The sequence of amino acids and the location of disulfide bonds

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

Answer: (B)

The main idea here is that the primary structure is the linear sequence of amino acids in a protein. This order not only specifies which residues are present, but also where cysteines lie, and therefore where disulfide bonds can form between them. Since disulfide bonds connect cysteine residues that are encoded by the sequence, the sequence of amino acids directly determines the potential pattern of these covalent linkages. That’s why this item points to the primary structure. By contrast, secondary structure deals with local folds like helices and sheets, tertiary structure is the overall 3D shape of a single polypeptide (stabilized by various interactions, including disulfide bonds), and quaternary structure involves how multiple polypeptide chains come together.