Which of the following lists the four levels of protein structure?

• A. Primary, Secondary, Tertiary, and Quaternary structure.
• B. Primary, Tertiary, Quaternary, and Quinary structure
• C. Secondary, Tertiary, Quaternary, and Quintary structure
• D. Primary, Secondary, Tertiary, and Quinary structure.

Answer: (A)

Proteins are described by a hierarchy of structure that starts with the sequence of amino acids and builds up to the full assembly. The first level is the primary structure, the linear order of amino acids. The next level comes from local folding patterns stabilized by backbone hydrogen bonds, giving structures like alpha helices and beta sheets—that’s the secondary structure. The third level is the overall three-dimensional shape of a single polypeptide, shaped by interactions among side chains and the backbone, including hydrophobic effects, ionic bonds, hydrogen bonds, and disulfide bridges. The fourth level describes how multiple polypeptide chains come together to form a functional protein complex, known as quaternary structure.

This four-part set—primary, secondary, tertiary, and quaternary—is the standard way to list protein structure levels. The other options either replace or omit one of the classical levels or introduce nonstandard terms like quinary or quintary, which aren’t part of the traditional four-tier framework.