Which level of protein structure is defined by the sequence of amino acids in a polypeptide?

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

Answer: (A)

The sequence of amino acids in a polypeptide defines the primary structure. This level is simply the linear order of the amino acids linked by peptide bonds, which acts as the blueprint for everything else the protein can become. The other levels describe how that chain folds and assembles: secondary structure refers to local patterns like alpha helices and beta sheets formed by hydrogen bonds along the backbone; tertiary structure is the overall three-dimensional shape resulting from interactions among the side chains; quaternary structure involves how multiple polypeptide subunits come together. The key idea is that only the primary structure is defined by the exact order of amino acids, while the higher levels arise from how that sequence folds and interacts.