Models for cooperative oxygen binding in hemoglobin

Equilibrium measurements of oxygen binding by iron(II) and cobalt(II) picket fence porphyrins exhibit pio1/2 2 ΔH°, and ΔS° values close to those of myoglobin and cobalt myoglobin respectively. In contrast the CO affinities of simple iron(II) porphyrins are much greater than those of the hemoproteins, hemoglobin (Hb) and myoglobin (Mb). This difference is apparently caused by distal residues in Hb and Mb. With sterically constrained axial bases iron(II) and cobalt(II) picket fence porphyrins exhibit lower oxygen affinities in solution–thus modeling the “T” form of Hb. In the solid state two picket fence iron(II) porphyrins exhibit reversible cooperative oxygen binding.