PHYSICOCHEMICAL STUDIES OF THE RELATION BETWEEN STRUCTURE AND FUNCTION IN HEMOGLOBIN HIROSHIMA (HC3β, HISTIDINE →ASPARTATE)

摘要

Various physicochemical properties of Hemoglobin Hiroshima (HC3β, histidine-aspartate) were studied to clarify the mechanism for the alte ed oxycen equilibrium functions of the hemoglobin and the sole of the C-terminal histidine residues of the β chains. Thecooperativity in oxygen binding of Hb Hiroshima is markedly diminished by stripping it of 2,3-diphos-phoglycerate (DPG). The effect of DPG on the oxygen equilibrium of Hb Hiroshima is, however, apparently normal because its overall oxygen affinity exhibits the same dependence on the concentration of DPG as that of Hb A and the overall free energy of interaction among the binding sites of oxygen of Hb Hiroshima increases on the addition of DPG by an extent similar to that of Hb A. Ultraviolet difference spectra suggest that the conformational changes of the a β 2 contacts that occur during oxygenation are impaired in Hb Hiroshima corresponding to its diminished coopera-tivity and that (he penultimate tyrosine residues of the β chains in lib Hiroshima undergo environ¬mental changes similar to those which occur in Hb A during oxygenation. The intrinsic microscopic equilibrium constant for the 4th stage of oxygena¬tion, k4 for Hb Hiroshima is insensitive to the concentration of DPG and nearly equal to that for Hb A, whereas the constant for the first stage of oxygenation, k1, decreases on the addition of DPG and is always larger than that of Hb A, irrespective of DPG concentration, indicating that deoxy-Hb Hiroshima probably assumes a more unconstrained form than deoxy-Hb A. The reaction rates of 4,4'-dipyridinedisulfide with the sulfhydryl groups of oxy- and deoxy-Hb Hiroshima were more rapid than for Hb A, particularly that of the deoxy form. Electron paramagnetic resonance spectra of NO Hb (nitric oxide derivative of hemoglobin) Hiroshima are identical with those of NO Hb A.