Question

In polypeptide chains, the proline residue is unique in

• A. having an aromatic ring
• B. being abundantly present in most proteins
• C. being the site for phosphorylation on proteins
• D. having its side chain connected to the peptide backbone twice

Hint:

Proline's cyclic side chain gives it unique structural properties, influencing protein folding and stability.

Answer 1

The Correct Option is D

Step 1: Understanding proline in polypeptides.
Proline is an amino acid that is unique among the standard amino acids because its side chain is covalently bonded to the nitrogen of the amide group, forming a cyclic structure. This means the side chain is attached to the backbone of the polypeptide twice.

Step 2: Analyzing the options.
(A)having an aromatic ring: Incorrect, as proline does not contain an aromatic ring.
(B)being abundantly present in most proteins: While proline is common, it is not necessarily abundant in all proteins, so this is incorrect.
(C)being the site for phosphorylation on proteins: This is incorrect; serine, threonine, and tyrosine are more commonly phosphorylated.
(D)having its side chain connected to the peptide backbone twice: Correct — Proline's unique cyclic structure results in its side chain being attached to the peptide backbone twice.

Step 3: Conclusion.
The correct answer is (D)having its side chain connected to the peptide backbone twice, as this unique feature distinguishes proline from other amino acids.