Reactivity differences between haemoglobins I. The ionization of human methaemoglobins A, S and C

Abstract

The ionization of human methaemoglobins A, S and C to their alkaline forms are measured at various ionic strengths and temperatures. An extrapolation function suggested by the theoretical work of Kirkwood, is used to calculate thermodynamic p K ’s, and hence standard free energies of ionization. Standard enthalpies and entropies of ionization are calculated from the temperature dependence of the ionization constants. Possible mechanisms by which the protein may affect the reactivity of the iron atoms are discussed, and it is concluded that the different therm odynamics of ionization of human methaemoglobins A, S and C are attributable to different electrostatic interactions in these haemoglobins arising from the known amino acid differences between them . The possible importance of electrostatic interactions in the reaction of different haemoglobins with oxygen and carbon monoxide is discussed.