Explain “why the presence of proline makes a kink in the alpha-helix structure and is present in beta turn during the beta-sheet formation of a protein? Please make the structure of a polypeptide chain having proline and explain your answer.
The Correct Answer and Explanation is:
Why Proline Causes a Kink in Alpha-Helix and Appears in Beta Turns
Proline is a unique amino acid in protein structure due to its rigid ring structure. Unlike other amino acids, proline’s side chain is bonded to the nitrogen of the amino group, forming a five-membered ring. This cyclic structure restricts the rotation of the N-Cα bond, making it structurally rigid. As a result, proline disrupts the regular hydrogen bonding pattern necessary for the formation of an alpha-helix.
In an alpha-helix, each backbone N-H group typically forms a hydrogen bond with the C=O group of the amino acid four residues earlier. However, proline lacks a hydrogen atom on its backbone nitrogen due to its secondary amine structure, which prevents it from donating a hydrogen bond. This disrupts the hydrogen bonding network and introduces a kink or bend in the helix, often terminating or breaking the helix entirely.
Conversely, proline is commonly found in beta-turns, which are tight turns that reverse the direction of the polypeptide chain in beta-sheet structures. The rigidity of proline stabilizes these sharp turns, and since proline naturally induces a bend due to its fixed angle, it is ideal for beta-turn formation. Additionally, its cyclic structure helps lock the turn in place, providing conformational stability.
Polypeptide Chain with Proline (Structural Representation):
Here’s a simplified representation of a polypeptide segment with a proline:
markdownCopyEdit...—NH—CHR1—CO—NH—C(Pro)—CO—NH—CHR2—...
|
Ring
In the proline residue:
- The nitrogen (N) is part of a five-membered ring.
- The ring limits rotation and lacks the NH hydrogen for bonding.
- This structural constraint causes a kink in helices but fits well in tight beta-turns.
Conclusion:
Proline causes a kink in alpha-helices due to its rigid ring and inability to donate a hydrogen bond. However, this same rigidity makes it perfect for beta-turns, where a sharp directional change is needed in the protein’s structure.
