a) Glycan biosynthesis [1]
　Glycans are synthesized by enzymes called glycosyltransferases which transfer a sugar residue from an activated nucleotide sugar donor to specific acceptor molecules, 　　forming glycosidic bonds (Breton et al.(2006)). Transfer of the sugar residue occurs with either the retention or the inversion of the configuration of the anomeric carbon (McNaught and Wilkinson (1997)). These enzymes can be found in both prokaryotes and eukaryotes, having high specificity for both the glycosyl donor as well as the acceptor substates. In this section, the major biosynthesis pathways of glycans will be introduced.
　Glycosylation produces different types of glycans that are typically attached to proteins or lipids. Protein glycosylation includes N-glycans, O-glycans and glycosaminoglycans (GAGs). Lipid glycosylation includes glycolipids (glycosphingolipids) and glycosylphosphatidylinositol (GPI)-anchors. In mammalian systems, these glycans are constructed in an ordered manner through the workings of glycosyltransferases and also glycosidases, which are enzymes that remove specific glycosidic linkages from a glycan structure. 　　 　　 　　 　　 　





























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