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: Understanding the Concept:
The process of translation is initiated at the start codon AUG, which codes for the amino acid methionine (or a modified form, fMet, in bacteria). The question asks about the fate of this initial methionine in the final protein product.
Step 2: Detailed Explanation:
Let's evaluate each statement:


(A) All proteins have methionine as the first amino acids. This is incorrect. While protein synthesis always begins with methionine, it is very often removed from the N-terminus of the polypeptide chain after translation.

(B) Methionine never added at the beginning of protein synthesis. This is incorrect. The start codon AUG explicitly directs the incorporation of methionine as the first amino acid.

(C) An enzyme can cleave the methionine amino acid from the polypeptide after synthesis. This is correct. This process is a common post-translational modification called N-terminal methionine excision (NME). It is carried out by enzymes called methionine aminopeptidases (MAPs). Whether the methionine is cleaved or not depends on the nature of the second amino acid in the chain.

(D) Methionine fall off automatically from the polypeptide... This is incorrect. The removal of the N-terminal methionine is an active, enzymatic process and does not happen automatically.

Step 3: Final Answer:
The correct statement is that the initial methionine can be enzymatically removed from the polypeptide after it has been synthesized.