Cooperative Ligand Binding by Ferrihemoglobin

Abstract

Reactions of human ferrihemoglobin with azide and fluoride ions have been reinvestigated, particularly with a view to compare the effect of pH on the Hill coefficient and to study its dependence on observation wavelength. The results at pH 6 (with initial high‐spin aquo form) show the absence of cooperativity for the fluoride reaction but the existence of significant co‐operativity for the azide reaction, in agreement with the conclusions drawn by some previous authors. At pH 9, where the ferrihemoglobin is an equilibrium mixture of high and low‐spin states, the same reactions give different Hill coefficients; cooperativity appears for fluoride binding and diminishes for the azide reaction. The possible significance of concomitant changes of iron spin state and the appearance of cooperativity in hemoglobin reactions is discussed.