Question

Which of the following statements are TRUE with regard to the process of post-tranlational modifications?
(A) Cleavage of polyprotein to their component protein.
(B) Hydroxylation of praline/lysine amino acid residues of collagen protein.
(C) Formylation of 'Met' amino acid residues.
(D) Formation of disulphide bridges.
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (C) and (D) only.
• C. (A), (B), (C) and (D).
• D. (B), (C) and (D) only.

Hint:

To classify modifications:
- Post-translational = Occurs on the completed polypeptide chain (e.g., Cleavage, Hydroxylation, Disulfide bonds).
- Pre-translational Co-translational = Occurs before or during synthesis (e.g., Formylation of initiator Met-tRNA in prokaryotes).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
After translation is completed on the ribosome, many polypeptides must undergo covalent modifications to become functionally active.
These modifications are called post-translational modifications (PTMs).

Step 2: Detailed Explanation:
Let us analyze each statement to determine if it is a true post-translational modification:
- Statement (A) is true:
Many proteins (such as insulin or viral proteins) are synthesized as larger, inactive precursor polyproteins.
Specific proteases cleave these precursors to yield mature, active individual proteins.
- Statement (B) is true:
Collagen synthesis requires the hydroxylation of specific proline and lysine residues to form hydroxyproline and hydroxylysine.
This modification occurs post-translationally in the endoplasmic reticulum and is critical for stabilizing the collagen triple helix.
- Statement (C) is false:
Formylation of the initiator Methionine (Met) to form fMet occurs *prior* to the initiation of translation in prokaryotes.
It is a co-translational or pre-translational modification where Methionine is formylated while attached to its initiator tRNA (\(\text{tRNA}^{\text{fMet}}\)), not a post-translational modification of a synthesized protein.
- Statement (D) is true:
The oxidation of sulfhydryl groups on cysteine residues to form covalent disulfide bridges is a classic post-translational modification occurring in the lumen of the rough endoplasmic reticulum.
Therefore, statements (A), (B), and (D) are true post-translational modifications, while (C) is not.

Step 3: Final Answer:
The true statements with regard to post-translational modifications are (A), (B), and (D) only.