Which statement defines the quaternary structure of a protein?

• A. Overall protein structure, combining 2 or more polypeptides known as subunits
• B. The sequence of amino acids
• C. Local folding into alpha helices and beta sheets
• D. The arrangement of beta sheets within a single chain

Answer: (B)

Quaternary structure describes how two or more polypeptide chains come together and interact to form a functional protein. The overall shape and function arise from how these subunits arrange themselves and the bonds between them—hydrophobic interactions, hydrogen bonds, ionic interactions, and sometimes disulfide bridges. Hemoglobin is a classic example, with four subunits arranged to enable cooperative binding of oxygen.

In contrast, the sequence of amino acids defines the primary structure, the local folding into alpha helices and beta sheets defines the secondary structure, and the arrangement of beta sheets within a single chain corresponds to aspects of the tertiary structure, which is the overall 3D shape of one polypeptide.