Question

Choose the correct answer regarding the translation of mRNA to proteins when AUG is the start codon and codes for methionine.

• A. All proteins have methionine as the first amino acids.
• B. Methionine never added at the beginning of protein synthesis.
• C. An enzyme can cleave the methionine amino acid from the polypeptide after synthesis.
• D. Methionine fall off automatically from the polypeptide just after termination step of the protein synthesis.

Hint:

Think of the initial methionine as a temporary "start signal" tag. Just like you might remove a price tag from new clothes before wearing them, the cell often removes the starting methionine with a specific enzyme after the protein is made.

Answer 1

The Correct Option is C

Step 1: Concept Overview:
Translation starts at the AUG codon, encoding methionine (or fMet in bacteria). The question concerns the fate of this initial methionine in the final protein.
Step 2: Detailed Analysis:
Evaluating each option:


(A) All proteins begin with methionine. Incorrect. While synthesis starts with methionine, it's often removed post-translationally from the N-terminus.

(B) Methionine is never the first amino acid. Incorrect. AUG signals the incorporation of methionine as the initial amino acid.

(C) An enzyme can cleave methionine post-synthesis. Correct. N-terminal methionine excision (NME), via methionine aminopeptidases (MAPs), is a common post-translational modification. Cleavage depends on the second amino acid.

(D) Methionine spontaneously falls off. Incorrect. N-terminal methionine removal is enzymatic, not spontaneous.

Step 3: Conclusion:
The initial methionine can be enzymatically removed from the polypeptide after synthesis.