Final answer:
Hemoglobin, composed of four subunits, exhibits a quaternary structure integral to its role in transporting oxygen in the bloodstream.
Step-by-step explanation:
A protein such as hemoglobin, which is composed of more than one protein subunit, is said to have a quaternary structure. Hemoglobin is a globular protein important for oxygen transport found in red blood cells. It is made up of four polypeptide chains (α1, α2, β₁, and β₂ subunits), and each of these subunits binds to a heme group that contains one iron ion. This iron ion is crucial for the binding and release of oxygen. The four subunits of hemoglobin fit together in a precise manner, allowing it to carry out its function effectively.