The O2 equilibria of human adult hemoglobin have been measured in a wide range of solution conditions in the presence and absence of various allosteric effectors in order to determine how far hemoglobin can modulate itsO2 affinity. TheO2 affinity, cooperative behavior, and the Bohr effect of hemoglobin are modulated principally by tertiary structural changes, which are induced by its interactions with heterotropic allosteric effectors. In their absence, hemoglobin is a high affinity, moderately cooperative O2 carrier of limited functional flexibility, the behaviors of which are regulated by the homotropic, O2-linked T/R quaternary structural transition of the Monod-Wyman-Changeux/Perutz model. However, the interactions with allosteric effectors provide such “in- ert” hemoglobin unprecedented magnitudes of func- tional diversities not only of physiological relevance but also of extreme nature, by which hemoglobin can be- have energetically beyond what can be explained by the Monod-Wyman-Changeux/Perutz model. Thus, the het- erotropic effector-linked tertiary structural changes rather than the homotropic ligation-linked T/R quater- nary structural transition are energetically more signif- icant and primarily responsible for modulation of func- tions of hemoglobin.
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