Quaternary structure of a protein is defined as

• A. A single polypeptide chain folded into a three-dimensional shape
• B. A number of polypeptide chains linked together to form one protein
• C. The linear sequence of amino acids
• D. The overall three-dimensional shape of a single polypeptide

Answer: (B)

Quaternary structure describes how multiple polypeptide chains come together to form one functional protein. Each chain may already have its own tertiary shape, and the overall protein’s properties come from how these subunits interact and assemble—through noncovalent bonds and sometimes disulfide linkages. This level of structure only exists when more than one polypeptide chain is involved. A classic example is hemoglobin, which has four subunits arranged to create its functional form, while collagen involves three polypeptide chains in a distinct triple-helix arrangement.

The other descriptions refer to lower levels of structure. The overall three-dimensional shape of a single polypeptide is its tertiary structure, and the linear sequence of amino acids is the primary structure. So, the statement describing multiple chains linked together to form one protein best captures the meaning of quaternary structure. Some proteins are monomeric and lack quaternary structure entirely, consisting only of a single polypeptide with tertiary structure.