An α2,3-Sialyltransferase from Photobacterium phosphoreum with Broad Substrate Scope: Controlling Hydrolytic Activity by Directed Evolution

Defined sialoglycoconjugates are important molecular probes for studying the role of sialylated glycans in biological systems. We show that the α2,3-sialyltransferase from Photobacterium phosphoreum JT-ISH-467 (2,3SiaTpph ) tolerates a very broad substrate scope for modifications in the sialic acid part, including bulky amide variation, C5/C9 substitution, and C5 stereoinversion. To reduce the enzyme's hydrolytic activity, which erodes the product yield, an extensive structure-guided mutagenesis study identified three variants that show up to five times higher catalytic efficiency for sialyltransfer, up to ten times lower efficiency for substrate hydrolysis, and drastically reduced product hydrolysis. Variant 2,3SiaTpph (A151D) displayed the best performance overall in the synthesis of the GM3 trisaccharide (α2,3-Neu5Ac-Lac) from lactose in a one-pot, two-enzyme cascade. Our study demonstrates that several complementary solutions can be found to suppress the common problem of undesired hydrolysis activity of microbial GT80 sialyltransferases. The new enzymes are powerful catalysts for the synthesis of a wide variety of complex natural and new-to-nature sialoconjugates for biological studies.

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 Highly Efficient Glycosidation of Glycosyl Chlorides by Using Cooperative Silver(I) Oxide-Triflic Acid Catalysis

Following our discovery that silver(I) oxide-promoted glycosylation with glycosyl bromides can be greatly accelerated in the presence of catalytic TMSOTf or TfOH, we report herein a new discovery that glycosyl chlorides are even more effective glycosyl donors under these reaction conditions. The developed reaction conditions work well with a variety of glycosyl chlorides. Both benzoylated and benzylated chlorides have been successfully glycosidated, and these reaction conditions proved to be effective in coupling substrates containing nitrogen and sulfur atoms. Another convenient feature of this glycosylation is that the progress of the reaction can be monitored visually; its completion can be judged by the disappearance of the characteristic dark color of Ag2 O.

Halogen-directed chemical sialylation: pseudo-stereodivergent access to marine ganglioside epitopes

Sialic acids are conspicuous structural components of the complex gangliosides that regulate cellular processes. Their importance in molecular recognition manifests itself in drug design (e.g. Tamiflu®) and continues to stimulate the development of effective chemical sialylation strategies to complement chemoenzymatic technologies. Stereodivergent approaches that enable the α- or β-anomer to be generated at will are particularly powerful to attenuate hydrogen bond networks and interrogate function. Herein, we demonstrate that site-selective halogenation (F and Br) at C3 of the N-glycolyl units common to marine Neu2,6Glu epitopes enables pseudo-stereodivergent sialylation. α-Selective sialylation results from fluorination, whereas traceless bromine-guided sialylation generates the β-adduct. This concept is validated in the synthesis of HLG-1 and Hp-s1 analogues.

Sialic acids are conspicuous structural components of the complex gangliosides that regulate cellular processes.

Advances in molecular mechanisms of drugs affecting abnormal glycosylation and metastasis of breast cancer

Breast cancer remains the leading cause of cancer-related death among women worldwide, and its incidence is also increasing. High recurrence rate and metastasis rate are the key causes of poor prognosis and death. It is suggested that abnormal glycosylation plays an important role in the growth, invasion, metastasis and resistance to therapy of breast cancer cells. Meanwhile, it can be used as the biomarkers for the early detection and prognosis of breast cancer and the potential attractive targets for drug treatment. However, only a few attentions have been paid to the molecular mechanism of abnormal glycosylation in the epithelial-mesenchymal transition (EMT) of breast cancer cells and the related intervention of drugs. This manuscript thus investigated the relationship between abnormal glycosylation, the EMT, and breast cancer metastasis. Then, the process of abnormal glycosylation, the classification and their molecular regulatory mechanisms of breast cancer were analyzed in detail. Last, potential drugs are introduced in different categories, which are expected to reverse or intervene the abnormal glycosylation of breast cancer. This review is conducive to an in-depth understanding of the metastasis and drug resistance of breast cancer cells, which will provide new ideas for the clinical regulation of glycosylation and related drug treatments in breast cancer.

Efficient diversification of GM3 gangliosides via late-stage sialylation and dynamic glycan structural studies with 19F solid-state NMR

GM3 gangliosides have been synthesized via late-stage α-sialylation using a macro-bicyclic sialyl donor. ¹⁹F solid-state NMR analysis of the C5-NHTFAc GM3 analog on a model membrane revealed the influence of cholesterol on glycan dynamics.

SnCl4-catalyzed solvent-free acetolysis of 2,7-anhydrosialic acid derivatives

Sialic acid-containing glycans are found in different sialic acid forms and a variety of glycosidic linkages in biologically active glycoconjugates. Hence, the preparation of suitably protected sialyl building blocks requires high attention in order to access glycans in a pure form. In line with this, various C-5-substituted 2,7-anhydrosialic acid derivatives bearing both electron-donating and -withdrawing protecting groups were synthesized and subjected to different Lewis acid-catalyzed solvent-free ring-opening reactions at room temperature in the presence of acetic anhydride. Among the various Lewis acids tested, the desired acetolysis products were obtained in moderate yields under tin(IV) chloride catalysis. Our methodology could be extended to regioselective protecting group installations and manipulations towards a number of thiosialoside and halide donors.

