What defines quaternary structure?

• A. Association of two or more polypeptides.
• B. 3D shape from R-group interactions.
• C. Sequence order of amino acids.
• D. Regular patterns from hydrogen bonds.

Answer: (A)

Quaternary structure describes how multiple polypeptide chains come together to form a functional protein complex. It is defined by the association of two or more polypeptide subunits, held together by various interactions such as hydrophobic contacts, ionic bonds, hydrogen bonds, and sometimes disulfide bridges. A classic example is hemoglobin, which consists of four subunits arranged to work as a unit.

In contrast, the 3D shape arising from interactions among R-groups within a single polypeptide corresponds to tertiary structure, not the assembly of subunits. The sequence order of amino acids is the primary structure, and regular hydrogen-bond patterns that produce helices or sheets describe secondary structure.