ABSTRACT
Many proteins synthesized by the ribosomes of the rough endoplasmic reticulum (RER) contain short chains of carbohydrates and are called glycoproteins. The oligosaccharides are of two main types; O-linked and N-linked. O-linked oligosaccharides are commonly attached to the protein via O-glycosidic bonds to OH groups of Ser or Thr side chains. N-linked oligosaccharides are linked to the protein via N-glycosidic bonds, to the NH2 groups of Asn side chains where the Asn occurs in the sequence Asn-X-Ser where X is any amino acid except Pro. The synthesis of O-linked oligosaccharides occurs by the sequential addition of monosaccharide units to the newly synthesized protein as it passes through the Golgi complex. In contrast to O-linked oligosaccharides which are built up sequentially on the protein, N-linked oligosaccharides are synthesized as a large, branched precursor structure that is then added en bloc to the acceptor Asn residue.
