Reactivity differences between haemoglobins. Part XVI. A correlation between the amino-acid composition of methaemoglobins and their reactivity towards azide ion

Abstract

The standard free-energies and enthalpies of formation of the azide complex of three human abnormal methaemoglobins, human fetal methaemoglobin F_II, and five animal methaemoglobins have been determined as a function of pH. For all the methaemoglobins –ΔH° passes through a maximum at a pH, pH_ch, characteristic of the particular methaemoglobin. There is a correlation between pH_ch and a simple function of the number of lysine, arginine, glutamic acid, and aspartic acid residues in the molecules. The pK values of ionization of methaemoglobins F_II, D_Ibadan, and N_Baltimore have been determined as a function of temperature and ionic strength. The variation of pK with ionic strength and the difference between the values of the pK values at zero ionic strength are shown to be consistent with the dielectric cavity model for the protein used in earlier papers in the series.