Tertiary structure is stabilized by which factors?

• A. Results from multiple polypeptide chains
• B. Stabilized by side chain interactions including hydrophobic core and disulfide bridges
• C. Only backbone hydrogen bonds
• D. Defined solely by the amino acid sequence

Answer: (B)

Tertiary structure is stabilized by interactions among the side chains of the amino acids within a single polypeptide. A hydrophobic core forms as nonpolar residues tuck away from water, driving folding and the compact 3D shape. Covalent disulfide bridges between cysteine residues can lock distant parts of the chain together, adding substantial stability. In addition, ionic interactions between charged groups, hydrogen bonds, and van der Waals contacts among side chains refine and maintain the overall fold. This combination of forces shapes the final three-dimensional arrangement of the protein.

Other statements point to different concepts or incomplete views: quaternary structure involves multiple polypeptide chains, backbone hydrogen bonds mainly support secondary structure, and the amino acid sequence guides folding but does not alone define the final 3D structure.