In recent months, Europe has witnessed a resurgence of highly pathogenic H5N1 avian influenza in both wild and domestic bird populations. These outbreaks raise concerns about potential transmission to humans, particularly if the virus adapts to recognize human-type receptors.
Terminal sialic acid plays a pivotal role in determining how avian influenza viruses, particularly the highly pathogenic H5N1 strain, initiate infection in their hosts. Understanding this receptor-virus interaction is essential not only for comprehending viral pathogenesis but also for developing effective surveillance strategies and therapeutic interventions in the context of emerging pandemic threats.
The specificity of avian influenza viruses for α2,3-linked sialic acid receptors, rather than the α2,6-linked variants found in human airways, represents one of the most important biological barriers protecting humans from widespread avian flu transmission. However, ongoing viral evolution and adaptation in novel mammalian hosts threaten to compromise this natural defense mechanism.
The Molecular Mechanism: How Terminal Sialic Acid Powers Viral Entry
Understanding Alpha 2,3 and Alpha 2,6 Sialic Acid Linkages
Terminal sialic acids exist in two primary structural configurations based on how they are glycosidically linked to galactose molecules. These chemical linkages fundamentally determine which viruses can recognize and bind to them.
Alpha 2,3-linked sialic acid (SAα2,3Gal) represents the predominant sialic acid form on epithelial cells in the avian respiratory tract and gastrointestinal tract. Sialic acids in avian hosts, particularly in ducks and other waterfowl, are linked to galactose through an α-2,3 glycosidic bond catalyzed by α-2,3 sialyltransferase (ST3Gal). Avian influenza viruses possess hemagglutinin (HA) proteins specifically adapted to recognize and bind this receptor configuration with high affinity.
Alpha 2,6-linked sialic acid (SAα2,6Gal), by contrast, predominates in the human upper respiratory tract, particularly in the nasal passages and pharynx. This receptor type is catalyzed by α-2,6 sialyltransferase (ST6Gal). Human-adapted influenza viruses have evolved to recognize this receptor variant exclusively or preferentially, explaining their natural tropism for human airway epithelium.
Figure 1. Comparison of avian (blue) versus human (red) sialic acid receptors.
The hemagglutinin (HA) protein serves as the primary attachment molecule for influenza viruses. The receptor-binding site (RBS) located at the distal end of the HA protein determines which terminal sialic acid configurations a particular virus strain can recognize. This binding specificity is encoded by specific amino acid sequences within the HA protein structure, particularly within the receptor binding domain.
Recent H5N1 Variants and Receptor Specificity
In March 2024, the first cases of transmission of H5N1 avian influenza to U.S. dairy cattle were detected, raising the alarm of possible zoonotic pandemic. H5N1 replicated efficiently in the mammary gland, which expresses both α2,3- and α2,6-linked sialic acids. However, haemagglutinin of a the new H5N1 virus strain exhibited weak binding to glycans terminating in α2,6-linked sialic acids. This observation was significant because α2,6-linked sialic acid is prevalent in the human upper respiratory tract, and the acquisition of α2,6 sialic acid receptor specificity is considered essential for efficient human-to-human transmission of influenza viruses. However, point mutations in haemagglutinin can enable cross-recognition of α2,6-linked sialic acids, thereby increasing the likelihood of human infection.
Continued surveillance of H5N1 in cattle and other species is essential to monitor the virus’s potential to acquire human-type receptor specificity, which could elevate the risk of its emergence as a pandemic strain.
Diagnostic Applications: Sialic Acid as a Biomarker for Pandemic Risk
Glycan Microarray Technology for Viral Characterization
Understanding terminal sialic acid specificity of viral proteins has enabled development of sophisticated diagnostic tools for pandemic surveillance. Glycan microarrays containing diverse sialic acid-terminated glycan structures allow researchers to characterize the receptor-binding specificity of newly emerged viral isolates with unprecedented comprehensiveness.
Figure 2 . Example of glycan array
Glycan microarray analysis has become a standard component of pandemic risk assessment for novel influenza viruses. Viruses demonstrating preferential or new binding to SAα2,6Gal receive heightened surveillance attention and investigation for potential human-to-human transmission capability.
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Therapeutic and Vaccine Development
Neuraminidase Inhibitors: Targeting Sialic Acid Cleavage
Antiviral medications including oseltamivir (Tamiflu) and zanamivir (Relenza), which are sialic acid mimetics, function by inhibiting the viral neuraminidase enzyme, the sialic acid-cleaving protein. By preventing terminal sialic acid cleavage, these drugs block viral release from infected cells and hinder spread to neighboring tissues.
