What does the primary structure of a protein refer to?

• A. The three-dimensional shape
• B. The amino acid sequence
• C. The overall folding
• D. The interaction with ligands

Answer: (B)

The primary structure is the linear sequence of amino acids in a polypeptide, held together by peptide bonds. This order is set by the gene and determines how the chain will behave chemically—hydrophobic and hydrophilic patterns, charges, and potential disulfide bonds—influencing where local helices and sheets form and how the chain begins to fold. All higher levels of structure—how the chain bends and stacks into a three-dimensional shape (tertiary and quaternary structure)—arise from this sequence, so the primary structure is fundamentally the amino acid order itself. The three-dimensional shape describes the folded architecture, while interactions with ligands depend on that final structure and its active sites, not on the sequence alone.