120. Proteins are glycosylated in the endoplasmic reticulum. In the final structures, the glycan
moieties are typically present:
A. on the surface of these proteins.
B. in the core of these proteins.
C. on the surface as well as in the core of these proteins.
D. on the membrane inserted segments of these proteins.

Detailed Explanation:

Protein glycosylation is a crucial post-translational modification that occurs primarily in the endoplasmic reticulum (ER) and the Golgi apparatus. During this process, carbohydrate groups (glycans) are covalently attached to specific amino acid residues of proteins.

Where Glycosylation Occurs:

• In the ER, glycosylation begins with the addition of oligosaccharides to asparagine residues (N-linked glycosylation).

• Further trimming and modification of glycans happen in the Golgi.

Location of Glycan Moieties in Final Protein Structures:

• Glycan groups are typically found on the surface of proteins.

• This is because glycosylation serves several surface-related functions:

  • Protection against proteases

  • Cell-cell recognition

  • Receptor binding

  • Protein folding and stability

Why Not in the Core or Membrane Regions?

• The protein core is usually hydrophobic and tightly packed, not suitable for bulky hydrophilic glycans.

• The membrane-inserted segments are embedded within the lipid bilayer and thus also not accessible for glycosylation.

• The ER machinery recognizes solvent-exposed asparagine residues, facilitating glycan addition only on extracellular or luminal-facing regions.

Correct Answer:

A. on the surface of these proteins.

This reflects the biological role of glycosylation and the physical accessibility of glycosylation sites during protein folding and trafficking through the ER and Golgi.