Why does the hemoglobin molecule qualify as a quaternary protein structure?

Hemoglobin qualifies as a quaternary protein structure because it is composed of four polypeptide subunits—two alpha and two beta chains—that come together to form a functional protein. This arrangement allows for cooperative binding of oxygen, enhancing its efficiency in oxygen transport. The interactions between the subunits are crucial for the protein's stability and functionality, characteristic of quaternary structures. Additionally, hemoglobin's ability to change conformation upon oxygen binding further exemplifies its complex quaternary nature.