Synthesis of O-linked oligosaccharides:

The synthesis of O-linked oligosaccharides occurs through the sequential addition of monosaccharide units to the newly synthesized protein as it passes by the Golgi complex. N-acetylgalactosamine (GalNAc)   is transferred   to the

Figure: Structures of oligosaccharide  linkages. (a) O-linked glycosidic bond between GalNAc and Ser (Thr) residue. (b) N-linked glycosidic bond between GlcNAc and an Asn residue.

relevant Ser or Thr residue of the protein through GalNAc transferase an enzyme which uses UDP-GalNAc  as the precursor described in above figure. Other monosaccharides [galactose, N-acetylglucosamine (GlcNAc), sialic acid, fucose] are then added using the corresponding sugar nucleotides as precursors.  The precise kind and number (commonly only 4-5) of monosaccharides added depends on the protein which is being modified.