A protein composed of more than one polypeptide chain held together by non-covalent interactions has which level of structure?

• A. Primary
• B. Secondary
• C. Tertiary
• D. Quaternary

Answer: (D)

Quaternary structure. When a protein has more than one polypeptide chain, the overall architecture is defined by how those subunits come together. The subunits are held together mainly by non-covalent interactions—hydrogen bonds, ionic attractions, hydrophobic effects, and van der Waals forces—to form the functional protein complex. This is distinct from primary (amino acid sequence), secondary (local folds like alpha helices and beta sheets stabilized by backbone hydrogen bonds), and tertiary (the 3D shape of a single polypeptide) structures. An example is hemoglobin, which consists of four subunits that assemble into a functional tetramer through non-covalent interactions.