Water, Ions, and Hemoglobin: Effects on Allostery and Polymerization

Proteins that function in aqueous solution can be perturbed by the solvent. Here we present experimental studies on two such interactions in the hemoglobin molecule. (1) Hemoglobin's oxygen binding is altered by introduction of crowding species or osmoticants, such as sucrose, through the linked binding of ions such as Cl or CO2, but not otherwise. This rules out a significant role of buried surface in the allosteric energetics. (2) Sickle hemoglobin (HbS) polymerizes more readily in high concentrations of phosphate buffer. Such polymerization is analyzed quantitatively here for the first time in terms of the double nucleation mechanism. The changes in solubility are found to account for the increase in monomer addition rates and nucleation rates without requiring additional parameter adjustments. In the analysis, we also show how the analytical formulation of HbS nucleation may be adapted to include water that occupies the interstices between the assembled molecules. While such a "correction" has been applied to the equilibrium process, it has not previously been applied to the nucleation process.