Adult hemoglobin (iron-containing protein) can carry up to four oxygen molecules per hemoglobin molecule, with the addition of oxygen proceeding in a step-wise fashion. For the third step, the value of the equilibrium constant Keq=5.86X10-4 although the reaction has a ΔH∘ value of -2.9 KJ/mol. (Remember that the system is not at room temperature)

Heme-Fe2+-(O2)2 + O2⇌ Heme-Fe