Which level of protein structure describes the assembly of two or more polypeptide subunits?

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

Answer: (A)

Quaternary structure describes how two or more polypeptide chains come together to form a functional protein. When a protein has several subunits, their specific arrangement and the interactions between them—such as hydrogen bonds, ionic interactions, van der Waals forces, and sometimes disulfide bridges—define this level of structure. Examples like hemoglobin show how multiple subunits cooperate to achieve function, illustrating why the assembly of subunits is the hallmark of quaternary structure. In contrast, primary structure is just the amino acid sequence, secondary refers to local folding patterns within a single chain, and tertiary describes the full three-dimensional shape of one polypeptide.