Question

The amino acid that hinders the formation of \(\alpha\)-helical structure is

• A. His
• B. Phe
• C. Pro
• D. Arg

Hint:

Proline is known as a "helix breaker" because its cyclic side chain restricts rotation and lacks the amide hydrogen necessary for the hydrogen-bonding network of an \(\alpha\)-helix. Glycine is also a weak helix former due to its high conformational flexibility.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The \(\alpha\)-helix is a common secondary structure in proteins stabilized by hydrogen bonds between the backbone amino and carbonyl groups. Some amino acids act as "helix breakers" because they cannot accommodate this conformation.

Step 2: Detailed Explanation:
1. Proline (Pro): Is an imino acid with a cyclic pyrrolidine side chain that is covalently bonded to the nitrogen of the peptide backbone.
2. This rigid ring structure restricts rotation around the \(\text{N}-\text{C}_\alpha\) bond, preventing the polypeptide chain from adopting the proper helical angles (\(\phi\) and \(\psi\)).
3. Additionally, the nitrogen in a proline peptide bond lacks an amide hydrogen, making it unable to form the essential hydrogen bonds needed to stabilize the \(\alpha\)-helix.
4. Therefore, proline creates a kink or bend in the chain and is excluded from \(\alpha\)-helices.

Step 3: Final Answer:
Proline is the amino acid that hinders \(\alpha\)-helix formation.