Which level of protein structure involves interactions that lead to unique 3D shapes?

The tertiary structure of a protein is characterized by the overall three-dimensional shape formed by the interactions and bonding between various side chains (R groups) of the amino acids. These interactions include hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges. This level of structure is crucial because it determines how the protein will function and interact with other molecules.

In contrast, the primary structure refers simply to the linear sequence of amino acids in a protein, which does not account for any folding or 3D shape. The secondary structure entails localized folding patterns, such as alpha helices and beta sheets, which are stabilized by hydrogen bonding, but it does not encompass the full spatial arrangement of all amino acid side chains. The quaternary structure arises when multiple polypeptide chains (subunits) come together to form a complex, which is relevant in some proteins but not directly about the unique 3D shape of a single polypeptide chain. Therefore, the tertiary structure is indeed the correct answer, as it is the level at which a protein folds into its proper functional shape.