Collagen's Structural Levels

Steps:

• Define protein structure levels: primary (amino acid sequence), secondary (local folding like helices), tertiary (3D folding of one chain), quaternary (multiple chains).
• Examine collagen: fibrous protein with repeating Gly-X-Y sequence (primary structure).
• Note collagen forms tropocollagen from three intertwined chains (quaternary structure).
• Confirm no typical tertiary folding; secondary is the triple helix, but primary and quaternary are key descriptors.

Why B is correct:

• Collagen's primary structure is its unique amino acid sequence enabling stability, and quaternary structure assembles three chains into a triple helix, as per standard biochemistry definitions.

Why the others are wrong:

• A: Secondary structure (triple helix) is present but not paired with primary in core descriptions.
• C: Tertiary structure absent in collagen's linear, fibrous form.
• D: Quaternary exists, but tertiary does not apply to its non-globular chains.

Final answer: B