Biosynthesis of O-linked glycoproteins

This process is the step-wise addition of the sugar residues directly onto the polypeptide chain. It occurs in the Golgi.

1. At first the N-Acetylgalactosamine residue is transferred from UDP-GalNAc to the hydroxyl group of Ser or Thr residue. It is catalyzed by N-Acetylgalactosaminyltransferase.
   UDP-GalNAc is synthesized from UDP-glucose.
   
2. Then the protein moves to the trans-Golgi vesicles where the carbohydrate chain is elongated. The specific glycosyltransferase adds the galactose residue to the GalNAc.
3. The last steps in biosynthesis of typical O-glycans are the additions of two N-Acetylneuramic acid (sialic acid) residues in the trans-Golgi reticulum.

Biosynthesis of glycolipids also occurs in the Golgi vesicles.This process utilize the same glycosyltransferases (for example several oligosaccharides like ABO blood group antigens are found attached to both glycoproteins and glycolipids).

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Biosynthesis of N-linked oligosaccharides

Biosynthesis of GPI membrane anchors

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