Sialic Acids and Other Nonulosonic Acids

Sialic acids are abundant on vertebrate glycoproteins and have diverse functional roles. Originally discovered within the Deuterostome lineage of animals and associated microbes, they are actually a subset of a more ancient family of 9-carbon backbone monosaccharides called nonulosonic acids, which are also found in some Eubacteria and Archaea. All nonulosonic acids share unusual biosynthetic pathways and are remarkable for the number of molecular components carried on one monosaccharide, including a carboxylic acid, a 3-carbon exocyclic side chain, and often one or more acylated amino groups. Further complexity arising from various modifications, as well as diverse linkages at terminal positions of glycans, makes them well suited to carry information for glycan–protein, cell–cell, and pathogen–cell recognition. Given their high density and location on vertebrate cells, sialic acids also exert many functions via electronegative charge, such as repulsion of cell–cell interactions, protein stabilization, ion binding, and ion transport. They are among the most rapidly evolving classes of glycans in nature.