| O-linked oligosaccharides
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| The biosynthesis of O-linked oligosaccharides occurs in the Golgi apparatus by the stepwise addition of sugars from their sugar nucleotide donors. No lipid intermediates are involved. Figure 25.12 describes the straightforward sequence of reactions for assembly of the oligosaccharide chains of salivary mucins. GalNAc is first transferred from UDP-GalNAc to serine or threonine residues on the protein by a GalNAc transferase in the Golgi apparatus. The GalNAc-serine-(protein) serves as the acceptor for galactose and then sialic acid, transferred from their sugar nucleotides by Golgi galactosyl and sialyl transferases, to form the final trisaccharide sequence on the mucin. Other Golgi glycosyl transferases are involved in the stepwise biosynthesis of oligosaccharides on proteoglycans and collagen. There are more than 100 glycosyl transferases involved in glycoconjugate biosynthesis in a typical cell.
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