What characterizes tertiary structure?

• A. 3D shape arises from interactions (ionic, h-bonds, or hydrophobic) between R-groups.
• B. Disulfide bonds only.
• C. Peptide bonds linking amino acids.
• D. Hydrogen bonding between the backbone.

Answer: (A)

Tertiary structure is the overall 3D arrangement of a single polypeptide, stabilized by interactions among the amino acid side chains (R groups). These interactions include ionic bonds between charged residues, hydrogen bonds among polar side chains, and hydrophobic interactions that drive nonpolar residues to the protein’s interior. Covalent disulfide bridges between cysteine residues can also help lock in the fold. Together, these side-chain interactions shape the final, functional form of the protein.

Peptide bonds link amino acids to form the backbone and define the primary structure, while hydrogen bonding between the backbone creates the local patterns of secondary structure like helices and sheets. R-group–driven interactions, not backbone hydrogen bonds or a single bond type, determine the overall 3D shape characteristic of the tertiary level.