Collagen's Composition and Triple Helix Structure

Steps:

• Recall collagen's primary structure: rich in glycine (small R group), proline, and hydroxyproline.
• Note glycine occurs every third residue, enabling tight packing in the triple helix.
• Examine options for accuracy: A fits composition; B, C, D distort assembly or bonding.
• Verify via definition: collagen molecule is three intertwined polypeptides, not fibres or alpha helices.

Why A is correct:

• Collagen has ~33% glycine, whose minimal R group (hydrogen atom) allows the three chains to pack closely in the triple helix, as per its biochemical structure.

Why the others are wrong:

• B: The triple helix forms from three polypeptide chains (molecules), not fibres, which are larger aggregates.
• C: Each polypeptide adopts a polyproline II helix, not alpha helices, due to proline's rigidity.
• D: Peptide bonds form within each polypeptide chain; inter-polypeptide links are hydrogen and covalent cross-links, not peptide bonds.

Final answer: A