Which level of protein structure is defined by the three-dimensional arrangement of a single polypeptide chain, including any disulfide bonds?

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

Answer: (C)

Think about how a protein reaches its functional shape when a single chain folds in three dimensions. That overall folding pattern is the tertiary structure, which arises from interactions among the amino acid side chains—hydrophobic packing, hydrogen bonds, ionic interactions, van der Waals forces, and, importantly, disulfide bonds that covalently link cysteines to stabilize the fold. This level describes the complete, three-dimensional arrangement of one polypeptide chain.

In contrast, the primary structure is just the linear sequence of amino acids, and the secondary structure refers to local motifs like alpha helices and beta sheets formed mainly by backbone hydrogen bonds. If multiple polypeptide chains come together to form a functional protein, that arrangement is quaternary structure.