The association reaction between hemoglobin and carbon monoxide as studied by the isolation of the intermediates : implications on the mechanism of cooperativity

The concentrations of the intermediates in the association reaction between human hemoglobin and CO at 20 degrees C, pH 7, under conditions of negligible dissociation of the ligand, were measured by cryogenic techniques. The monoligated species were predominant at all values of overall ligand bound studied. The analysis of the experimental data assuming a scheme of four consecutive reactions indicated that the binding rates increased in a continuous fashion. A significant acceleration after the binding of the second molecule of ligand occurred in the presence of 0.1 M KCl, but not with the addition of an excess of inositol hexaphosphate, indicating that major functional, and possibly structural, transitions occur at the diligated state. Differences in the concentrations of the intermediates in the same state of ligation were observed under all conditions. The analyses of the data on the basis of schemes of multiple pathways of reaction indicated that the beta subunits reacted about 1.5 times faster than the alpha subunits in the first ligation reaction. After the addition of inositol hexaphosphate, the alpha subunits reacted about 1.5 times faster than the beta subunits in the first ligation step, but the overall rate of the first CO binding step was unchanged.