Oxygen affinity of hemoglobin.
The quaternary hemoglobin structure of hemoglobin determines it's affinity to oxygen. In deoxygenated hemoglobin, globin chains are tightly bound thus they close the oxygen-binding sites of hemoglobin (Tensed hemoglobin). When the first molecule binds to hemoglobin, globin chains open thus exposing the oxygen binding site of hemoglobin. So binding of oxygen increases the affinity to it by 500. Combination of the first heme in the Hemoglobin molecule with O2 increases the affinity of the second heme for O2 , and oxygenation of the second increases the affinity of the third, and so on, so that the affinity of Hb for the fourth O2 molecule is many times that for the first. In tissues, this reaction is reversed.