Which statement best describes quaternary structure?

• A. It is the primary sequence of amino acids.
• B. It describes a single polypeptide's 3D fold.
• C. It involves multiple polypeptide chains assembling into a functional protein.
• D. It refers to the type of bonds between amino acids.

Answer: (C)

Quaternary structure is about how more than one polypeptide chain come together and interact to form a functional protein. When a protein consists of multiple subunits, the way those subunits assemble and arrange themselves—their interfaces, contacts, and overall geometry—defines its quaternary structure. Hemoglobin is a classic example: four subunits assemble into a tetramer that works together to carry oxygen, and that arrangement is essential for its function.

This is different from the primary sequence, which is simply the linear order of amino acids in a single chain. It’s also different from the tertiary structure, which describes the three-dimensional fold of one polypeptide chain. The notion of bonds between amino acids refers to the chemical links and interactions that stabilize the chain or interfaces, but it doesn’t specify how multiple chains come together to form a functional assembly.