Mundo De La Celulas Cap

Lecture 11: Glycosylation

Roles of the ER and Golgi Complex in Protein Glycosylation:
Glycosylation is the addition of carbohydrate side chains to specific amino acid residues of proteins,forming glycoproteins.
There are two kinds of glycosylation in cells:
- N-linked glycosylation which involves the addition of a specific oligosaccharide unit to the nitrogen atom on the terminal aminogroup of certain asparagine residues.
- O-linked glycosylation which involves addition of an oligosaccharide to the oxygen atom on the hydroxyl group of certain serine or threonine residues.Initial Glycosylation Occurs in the ER:
The initial steps of N-glycosylation take place on the cytosolic surface of the ER membrane and later steps occur in the ER lumen.
Despite the variety ofoligosaccharides found in mature glycoproteins, all the carbohydrate side chains added to proteins in the ER initially have a common core oligosaccharide consisting of two units of N-acetylglucosamine,nine mannose units, and three glucose units.
- Glycosylation:
1.- It begins as dolichol phosphate, an oligosaccharide carrier, is inserted into the ER membrane.
2.- GlcNAc and mannose groups arethen added to the phosphate group of dolichol phosphate.
3.- The growing core oligosaccharide is then translocated from the cytosol to the ER lumen by flippase.
4.- Once inside the ER lumen, moremannose and glucose units are added.
5.- The complete core oligosaccharide is then transferred as a single unit from dolichol to an asparagine residue of the recipient protein.
6.- Finally, thecore oligosaccharide attached to the protein is trimmed and modified.
* The core oligosaccharide is added to the protein as the polypeptide is being synthesized by a ribosome bound to the ER. We knowthat this contranslational glycosylation helps to promote proper protein folding.

Further Glycosylation Occurs in the Golgi Complex:
Further glycosylation happens as the glycoproteins move from...