Which protein is an example of a quaternary structure?

• A. Myoglobin
• B. Hemoglobin
• C. Collagen
• D. Albumin

Answer: (B)

Quaternary structure comes from assembling multiple polypeptide chains into a functional protein complex. Hemoglobin is the classic example because it consists of four polypeptide subunits (two alpha and two beta) that come together to form a tetramer. These subunits interact with each other, and this arrangement enables cooperative binding of oxygen—each subunit’s state can influence the others, which is a hallmark of quaternary structure.

Myoglobin, by contrast, is a single polypeptide chain, so it only has tertiary structure and no multi-subunit assembly. Albumin is also a single-chain protein, not a complex of separate subunits. Collagen contains three polypeptide chains, forming a triple-helix within one molecule, which is a special higher-order arrangement but not the classic multi-subunit quaternary protein complex that hemoglobin showcases.