Figure 1

Natural diversity in Sia presentation, structure and function. (a) Diversity in presentation. Sias are typically found at the terminal position of N- and O-linked glycans attached to the cell surface and to secreted glycoproteins, as well as on glycosphingolipids expressed at the cell surface. Ac, O-acetyl ester; Fuc, fucose; Gal, galactose; GalNAc, N-acetylgalactosamine; Glc, glucose; GlcNAc, N-acetylglucosamine; Man, mannose; S, sulfate ester. Reproduced with permission from Varki.¹¹ (b) Structural diversity. All Sias share the common feature of having nine carbons, a carboxylic acid residue at the 1-position, and a variety of linkages to the underlying sugar chain from the 2-position. Various types of substitutions at the 4, 5, 7, 8 and 9 positions combine with the linkage variation to generate the diversity of Sias found in nature. Only a portion of this diversity is represented here. Some examples of the binding specificity of probes for detection of Sia types and linkages are discussed in the Box 1. (c) Examples of pathobiological interactions involving Sias. This cartoon shows examples of the types of interactions involving recognition of Sia diversity. The molecules and cells are obviously not drawn to scale.