Question

The imino acid found in proteins is:

• A. Arginine
• B. Tyrosine
• C. Phenylalanine
• D. Proline

Hint:

Proline is often referred to as a "helix breaker" because its rigid pyrrolidine ring lacks the hydrogen atom needed to form stabilizing hydrogen bonds within an alpha-helix.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Standard amino acids contain a primary amino group ($-NH_2$) and a carboxylic acid group ($-COOH$) attached to the same alpha-carbon.
However, one of the twenty standard proteinogenic amino acids contains a cyclic structure that alters this chemistry.

Step 2: Detailed Explanation:
Proline is unique because its aliphatic side chain is bonded back to its own nitrogen atom.
This forms a five-membered pyrrolidine ring that incorporates the alpha-amino nitrogen.
Because the nitrogen atom is bonded to two carbon atoms within this ring, it is a secondary amine rather than a primary amine.
Consequently, proline is chemically classified as a cyclic $\alpha$-imino acid rather than an amino acid.
This rigid ring structure puts conformational constraints on the polypeptide backbone, often disrupting alpha-helices and leading to the formation of turns or kinks in protein secondary structure.

Step 3: Final Answer:
Proline is the unique cyclic secondary amine that is classified as an imino acid among the twenty standard amino acids.