Oxygen affinity of bovine haemoglobin. Relevance of electrostatic and hydrophobic interactions.

We have studied the effects of organic cosolvents (monohydric alcohols and formamide) on the oxygen equilibrium of bovine haemoglobin and have compared them with the effects of the same cosolvents on the oxygen equilibrium of human haemoglobin. Our results indicate: (1) that in agreement with previous suggestions, the lower affinity of bovine haemoglobin for oxygen is not due to an increased number of salt bridges stabilizing the T structure; (2) that, following T----R transition, more hydrophobic surface is exposed to the solvent by bovine than by human haemoglobin. We suggest, therefore, that a relevant role in keeping the oxygen affinity of bovine haemoglobin lower than that of human haemoglobin is played by the higher free energy needed to expose this more hydrophobic surface to the solvent. We stress, however, that our analysis does not enable us to say which particular amino acid residues are concerned in these effects.