The tertiary structure of a protein is the 3D structure due to what interactions?

• A. Sequence of amino acids
• B. 2D arrangement of backbone
• C. 3D structure due to R-group interactions
• D. Presence of multiple subunits

Answer: (A)

The main idea is that the tertiary structure is the overall 3D shape of a single polypeptide stabilized by interactions among its side chains (R groups) and, when present, disulfide bonds. These interactions—hydrophobic effects that bury nonpolar side chains, hydrogen bonds, ionic interactions between charged side chains, and covalent disulfide bridges—work together to fold the protein into its final, functional form. The sequence of amino acids sets up which side chains are present and where they are, but the actual 3D arrangement is driven by these R-group interactions.

The other options don’t capture this stabilization mechanism. The amino acid sequence describes which residues exist but doesn’t by itself specify the 3D fold. The 2D arrangement of the backbone refers to secondary structure like helices and sheets formed by backbone hydrogen bonding, not the full 3D tertiary shape. The presence of multiple subunits describes quaternary structure, not the tertiary structure of a single polypeptide.