Synthetic O-Acetylated Sialosides and their Acetamido-deoxy Analogues as Probes for Coronaviral Hemagglutinin-esterase Recognition

Synthesis of 4-azido sialic acid for testing against Siglec-7 and in metabolic oligosaccharide engineering

An important approach for tracking and visualizing sialic acid-containing glycans involves using sialic acid reporters functionalized with bioorthogonal handles. More specifically, metabolic oligosaccharide engineering (MOE) commonly employs monosaccharides with an alkyne or azide handle for incorporation into cellular glycans, followed by a subsequent click reaction to elaborate with a biotin or fluorophore handle. For sialic acid, this has been carried out extensively, with an azide or alkyne appended to the C5 N-acetamido group being the most common location for the handle. However, circumstances may require the handle to be at different positions and, to date, the C7 and C9 positions have been shown to work to varying degrees. Herein, we synthesized protected 4AzNeu5Ac that could be incorporated into cellular glycans nearly as efficiently as Neu5Az and targeted with DBCO-biotin through strain promoted azide-alkyne cycloaddition. Owing to the good incorporation of 4AzNeu5Ac into cellular glycans, we followed up this ability by first synthesizing the deprotected form of 4AzNeu5Ac, using a thioglycoside to lock the anomeric center during deprotection of the acetyl groups. Activation of 4AzNeu5Ac to CMP-4AzNeu5Ac then enabled the use of this donor by human sialyltransferase ST3GAL1 to transfer CMP-4AzNeu5Ac to β-Galp-(1→3)-α-GalpNAc. With purified α-4AzNeup5Ac-(2→3)-β-Galp-(1→3)-α-GalpNAc in hand, we tested it as a ligand for Siglec-7 and found that the C4-Az modification is tolerated, opening future possibilities to exploit this position to generate high affinity and selective ligands. These findings expand the repertoire of metabolic oligosaccharide engineering agents and show that azide modifications are tolerated at the C4 position of sialic acid.

Catch, Cut, or Block? Versatile 4-N-Derivatized Sialyl Glycosides for Influenza Virus Neuraminidase Detection and Purification

Seasonal influenza continues to threaten human lives and pose a significant burden to healthcare systems and the economy, emphasizing the need for developing better influenza vaccines, diagnostics, and antiviral therapeutics. To address these challenges, we generated a library of structurally diverse sialyl glycosides containing 4-N-derivatized sialic acids by a highly efficient one-pot two-enzyme chemoenzymatic sialylation strategy. Sialosides containing 4-azido-substituted sialic acid were selectively cleaved by sialidases from influenza viruses, whereas sialosides containing 4-acetamido-modified sialic acid were resistant to sialidase cleavage. Interestingly, sialosides containing 4-amino- or 4-guanidino-substituted sialic acid were effective inhibitors moderately or highly resistant to cleavage by influenza sialidases (also called neuraminidases). The sialosides containing the 4-guanidino-substituted sialic acid represent a new class of sialidase substrate analog-based inhibitors. We took advantage of this unique property to create a ligand-based approach for efficiently isolating influenza virions from egg allantoic fluid with high purity. Together, these compounds are versatile probes and ligands for developing new approaches to detect, profile, isolate, and characterize influenza viruses via neuraminidases on their surface.

NMR investigations of glycan conformation, dynamics, and interactions

Glycans are ubiquitous in nature, decorating our cells and serving as the initial points of contact with any visiting entities. These glycan interactions are fundamental to host-pathogen recognition and are related to various diseases, including inflammation and cancer. Therefore, understanding the conformations and dynamics of glycans, as well as the key features that regulate their interactions with proteins, is crucial for designing new therapeutics. Due to the intrinsic flexibility of glycans, NMR is an essential tool for unravelling these properties. In this review, we describe the key NMR parameters that can be extracted from the different experiments, and which allow us to deduce the necessary geometry and molecular motion information, with a special emphasis on assessing the internal motions of the glycosidic linkages. We specifically address the NMR peculiarities of various natural glycans, from histo-blood group antigens to glycosaminoglycans, and also consider the special characteristics of their synthetic analogues (glycomimetics). Finally, we discuss the application of NMR protocols to study glycan-related molecular recognition events, both from the carbohydrate and receptor perspectives, including the use of stable isotopes and paramagnetic NMR methods to overcome the inherent degeneracy of glycan chemical shifts.

Biocatalysis versus Molecular Recognition in Sialoside-Selective Neuraminidase Biosensing

Sialic acid recognition and hydrolysis are essential parts of cellular function and pathogen infectivity. Neuraminidases are enzymes that detach sialic acid from sialosides, and their inhibition is a prime target for viral infection treatment. The connectivity and type of sialic acid influence the recognition and hydrolysis activity of the many different neuraminidases. The common strategies to evaluate neuraminidase activity, recognition, and inhibition rely on extensive labeling and require a large amount of sialylated glycans. The above limitations make the effort of finding viral inhibitors extremely difficult. We used synthetic sialylated glycans and developed a label-free electrochemical method to show that sialoside structural features lead to selective neuraminidase biosensing. We compared Neu5Ac to Neu5Gc sialosides to evaluate the organism-dependent neuraminidase selectivity-sensitivity relationship. We demonstrated that the type of surface and the glycan monolayer density direct the response to either binding or enzymatic activity. We proved that while the hydrophobic glassy carbon surface increases the interaction with the enzyme hydrophobic interface, the negatively charged interface of the lipoic acid monolayer on gold repels the protein and enables biocatalysis. We showed that the sialoside monolayers can serve as tools to evaluate the inhibition of neuraminidases both by biocatalysis and molecular recognition.

Synthetic O-Acetyl-N-glycolylneuraminic Acid Oligosaccharides Reveal Host-Associated Binding Patterns of Coronaviral Glycoproteins

A panel of O-acetylated N-glycolylneuraminic acid oligosaccharides has been prepared by diversification of common synthetic precursors by regioselective de-O-acetylation by coronaviral hemagglutinin-esterase (HE) combined with C7-to-C9 acetyl ester migration. The resulting compound library was printed on streptavidin-coated glass slides to give a microarray to investigate receptor binding specificities of viral envelope glycoproteins, including spike proteins and HEs from animal and human coronaviruses. It was found that the binding patterns of the viral proteins for N-glycolylated sialosides differ considerable from those of the previously synthesized N-acetylated counterparts. Generally, the spike proteins tolerate N-glycolyl modification, but selectivities differ among viruses targeting different hosts. On the other hand, the lectin domain of the corresponding HEs showed a substantial decrease or loss of binding of N-glycolylated sialosides. MD simulations indicate that glycolyl recognition by HE is mediated by polar residues in a loop region (109-119) that interacts with the 5-N-glycolyl moiety. Collectively, the results indicate that coronaviruses have adjusted their receptor fine specificities to adapt to the sialoglycome of their host species.