O2-BINDING PROPERTIES OF SOME LARVAL HEMOGLOBINS OF CHIRONOMUS TH THUMMI

Some chromatographically separated hemoglobins from Chironomus th. thummi were tested for their oxygen binding properties. In all cases the O2‐affinity showed a characteristic pH‐dependency which can be described as a “simple” Bohr effect. The amplitude of the Bohr effect curves differs from hemoglobin to hemoglobin. The degree of polymerization of these hemoglobins is known from experiments of the Braunitzer group and it becomes evident that this Bohr effect is found both in monomer and in dimer hemoglobins. From the correspondence of the mathematical model with the experiment it can be shown that the Bohr effect is caused by proton dissociation of only one group, having negative interaction to O2‐binding. The possibility is discussed that this dissociable group is localized far from the O2‐binding site and influences the affinity constant through a change of conformation. The half‐value pressure determined experimentally in relation to the pH is therefore the half‐value pressure of the mixture of two “conformation isomers”, each of which is dependent on the pH. The mathematical examination of the model shows the disproportion between the steepness and the amplitude of the Bohr effect curves to be the consequence of a Hill coefficient deviating from n= 1. This coefficient proved to be independent from the pH and shows no relation to the degree of polymerization. The equation describing the Bohr effect curves can be applied also for other hemoglobins of the same type (Gyclostomata) as is shown by the lampetra hemoglobin. Copyright © 1969, Wiley Blackwell. All rights reserved