Glycoproteins are estimated to comprise 50% of eukaryotic proteomes. In contrast to DNA, RNA and proteins, the biosynthesis of complex carbohydrates is not driven by a “template”: the repertoire of glycan structures in a given cell type is primarily regulated by the activity of multiple and competitive enzymatic pathways. The resulting modifications are extremely diverse in structure: glycans have complex, branched structures and are intrinsically heterogeneous. Many glycoproteins are localized on the cell surface or secreted to the extracellular matrix, where they mediate molecular interactions critical to cell growth, inflammation, immune defense, fertilization and parasitic infection. These molecules reach their destinations by organized intracellular trafficking; it is during trafficking that they are modified by glycosylation
