Synthesis of N-linked oligosaccharides:

In contrast to O-linked oligosaccharides that are established up sequentially on the protein, N-linked oligosaccharides are synthesized as a huge, branched precursor structure which is then added en bloc to the acceptor Asn residue. The oligosaccharide is built on a lipid carrier known as dolichol phosphate. This consists of 22 isoprene C5 units with a terminal phosphate set and it is anchored to the RER membrane.

Synthesis of the oligosaccharide begins through the dolichol phosphate accepting monosaccharides from the cytosolic face of the RER membrane but when the (Man)5(GlcNac)2-dolichol phosphate intermediate has established, this flips orientation and now accepts further monosaccharides from the lumenal side of the RER membrane. All of these subsequent transfers are from dolichol phosphate-linked monosaccharides which are made on the cytoplasmic side of the RER membrane then likewise flipped across to act as donors. The last oligosaccharide, with the composition (Glc) 3(Man) 9(GlcNAc) 2 is associated to the dolichol by a high-energy pyrophosphate bond. This gives the energy for transfer of the oligosaccharide to the protein, a reaction catalyzed through a membrane-bound oligosaccharide transferase enzyme and that occurs in the RER.

Whilst the protein is still in the RER and the three glucose residues are erased that is shown in the figure. Interestingly, glucose residues are added back to the protein if it

Figure: Synthesis of O-linked oligosaccharide. The example shown is an O-linked oligosaccha- ride in human immunoglobulin A (IgA).

Figure: Synthesis of N-linked oligosaccharides on a dolichol phosphate carrier in the RER membrane.

Figure: Transfer of the N-linked oligosaccharide from the dolichol phosphate carrier to an acceptor Asn residue in a growing polypeptide chain by oligosaccharide transferase. The terminal three glucose residues and six mannose residues that are removed during trimming of the oligosaccharide are indicated.

 is wrongly folded or unfolded. Therefore, only when the protein is rightly folded are all glucose residues finally erased and the protein should continue along the secretory way. The folded glycoprotein with its high-mannose kind oligosaccharides is now transported to the Golgi complex through vesicles. As it moves by the Golgi complex, the oligosaccharide  is frequent 'trimmed' or 'processed' with the terminal six mannose residues being erased to leave the pentasaccharide  core. Further mannose residues and other monosaccharides may then be added to the oligosaccharide in the Golgi to produce the complex kind of oligosaccharide.

Figure:  (a) Transfer of the N-linked oligosaccharide to a protein and its further processing in the RER and Golgi (see text for details). (b) Structure of the pentasaccharide core of N-linked oligosaccharides.