Glycosylation Precursors

Excerpt

In nature, most glycans are synthesized by glycosyltransferases, enzymes that transfer activated forms of monosaccharides from nucleotide sugars and lipid-linked sugar intermediates to acceptors, including proteins, lipids, and growing glycan chains. Monosaccharide precursors are imported into the cell, salvaged from degraded glycans, or created enzymatically from other sugars within the cell. In eukaryotic cells, glycosylation occurs mostly in the endoplasmic reticulum (ER) and Golgi apparatus, whereas monosaccharide activation and interconversions occur mostly in the cytoplasm. Nucleotide sugar–specific transporters carry activated sugar donors into the Golgi and to a lesser extent into the ER. In some cases, nucleotide sugars are used to synthesize activated lipid-linked intermediates before glycan transfer. This chapter describes how cells accomplish these tasks, with an emphasis on animal cells.