Glycosylation

The addition of a carbohydrate moiety to a protein or a lipid is called glycosylation. Glycosylation is a common post translational modification for proteins involved in cell membrane formation. The glycosylation of a protein helps in proper protein folding, stability and the cell to cell adhesion commonly needed by cells in the immune system.

There are 2 main types of glycosylation:

a) N-linked glycosylation: It begins with the addition of a 14-sugar precursor to an asparagine amino acid. It contains glucose, mannose and n-acetylglucosamine molecules. This entity is then transferred to the ER lumen. The oligosaccharyl transferase enzyme attaches the oligosaccharide chain to asparagine that occurs in the tripeptide sequence, Asn-X-Ser or Asn-X-Thr. X can be any amino acid other than Proline. The oligosaccharide attached protein sequence now folds correctly and is now translocated to the Golgi body where the mannose residue is removed.

b) O-linked glycosylation: Glycosylation begins with an enzyme mediated addition of N-acetyl-galactosamine followed by other carbohydrates to serine or threonine residues. Studies reveal that O linked glycosylation occurs at a later stage in protein processing.

c) Protein glycosylation:Protein Glycosylation: Linking of monosaccharide units to the amino acid chains sets up the stage for a series of enzymatic reactions that lead to the formation of glycoproteins (n and o linked oligosaccharides that are found to a protein entity). A typical glycoprotein has at least 41 bonds which involve 8 amino acids and 13 different monosaccharide units and includes the glycophosphatidylinositol (GPI) and phosphoglycosyl linkages.

In all 16 known enzymes are supposed to mediate this reaction and the major sites of protein glycosylation in the body are ER, Golgi body, nucleus and the cell fluid.

Glycosylation