Question

. In what cellular compartment would a glycoprotein be expected to have its greatest:

a) sialic acid content?

b) mannose content?

c) N-Acetylglucosamine content?     

Answer 1

The cellular compartment where glycoprotein is expected to have greatest

mannose residues:-Lysosomes.

Greatest N-acetylglucosamine content:- cis Golgi complex.

Greatest sialic acid residues:- trans Golgi complex.

The N-linked carbohydrate units of glycoproteins are modified in each of the compartments of the Golgi complex. In the cis Golgi compartment, three mannose residues are removed from the oligosaccharide. The carbohydrate units of glycoproteins targeted to the lumen of lysosomes are further modified in the medial Golgi compartments of some cells, two more mannose residues are removed, and two N- acetylglucosamine residues and a fucose residue are added. Lastly , in the trans Golgi, another N-acetylglucosamine residue can be added, followed by galactose and sialic acid, to form a complex oligosaccharide unit. The sequence of N-linked oligosaccharide units of a glycoprotein is determined both by (1) the sequence and conformation of the protein undergoing glycosylation and by (2) the glycosyltransferases present in the Golgi compartment in which they are processed.Carbohydrate processing in the Golgi complex is called terminal glycosylation  to distinguish it from core glycosylation , which takes place in the ER.