Which statement about the alpha helix, the most common secondary structure in globular proteins, is correct?
- A The amino acid side chains project into the center of the helix, forming a hydrophobic core
- B It contains exactly 4.0 amino acid residues per complete turn
- C It is stabilized by hydrogen bonds between the C=O of residue n and the N-H of residue n+4, running nearly parallel to the helix axis ✓
- D Proline is the strongest naturally occurring helix-forming residue
Correct answer: C. It is stabilized by hydrogen bonds between the C=O of residue n and the N-H of residue n+4, running nearly parallel to the helix axis
Explanation
In an alpha helix each peptide A=O hydrogen bonds to the N-H of the residue four positions ahead, so the bonds lie roughly parallel to the axis. There are 3.6 residues per turn with a rise of 1.5 Angstrom per residue. Side chains point outward, not inward, and proline is a helix breaker because its rigid ring restricts backbone rotation and it lacks an amide hydrogen.
Reference: Lehninger Principles of Biochemistry, 7th ed.
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