IMAT 2012 Q56 [Glycoproteins]

The following organelles are involved in processing amino acids into glycoprotein:

  1. Golgi apparatus
  2. Ribsomes
  3. RER

Which sequence is correct for this process?

A. 3 {\rightarrow} 1 {\rightarrow} 2
B. 2 {\rightarrow} 3 {\rightarrow} 1
C. 1 {\rightarrow} 2 {\rightarrow} 3
D. 1 {\rightarrow} 3 {\rightarrow} 2
E. 2 {\rightarrow} 1 {\rightarrow} 3

The answer to this question is B, where the sequence is 2 → 3 → 1.

First, it is important to know what a glycoprotein is. These are proteins that contain oligosaccharide chains that are covalently bound to amino acid side chains. This glycosylation (or the addition of the carbohydrate to the protein) happens as a cotranslational or posttranslational modification.

How are glycoproteins synthesized?

  • Glycoprotein synthesis follows a sequence of events that involves ribosomes, RER, and the golgi apparatus.
  • Knowing that glycosylation is a cotranslational/posttranslational process, you need to think that firstly, you would need the protein, to have something to glycosylate in the first place. The RNA that codes for the protein enters the cytoplasm, where it binds to large RNA-protein complexes called ribosomes. This means that #2 is the first in the sequence, which only leaves choice B and E in your considerations.
  • The ribosomes then bind to an organelle called the endosplasmic reticulum, which is the organelle tasked to produce proteins. The protein chain then enters the lumen of the endoplasmic reticulum so that glycosylation can start, where a core oligosaccharide is added to the protein.
  • Additional posttranslational carbohydrate modifications will then be performed as the protein chain transitions from the RER to another organelle called the golgi apparatus where final carbohydrate modifications will be done. The function of this organelle in summary, is to perform final modifications, process, and sort the synthesized glycoproteins.

Thus, among the remaining considerations, only choice B is the correct sequence that describes glycoprotein biosynthesis.