N-Glycosidically Linked Oligosaccharide Chains

The use of [l-i CJglucosamine as a precursor of glycoprotein-bound sialic acid has been demonstrated in liver (e.g. Kohn et al. 1962, Lawford and Schachter 1966). Radioactivity was incorporated into microsomal glycoprotein (SARCiONEcr al. 1964), and the sialic acid label could be identified in Golgi fractions (Schachter 1974). However, glucosamine is converted into sialic acid and incorporated into 

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The presence of both O- and N-glycosidically linked oligosaccharides containing sialic acid was shown in proteoglycans from swarm rat chondrosarcoma and chick limb bud chondrocyte cultures by Lohmander et al. (1980) and De Luca et al. (1980). The 0-glycosidically linked chains were all related to the sialylated T-antigen structure (Lohmander et al. 1980). 

As an example of the carbohydrate component of a glycoprotein, the structure of one of the oligosaccharide chains of immunoglobulin G (IgG see p.300) is shown here. The oligosaccharide has an N-glycosidic link to the amide group of an asparagine residue in the Fc part of the protein. Its function is not known. 

N-linked oligosaccharide chain. An N-glycosidic bond to the side-chain... 

N-linked oligosaccharides are linked to the protein via N-glycosidic bonds, to the NH2 groups of asparagine side chains (Fig. lb) where the asparagine occurs in the sequence Asn-X-Ser (or Thr) where X is any amino acid except Pro and possibly Asp. 

Structural studies of the carbohydrate residues of e-galacto-sidase and related inhibitor glycoproteins indicate that the oligosaccharide chains contain glycosidically-bound mannose and N-acetylglucosamine residues linked in the manner shown below ... 

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