Protein Quaternary Structure via Disulfide Bonds

Steps:

• Recall protein structure levels: primary (amino acid sequence), secondary (local folding like helices), tertiary (overall 3D shape of one chain), quaternary (arrangement of multiple chains).
• Note the protein has two polypeptide chains, meaning multiple subunits.
• Identify disulfide bonds as covalent links between chains, not within one.
• Conclude this describes assembly of separate chains into a functional unit.

Why D is correct:

• Quaternary structure is defined as the spatial arrangement and interactions, including disulfide bonds, between two or more polypeptide chains in a protein complex.

Why the others are wrong:

• A: Primary structure is the linear sequence of amino acids in a single chain, without bonds between chains.
• B: Secondary structure involves hydrogen-bonded local patterns like alpha helices or beta sheets within one chain.
• C: Tertiary structure is the 3D folding of a single polypeptide chain, stabilized by bonds within that chain.

Final answer: D