FIGURE 3.

Hierarchical levels of sialome complexity. The sialome can be analyzed at the following complexity levels. A: sialic acid core and core modifications: esterification (with various groups), O-methylation, lactonization, or lactamization yielding >50 different structures. B: linkage to the underlying sugar (four major and many minor linkages). C: identity and arrangement of the underlying sugars that can also be further modified by fucosylation or sulfation. D: glycan class (N-linked, O-linked, or glycosphingolipids). E: spatial organization of the Sia in sialylated microdomains, which have been referred to as “clustered saccharide patches” (509) or “the glycosynapse” (182). Gal, galactose (Gal), GalNAc, N-acetylgalactosamine; Glc, glucose; GlcNAc, N-acetylglucosamine; Man, mannose; Sia, sialic acid; Fuc, fucose; Asn, asparagine; Ser, serine; Thr, threonine. [Adapted from Cohen and Varki (86), with permission from Mary Ann Liebert, Inc.]