MODELS FOR COOPERATIVE OXYGEN BINDING IN HEMOGLOBIN

Abstract- Equilibrium measurements of oxygen binding by iron(II) and cobalt(II) picket fence porphyrins exhibit p2, AH°, and AS ° values close to those of myoglobin and cobal 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. Recently we have isolated and fully characterized dioxygen adducts of "picket fence " iron porphyrins. Without the agency of a protein these dioxygen complexes are kinetically stable for prolonged periods as crystalline solids and for shorter periods in solution at ambient temperatures. The preparation and characterization of these analogues of the oxygen-binding hemoproteins 1ave been recently reviewed (1). Physical properties of these model dioxygen adducts such as their Mössbauer spectra, magnetic circular dichroism spectra, \)02 infrared bands, magnetic properties, and structural features about iron are closely congruent with those same features of oxyhemoglobin (Hb02), and oxymyoglobin (Mb02). This account summarizes oxygen binding equilibrium studies of iron and cobalt "picket fence " porphyrins and reveals the apparent relationships between the oxygen affinities of synthetic models and those of the hemoproteins. These results support a putative mechanism by which the protein globin could control oxygen affinity in the limiting "T " and "R " quaternary states of hemo-globin and thus effect cooperativity