30. Which is correct for α-helix of a protein

    (1) It has H-bonding in two or more parallel running chains
    (2) There is intra-chain H-bonding in single helix
    (3) No H- bonding is seen
    (4) is a tertiary structure

Alpha-Helix in Protein Structure: Hydrogen Bonding and Stability

The alpha-helix is one of the fundamental secondary structures in proteins. Understanding its formation, stability, and hydrogen bonding pattern is essential for students preparing for CSIR NET Life Science and other competitive exams.

Correct Answer:

The correct option is (2) There is intra-chain H-bonding in a single helix.

Explanation:

(1) It has H-bonding in two or more parallel running chains ❌

This statement describes beta-sheets, not alpha-helices. In beta-sheets, hydrogen bonding occurs between adjacent polypeptide chains, which can be parallel or antiparallel.

(2) There is intra-chain H-bonding in a single helix ✅

In an alpha-helix, hydrogen bonds form within the same polypeptide chain between the carbonyl oxygen (C=O) of one amino acid and the amide hydrogen (N-H) of another amino acid four residues ahead. This stabilizes the helical structure.

(3) No H-bonding is seen ❌

Hydrogen bonding is crucial for maintaining the alpha-helical structure. Without H-bonding, the helix would not be stable.

(4) It is a tertiary structure ❌

The alpha-helix is a secondary structure, not a tertiary structure. Tertiary structure refers to the overall three-dimensional folding of an entire polypeptide chain, including interactions between different secondary structures.

Nearby Topics for Better Understanding

1. Features of Alpha-Helix

• Right-handed coil
• 3.6 residues per turn
• Intra-chain hydrogen bonding for stabilization
• Found in structural and functional proteins (e.g., myoglobin, hemoglobin)

2. Factors Affecting Alpha-Helix Stability

• Amino Acid Composition:
  • Alanine promotes helix formation.
  • Proline and glycine disrupt helices.
• Hydrogen Bonding: Stabilizes the helical conformation.
• Electrostatic Interactions: Opposite charges stabilize, while like charges repel.
• Steric Hindrance: Bulky side chains can destabilize the helix.

3. Comparison of Alpha-Helix and Beta-Sheet

4. Importance of Alpha-Helix in Proteins

• Provides structural stability
• Plays a role in protein-protein interactions
• Found in membrane proteins (e.g., ion channels, receptors)

Conclusion

The alpha-helix is a crucial secondary structure in proteins, stabilized by intra-chain hydrogen bonds. It plays a vital role in protein folding and function, making it an essential topic for CSIR NET Life Science aspirants. Understanding its formation and comparison with beta-sheets helps in mastering protein biochemistry concepts.