Most proteins made by ribosomes on the RER (rough endoplasmic reticulum) are glycoproteins, which is they hold short chains of carbohydrates (oligosaccharides) covalently related to them during passage by the Golgi and RER complex. Two major types of oligosaccharide linkage exist:
? O-linked oligosaccharides are commonly attached to the protein through O-glycosidic bonds to OH groups of Thr or Ser side chains. The residues which are changed are frequently in a region of the polypeptide chain which is rich in Thr and Ser residues. In plant glycoproteins, the OH sets of hydroxyproline (Hyp) residues can be glycosylated, by the hydroxylysine (Hyl) reduce in the mammalian protein collagen can be O-glycosylated.
? N-linked oligosaccharides are related to the protein via N-glycosidic will bonds to the NH2 where the Asn occurs in the sequence Asn-X-Ser (or Thr) where X is many amino acids except Pro. If the Asn is N-glycosylated the Thr or Ser residue in this motif is not O-glycosylated.