The four structures of proteins are:

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

Answer: (D)

Protein structure is built in four levels. The primary structure is the linear sequence of amino acids linked by peptide bonds. The secondary structure describes local folding into regular patterns like alpha helices and beta sheets held together by hydrogen bonds. The tertiary structure is the overall three-dimensional shape of a single polypeptide, stabilized by various interactions among side chains and the backbone. When multiple polypeptide chains come together, their arrangement constitutes the quaternary structure.

All four levels together capture how a protein is organized from its sequence to its folded form and, if present, its multi-subunit assembly. The option that lists all four levels matches this complete framework, while the other choices leave out one or more essential levels.