What is the correct order of protein structure levels from smallest to largest?

• A. Secondary, Primary, Tertiary, Quaternary.
• B. Tertiary, Quaternary, Primary, Secondary.
• C. Primary, Secondary, Tertiary, Quaternary.
• D. Quaternary, Tertiary, Secondary, Primary.

Answer: (C)

Proteins are built in a stepwise hierarchy, and the size grows as you move from the linear sequence to the full multi-subunit assembly. The smallest level is the primary structure—the exact sequence of amino acids linked by peptide bonds. From that sequence, local patterns form in the backbone to give secondary structure, such as alpha helices and beta sheets, held together by hydrogen bonds along the peptide chain. The next level is tertiary structure, which is the overall three-dimensional shape of a single polypeptide, shaped by interactions among the side chains (R groups), including hydrophobic effects, ionic interactions, hydrogen bonds, and sometimes disulfide bridges. When a protein consists of more than one polypeptide chain, their arrangement and interactions constitute quaternary structure.

So the order from smallest to largest is primary, secondary, tertiary, and then quaternary. Some proteins don’t have a quaternary structure if they’re single-chain, but the progression up to a single chain remains the same.