Reactivity differences between hæmoglobins. Part VI. The reaction of human methæmoglobins A and C with azide ion, and the concept of “hæm-linked” groups

Abstract

The formation constants of the azide complexes of human methæmoglobins A and C have been measured as a function of pH and temperature. The hydrogen-ion uptake in these reactions has been measured as a function of pH and temperature. The customary interpretation of the pH-dependence of the affinity of a methæmoglobin for a ligand in terms of one or two “hæm-linked” groups is shown to be invalid, and an alternative mechanism is proposed. The wider implications of this mechanism are discussed.