Identifying Protein Structure Levels

Steps:

• Recall protein structures: primary (sequence), secondary (local folds like α-helix), tertiary (overall 3D shape of one chain), quaternary (multiple chains).
• Examine diagram: shows single polypeptide with loose spiral (α-helix, secondary) and distinct regions folding into 3D form.
• Match description: overall 3D arrangement of regions indicates tertiary structure, not just local helix.
• Eliminate mismatches: no multiple chains (rules out quaternary); helix alone is secondary, not full tertiary.

Why D is correct:

• Tertiary structure is the three-dimensional folding of a single polypeptide chain via side-chain interactions, forming a functional 3D shape from different regions.

Why the others are wrong:

• A: Quaternary involves multiple polypeptide subunits assembling, but diagram shows one chain.
• B: Tertiary encompasses full 3D folding, not limited to α-helix (which is secondary).
• C: Secondary structure refers only to local patterns like α-helix, not overall 3D shape.

Final answer: D