17. How many hydrogen bonds involving the backbone CO and NH can be observed in an a-helix consisting of 15 amino acid residues?
    (1) 10
    (2) 11
    (3) 12
    (4) 13

Hydrogen Bonding in an α-Helix of 15 Amino Acids

Protein secondary structures, like α-helices, rely on hydrogen bonding to maintain their stability and function. The number of hydrogen bonds in an α-helix depends on the total number of amino acids in the sequence.

Correct Answer:

The correct option is:

(2) 11

Explanation of the Calculation

1. Hydrogen Bonding Pattern in α-Helices

• Each hydrogen bond is formed between the C=O (carbonyl) of residue i and the N-H (amide) of residue i + 4.
• The first four NH groups at the N-terminal and the last four CO groups at the C-terminal cannot participate in hydrogen bonding.

2. Formula for Hydrogen Bonds in an α-Helix

• The number of hydrogen bonds (H) in an α-helix with N amino acid residues is: 
• For N = 15:

3. Why Are Other Options Incorrect?

• (1) 10: Incorrect, as it underestimates the number of hydrogen bonds.
• (3) 12 and (4) 13: Incorrect, as they overestimate the possible hydrogen bonds in a 15-residue α-helix.

Significance of Hydrogen Bonds in α-Helices

1. Role in Protein Structure

• Hydrogen bonds stabilize the helical structure, preventing unwinding.
• They align the backbone into a right-handed coil.

2. Importance in Protein Function

• Found in enzymes, receptors, and membrane proteins.
• Critical for protein folding and interactions.

3. Applications in Bioengineering

• Helps in designing synthetic peptides.
• Used in biomedical research for protein modeling.

Conclusion

An α-helix of 15 amino acids contains 11 hydrogen bonds, following the N – 4 rule. Understanding hydrogen bonding in helices is essential for biochemistry, molecular biology, and protein engineering.