Which structural level describes a protein comprising more than one polypeptide chain?

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

Answer: (D)

Quaternary structure describes how multiple polypeptide chains come together to form a functional protein complex. It’s about the arrangement and interactions of these subunits—how several chains associate, whether they are identical or different, to create the active molecule. For example, hemoglobin is built from two alpha and two beta chains that fit together to carry oxygen, illustrating a protein with more than one polypeptide subunit.

In contrast, the primary structure is just the linear sequence of amino acids linked by peptide bonds; the secondary structure refers to local folding patterns like alpha helices and beta sheets stabilized by hydrogen bonds; and the tertiary structure is the overall three-dimensional shape of a single polypeptide chain, including how its side chains fold and interact. Since this question specifies more than one polypeptide chain, the correct concept is quaternary structure.