Haemoglobin contains four polypeptide chains. This makes it an example of a protein with
- A. primary structure only
- B. secondary structure only
- C. tertiary structure only
- D. quaternary structure
Explanation
Quaternary structure exists whenever a functional protein is built from more than one polypeptide chain. Haemoglobin has two alpha and two beta chains, each holding a haem group, and their cooperative interaction is what produces the sigmoid oxygen dissociation curve. A single-chain protein such as myoglobin stops at tertiary structure.