Hemoglobin function in the water-ethylene glycol cosolvent system: linkage between oxygen binding and hydration

The effects of ethylene glycol (EG) on the oxygen binding properties of human hemoglobin are described in this report. Under the conditions used, the hemoglobin molecule remains in the intact tetrameric form in up to 70% (w/w) EG, corresponding to a mole fraction of EG of 0.4. Interaction between the cosolvent and the hemoglobin is quite weak. Only at high concentrations of EG are the effects on the oxygen binding curve detectable. In the range of mole fraction of EG up to 0.2, oxygen affinity is decreased. In the range of mole fraction of EG between 0.2 and 0.4 (corresponding to molar concentrations of 8-12 M EG), hemoglobin oxygen affinity increases, eventually becoming higher than the value obtained in the absence of EG. Experiments were carried out in the presence of 0.013, 0.10, and 1.0 M NaCl to evaluate the linkage between EG and chloride as allosteric effectors and the possible general effect of ionic strength on oxygen binding properties of hemoglobin in the presence of cosolvent. The effects of EG on hemoglobin ligation are discussed in terms of a model in which EG interacts with hemoglobin in a weak allosteric fashion at the lower concentration range (less than mole fraction of 0.2) while at the higher range (mole fraction of 0.2-0.4) perturbations of protein hydration lead to stabilization of the high-affinity form of hemoglobin.