Classes of glycosylation

Classes of glycosylation are defined according to their respective invariant core structures and linkage to carrier molecules, which are amino acids and lipidic compounds. These invariant cores are further elongated, with the exception of C-linked Man and O-linked GlcNAc. The diversity of glycosylation classes encountered in prokaryotes is immense and probably only partially documented. Even in eukaryotes, some classes of glycosylation have only been discovered very recently and further classes likely remain to be described. In any case, the claim that 13 classes of glycosylation are found in vertebrates is certainly very close to the actual count. These 13 classes represent 7 major groups, featuring: N-linked glycans, O-linked glycans, C-linked mannose, glycosaminoglycans, the glycosyl-phosphatidylinositol anchor, glycosphingolipids, and cytoplasmic/nuclear O-GlcNAc.

Figure 13. Classes of glycosylation in vertebrates. The invariant core structures are shown for each type of glycoconjugates.

In addition to the major classes mentioned here, further core structures are encountered in specific groups of organisms. For example, lipopolysaccharide, peptidoglycan, lipoarabinomannan are glycoconjugates encountered in bacteria, and Basidiolipids are glycolipids found in higher mushrooms. Some other glycans are not conjugated to lipids or proteins and therefore are referred to as oligosaccharides and polysaccharides. The first group include structures like colanic acid in Enterobacteriaceae and human milk oligosaccharides. Typical polysaccharides include hyaluronan, pectin, hemicellulose, and cellulose.