Glycan Core Modifications

The core structure of X-glycans is invariably the trimannosyl chitobiose unit, Man3GlcNAc2, which can be a6-fucosylated at the reducing end GlcNAc. In plants, this core fiicosylation is a3 where it occurs, with and without additional / 2-xylosylation at the /3-mannose. In insects, either or both types of core fiicosylation can occur but not /32-xylosylation, whereas in some other lower animals that have been looked at, including mollusks and nematodes, instances of all these core modifications could be found. 

Fucosylation of the Asn-linked GlcNAc is a common modification unique to A-linked glycans. In vertebrates, core FucT adds fucose in an al-6 linkage (59), whereas in plants and invertebrates al-3-linked fucose is observed (60). 

Biosynthesis of GPI anchors starts with the core structure assembly by sequential addition of UDP-GlcNAc (followed by iV-deacetylation), dolichol-phosphate-mannose, and phospho-ethanolamine to phosphatidylinositol and culminates in the en bloc transfer to protein shortly after the protein is synthesized. However, the biosynthetic pathways can differ strikingly between different organisms with respect to specific modifications and fatty acid remodeling occurring after completion of the core glycan. This also applies for the point when certain modifications are introduced, e. g. before or after the transfer of the GPI-moiety to the protein. GPI anchors can be cleaved at defined positions by an array of enzymatic and chemical methods, respectively (O Fig. 5). Thus, it becomes possible to identify GPI-anchored proteins and, moreover, analyze the structure and biosynthesis of GPI anchors [103]. 

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