A protein structure composed of more than one polypeptide chain is described as:

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

Answer: (D)

Quaternary structure describes how multiple polypeptide chains come together to form a functional protein. When a protein consists of more than one chain, these subunits interact and align in a specific arrangement to create the overall structure and activity of the complex. For example, hemoglobin has four subunits that work together to carry oxygen efficiently.

To place this in the bigger picture: the primary structure is the linear sequence of amino acids; secondary structure refers to local folding patterns like alpha helices and beta sheets; tertiary structure is the three-dimensional shape of a single polypeptide chain. When several chains are involved, their collective arrangement is what we call the quaternary structure. Since the protein in question is made of more than one polypeptide chain, it is described by its quaternary structure.