Interaction of hemoglobin with ions: Allosteric effects of the binding of anions

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Abstract

The influence of several anions, such as ATP, 2,3-diphosphoglycerate, inositol hexaphosphate, K4[Fe(CN)6] and K3[Co(CN)6], on the O2 binding of Hb A and the binding of n-propyl isocyanide to Hb A dependent upon pH was investigated. Known organic anions as well as inorganic polyanions and even inorganic phosphate ions decreased the affinity of Hb for ligands and increased the Bohr effect. The allosteric effectiveness of the anions investigated varied and was most pronounced with inositol hexaphosphate. By graphical extrapolation of the curves obtained from the pH dependence, which agree with a theoretical curve based on four equilibria, a shift of the ionization constant of a “heme-linked” group to a higher pK′ in the presence of allosteric anions was demonstrated.

Furthermore it was demonstrated by ESR measurements that in the presence of inorganic phosphate, the high-low-spin ratio is changed by enhanced high-spin proportion, suggesting that the contact of iron with the proximal histidine (F8) is weakened. Finally, a favoring of tetramerization in the presence of ATP was found indicating that the α1β2-contact plane might be changed by allosteric anions.

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