Conformational change and cooperative ligand binding in hemoglobin.

Although many studies have been carried out on the cooperativityly of ligand binding to hemoglobin, its molecular mechanism is not yet completely confirmed. Various models have been proposed on one hand, but on the other hand the direct judgement of the validity of each model is confronted with experimental difficulties. With this situation in mind, we re-examine carefully various kinds of experimental data, with the intention of finding any hints for future studies on the molecular mechanism of the cooperativity of ligand binding not only to hemoglobin but also to other allosteric proteins composed of subunits. As a result of these re-examinations most of the experimental data can be consistently explained from the viewpoint that the strain due to ligation is transmitted to the interfaces of the subunits and the degree of the strain stored in the interfaces between the subunits controls the cooperativity of of ligand binding. The subjects selected to the present study are molecular structural differences between the deoxy- and oxy-hemoglobin, temperature dependence of oxygen equilibrium constants, and recombination curves of carbon monoxide in flash photolysis experiments. In the first section, the difference of the interfaces of the subunits between deoxy- and oxy-hemoglobin is investigated on the basis of the atomic coordinates determined by Perutz et al. and we examine energetically which of the segment's pairs contributes mainly to the difference in the molecular structure between the two forms. On the basis of this investigation, it is shown in the next section that the oxygen equilibrium constants of human adult hemoglobin under normal physiological conditions are well explained by the following sequence of the conformational changes. The local changes in the interfaces of the subunits are reflected directly on the small differences among the first three Adair constants and the succeeding rearrangement to the four subunits causes the outstandingly larger value of the fourth Adair constant. In more alkaline pH, this rearrangement begins to occur at the earlier stages of ligation. In the succeeding section, it is pointed out that the property of this rearrangement can be studied in more detail through the recombination curves followed by flash photolysis under special conditions.