Denaturation leaves which level of protein structure unchanged?

• A. secondary
• B. primary
• C. tertiary
• D. quaternary

Answer: (B)

Denaturation disrupts the weaker, noncovalent interactions that hold the higher levels of protein structure together. Heat, extreme pH, or denaturing agents break hydrogen bonds, ionic interactions, and hydrophobic packing that define secondary, tertiary, and quaternary structure, causing the protein to unfold or lose its arranged shape. The backbone of the protein, held together by covalent peptide bonds, stays intact under typical denaturing conditions, so the sequence of amino acids—the primary structure—remains unchanged. This is why the primary structure is preserved even as the other levels are disrupted.