Tertiary structure: 3D folding driven by R-group interactions

Steps:

• Define tertiary structure as the spatial arrangement of a polypeptide chain stabilized by hydrophobic, ionic, H-bond, and disulfide interactions between side chains.
• Eliminate options describing other structure levels: primary (sequence), secondary (local folding like helices), quaternary (multi-subunit).
• Identify option matching R-group burial in folded protein core.
• Confirm D aligns with hydrophobic core formation in soluble proteins.

Why D is correct:

• In tertiary structure, hydrophobic R groups cluster inside the protein to minimize water contact, as per the hydrophobic effect principle.

Why the others are wrong:

• A describes quaternary structure, involving interactions between separate polypeptides like glutenin and gliadin.
• B refers to primary structure, the linear sequence rich in glutamine residues.
• C describes secondary structure, local α-helical conformations within the chain.

Final answer: D