Question:
What is the amino acid which does not contain $-\text{NH}_2$ group in its structure?}
What is the amino acid which does not contain $-\text{NH}_2$ group in its structure?}
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Proline is called the "imino acid" because its nitrogen is part of a ring structure and lacks a free $-\text{NH}_2$ group. It introduces bends in protein structures.
Updated On: Jun 12, 2026
- Tyrosine
- Alanine
- Proline
- Methionine
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The Correct Option is C
Solution and Explanation
Concept:
This question is based on the structure of amino acids.
All standard amino acids contain:
• $-\text{COOH}$ group
• $-\text{NH}_2$ group (primary amine)
• Hydrogen atom
• Side chain (R group) However, one exception exists due to cyclization of the side chain.
Step 1: Structure of Proline Proline is unique because its side chain forms a ring with the amino nitrogen. This forms a pyrrolidine ring, making the nitrogen a secondary amine ($-\text{NH}-$) instead of a primary amine ($-\text{NH}_2$). Hence, Proline does not contain a free $-\text{NH}_2$ group.
Step 2: Other amino acids
• Alanine: contains normal $-\text{NH}_2$
• Tyrosine: contains normal $-\text{NH}_2$
• Methionine: contains normal $-\text{NH}_2$ Thus, only Proline lacks a free amino group.
• $-\text{COOH}$ group
• $-\text{NH}_2$ group (primary amine)
• Hydrogen atom
• Side chain (R group) However, one exception exists due to cyclization of the side chain.
Step 1: Structure of Proline Proline is unique because its side chain forms a ring with the amino nitrogen. This forms a pyrrolidine ring, making the nitrogen a secondary amine ($-\text{NH}-$) instead of a primary amine ($-\text{NH}_2$). Hence, Proline does not contain a free $-\text{NH}_2$ group.
Step 2: Other amino acids
• Alanine: contains normal $-\text{NH}_2$
• Tyrosine: contains normal $-\text{NH}_2$
• Methionine: contains normal $-\text{NH}_2$ Thus, only Proline lacks a free amino group.
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