Synthesis and Immunological Evaluation of Disaccharide Bearing MUC-1 Glycopeptide Conjugates with Virus-like Particles

Mucin-1 (MUC1) is a highly attractive antigenic target for anticancer vaccines. Naturally existing MUC1 can contain multiple types of O-linked glycans, including the Thomsen-Friedenreich (Tf) antigen and the Sialyl Thomsen-nouveau (STn) antigen. In order to target these antigens as potential anticancer vaccines, MUC1 glycopeptides SAPDT*RPAP (T* is the glycosylation site) bearing the Tf and the STn antigen, respectively, have been synthesized. The bacteriophage Qβ carrier is a powerful carrier for antigen delivery. The conjugates of MUC1-Tf and -STn glycopeptides with Qβ were utilized to immunize immune-tolerant human MUC1 transgenic (MUC1.Tg) mice, which elicited superior levels of anti-MUC1 IgG antibodies with titers reaching over 2 million units. The IgG antibodies recognized a wide range of MUC1 glycopeptides bearing diverse glycans. Antibodies induced by Qβ-MUC1-Tf showed strongest binding, with MUC1-expressing melanoma B16-MUC1 cells, and effectively killed these cells in vitro. Vaccination with Qβ-MUC1-Tf first followed by tumor challenge in a lung metastasis model showed significant reductions of the number of tumor foci in the lungs of immunized mice as compared to those in control mice. This was the first time that a MUC1-Tf-based vaccine has shown in vivo efficacy in a tumor model. As such, Qβ-MUC1 glycopeptide conjugates have great potential as anticancer vaccines.

Constrained sialic acid donors enable selective synthesis of α-glycosides

Sialic acid is a sugar residue present in many biologically significant glycans of mammals, commonly as a terminal α-glycoside. The chemical structure of sialic acid, which features an anomeric center with carboxyl and methylene substituents, poses a challenge for synthesis of the α-glycoside, thus impeding biological and therapeutic studies on sialic acid–containing glycans. We present a robust method for the selective α-glycosidation of sialic acid using macrobicyclized sialic acid donors as synthetic equivalents of structurally constrained oxocarbenium ions to impart stereoselectivity. We demonstrate the power of our method by showcasing broad substrate scope and applicability in the preparation of diverse sialic acid–containing architectures.

Surface display of sialyltransferase on the outer membrane of Escherichia coli and ClearColi

α2,3-Sialyltransferase from Pasteurella multocida (PmST1) is an enzyme that transfers a sialyl group of donor substrates to an acceptor substrate called N-acetyl-d-lactosamine (LacNAc). In this study PmST1 was expressed on the outer membrane of wildtype Escherichia coli (BL21) with lipopolysaccharide (LPS) and ClearColi with no LPS, and then the enzyme activity and expression level of PmST1 were compared. As the first step, the expression levels of PmST1 on the outer membranes of wildtype E. coli (BL21) and ClearColi were compared according to the IPTG induction time, and the absolute amount of surface-displayed PmST1 was calculated using densitometry of SDS-PAGE. As the next step, the influence of LPS on the PmST1 activity was estimated by analyzing Michaelis-Menten plot. The enzyme activity of PmST1 was analyzed by measuring the concentration of CMP, which was a by-product after the transfer of the sialyl group of donor compounds to the acceptor compounds. From a Michaelis-Menten plot, the enzyme activity of the surface-displayed PmST1 and the maximum rate (V_max) of ClearColi were higher than those of wildtype E. coli (BL21). However, the K_M value, which represented the concentration of substrate to reach half the maximum rate (V_max), was similar for both enzymes. These results represented such a difference in enzyme activity was occurred from the interference of LPS on the mass transport of the donor and acceptor to PmST1 for the sialyl group transfer.

Lactonization Method To Assign the Anomeric Configuration of the 3,4-Unsaturated Congeners of N-Acetylneuraminic Acid

Assigning the correct configuration at C2 in sialosides is a standing problem because of the absence of an anomeric hydrogen. All different empirical rules that have been proposed over the years lack general applicability. In particular, the correct configuration of several 3,4-unsaturated derivatives of N-acetylneuraminic acid (Neu5Ac), which have been recently shown to be novel sialidase/neuraminidase inhibitors, could only be tentatively assigned by similarity with the reported 3,4-unsaturated 2O-methyl sialosides. In this work, we overcome this problem as we devised a rapid synthetic method to unequivocally resolve the anomeric configuration of the 3,4-unsaturated Neu5Ac derivatives through the synthesis of the corresponding unreported unsaturated 1,7-lactones. Moreover, we discovered a diagnostic ¹³C nuclear magnetic resonance signal that allows the formulation of a new empirical rule for the direct assignment of the C2 stereochemistry of these molecules, even when only one of the two C2 epimers is available.

Regioselective One-Pot Multienzyme (OPME) Chemoenzymatic Strategies for Systematic Synthesis of Sialyl Core 2 Glycans

O-GalNAc glycans or mucin-type glycans are common protein post-translational modifications in eukaryotes. Core 2 O-GalNAc glycans are branched structures that are broadly distributed in glycoproteins and mucins of all types of cells. To better understand their biological roles, it is important to obtain structurally defined Core 2 O-GalNAc glycans. We present here regioselective one-pot multienzyme (OPME) chemoenzymatic strategies to systematically access a diverse array of sialyl Core 2 glycans. Regioselectivity can be achieved by using OPME systems containing a glycosyltransferase with restricted acceptor specificity or by differentiating the branches using altered glycosylation sequences. This work provides a general regioselective strategy to access diverse Core 2 O-GalNAc glycans which can be extended for the synthesis of other complex branched glycans.

Gold(i)-promoted α-selective sialylation of glycosylortho-hexynylbenzoates for the latent-active synthesis of oligosialic acids

A gold(i)-promoted α-selective glycosylation approach with sialylortho-hexynylbenzoates as donors is developed for the latent-active synthesis of α-(2 → 9)-linked oligosialic acids.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.