Which term describes a protein composed of multiple polypeptide chains?

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

Answer: (D)

When a protein is made from more than one polypeptide chain, the way those chains come together and interact defines the level of structure. This is described by quaternary structure—the arrangement and interfaces of multiple subunits in a protein complex. The functional protein can rely on how these subunits cooperate, as seen in hemoglobin, which has two alpha and two beta chains working together to carry oxygen. In contrast, primary structure is just the exact amino acid sequence of a single chain, secondary structure refers to local folds like alpha helices and beta sheets within regions of a chain, and tertiary structure is the overall three-dimensional shape of a single polypeptide chain. Some proteins consist of a single polypeptide and do not exhibit quaternary structure, while others assemble multiple subunits to form a functional complex.