The α2,3-specific neuraminidase activity of H5N1 viruses creates both advantage and vulnerability. While this specificity enhances the virus’s ability to penetrate human mucus layers rich in α2,3-linked sialic acids, it also creates dependence on α2,3 cleavage for efficient replication. Neuraminidase inhibitors therefore represent particularly valuable therapeutic options for H5N1 infection.
Sialic Acid Decoy Therapeutics
Recombinant soluble sialic acid-terminated glycoproteins or engineered glycan structures can function as viral “decoys,” competitively inhibiting virus-cell interactions. By presenting high-density terminal sialic acids in solution, these decoys sequester virions away from cellular targets. While not yet widely deployed clinically, such approaches represent promising therapeutic avenues for severe H5N1 infection, potentially reducing viral burden before adaptive immune mechanisms become effective.
Monoclonal Antibodies Targeting Receptor-Binding Sites
Understanding how terminal sialic acids interact with HA receptor-binding sites has enabled design of highly specific monoclonal antibodies. Some antibodies target epitopes that overlap with the sialic acid-binding pocket, sterically preventing viral attachment. Others recognize conformational changes in HA induced by sialic acid binding, targeting intermediate states during viral entry.
Asparia Glycomics has extensive expertise in synthesizing sialic acid derivatives for antiviral applications. Contact us to explore collaboration opportunities, custom syntheses, or tailored solutions for your research and development needs.
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Identifying α2,6 and α2,3 Sialic Acid-Containing Glycans
Distinguishing α2,6 and α2,3 sialic acid linkage isomers presents significant analytical challenges, primarily due to their structural similarity and mass equivalence. Both linkage types result in identical molecular weights, making conventional mass spectrometry unable to differentiate them directly. Additionally, sialic acids are inherently unstable, featuring labile glycosidic bonds that readily fragment during ionization and mass spectrometric analysis, leading to loss of sialic acid residues and inaccurate structural characterization.
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Conclusion
Terminal Sialic Acid as the Molecular Gatekeeper of Influenza Pandemic Risk
Terminal sialic acid, specifically the distinction between α2,3 and α2,6 glycosidic linkages, along with increasingly recognized complexity in glycan structure, length, and modifications, represents the molecular foundation of influenza host specificity. The remarkable specificity of avian influenza viruses for α2,3-linked sialic acids has historically protected humans from widespread pandemic transmission despite decades of frequent viral spillover events.
However, this natural barrier remains impermanent. The documented capacity of H5N1 viruses to spontaneously acquire human-type receptor specificity, combined with accelerating viral circulation in novel mammalian hosts like dairy cattle, necessitates continuous surveillance and scientific attention to terminal sialic acid binding properties.
Understanding how terminal sialic acids control influenza virus tropism and pandemic potential remains among the most critical bridges between basic molecular biology and global health security. As avian influenza viruses continue evolving in an increasingly complex landscape of avian, mammalian, and human hosts, this knowledge only grows in strategic importance. The next major influenza pandemic may well be determined by changes in how viral hemagglutinin recognizes these nine-carbon sugar molecules.
References
Heida, R.; Bhide, Y. C.; Gasbarri, M.; Kocabiyik, Ö.; Stellacci, F.; Huckriede, A. L. W.; Hinrichs, W. L. J.; Frijlink, H. W. Advances in the Development of Entry Inhibitors for Sialic-Acid-Targeting Viruses. Drug Discov. Today 2021, 26 (1), 122–137. https://doi.org/10.1016/j.drudis.2020.10.009
Santos, J. J. S.; Wang, S.; McBride, R.; Adams, L.; Harvey, R.; Zhao, Y.; Wrobel, A. G.; Gamblin, S.; Skehel, J.; Lewis, N. S.; Paulson, J. C.; Hensley, S. E. Bovine H5N1 Binds Poorly to Human-Type Sialic Acid Receptors. Nature 2025, 640 (8059), E18–E20. https://doi.org/10.1038/s41586-025-08821-6.
Fabrizio, T. P.; Kandeil, A.; Harrington, W. N.; Jones, J. C.; Jeevan, T.; Andreev, K.; Seiler, P.; Fogo, J.; Davis, M. L.; Crumpton, J. C.; Franks, J.; DeBeauchamp, J.; Vogel, P.; Daniels, C. S.; Poulson, R. L.; Bowman, A. S.; Govorkova, E. A.; Webby, R. J. Genotype B3.13 Influenza A(H5N1) Viruses Isolated from Dairy Cattle Demonstrate High Virulence in Laboratory Models, but Retain Avian Virus-like Properties. Nat. Commun. 2025, 16 (1), 6771. https://doi.org/10.1038/s41467-025-61757-3.