Osmometric study of the subunit dissociation of hemoglobin Porto Alegre [beta 9(A6)Ser----Cys] dissulfide polymer

The hemoglobin Porto Alegre (HbPA) disulfide polymer dodecamer and the HbPA, HbA disulfide polymer octamer subunit dissociation by NaCl was studied by measuring the osmotic pressure of CO-hemoglobin solutions at pH = 6.9 and 20 degrees C. The dissociation equilibrium constants were evaluated from the osmotic pressure data. The subunit dissociation of the two types of disulfide polymers follows a reaction of the type A3 in equilibrium 3A. The HbPA disulfide polymer dodecamer appears to be more susceptible to NaCl induced dissociation than is normal HbA and less resistant to dissociation on protein dilution. The HbPA, HbA disulfide polymer octamer appears to be much more susceptible to dissociation on protein dilution than is the HbPA disulfide polymer dodecamer and normal HbA. The standard free energy of dissociation of the HbPA disulfide polymer at 2 M NaCl is delta GOD(25 degrees C) = 12 kcal/mole. The type of dissociation reaction (A3 in equilibrium with 3A) support the conclusion that the HbPA disulfide polymer has a quaternary molecular structure of a closed ring of three tetramers.