Question

Which of the following is an imino acid

• A. Cysteine
• B. Histidine
• C. Proline
• D. Glycine

Hint:

Because of its rigid imino ring, Proline acts as a "helix breaker" in proteins.
It cannot fit into standard alpha-helical structures and is often found in the turns or bends of folded proteins.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Standard amino acids have a primary amino group ($-\text{NH}_2$) attached to their alpha-carbon.
However, one standard amino acid has a cyclic side chain that bonds back to its nitrogen atom, forming a secondary amine instead of a primary amine.
This unique structure is classified as an imino acid.

Step 3: Detailed Explanation:
Let us analyze the molecular structures of the given options:
1. Cysteine, Histidine, Glycine:
These are standard primary amino acids.
They contain a free primary amino group ($-\text{NH}_2$) attached to the alpha-carbon.
Thus, Options (A), (B), and (D) are incorrect.
2. Proline:
Proline is unique among the 20 standard proteinogenic amino acids.
Its side-chain propyl group is covalently bonded back to the nitrogen atom of the amino group.
This forms a rigid five-membered pyrrolidine ring, creating a secondary amine structure (an imino group, $=\text{NH}$) rather than a primary amine group ($-\text{NH}_2$).
As a result, Proline is classified as an imino acid.
This rigid ring structure significantly limits Proline's conformational flexibility, which plays a major role in forming secondary protein structures like alpha-helices and beta-turns.

Step 4: Final Answer:
Proline is the imino acid among the options.
Therefore, the correct option is (C).