Sialosides Containing 7-N-Acetyl Sialic Acid Are Selective Substrates for Neuraminidases from Influenza A Viruses

[Image: see text] Sialidases or neuraminidases are sialic-acid-cleaving enzymes that are expressed by a broad spectrum of organisms, including pathogens. In nature, sialic acids are monosaccharides with diverse structural variations, but the lack of novel probes has made it difficult to determine how sialic acid modifications impact the recognition by sialidases. Here, we used a chemoenzymatic synthon strategy to generate a set of α2–3- and α2–6-linked sialoside probes that contain 7-N-acetyl or 7,9-di-N-acetyl sialic acid as structure mimics for those containing the less stable naturally occurring 7-O-acetyl- or 7,9-di-O-acetyl modifications. These probes were used to compare the substrate specificity of several sialidases from different origins. Our results show that 7-N-acetyl sialic acid was readily cleaved by neuraminidases from H1N1 and H3N2 influenza A viruses, but not by sialidases of human or bacterial origin, thereby indicating that the influenza enzymes possess a distinctive and more promiscuous substrate binding pocket.