Heterotropic effectors control the hemoglobin function by interacting with its T and R states—a new view on the principle of allostery

Abstract

Careful analyses of precise oxygenation curves of hemoglobin (Hb) clearly indicate that, contrary to the common belief, allosteric effectors exert a dramatic control of the oxygenation characteristics of the protein by binding not only to the T (unligated), but also to the R (ligated) state, in a process that is proton-driven and involves proton uptake. The most striking functional changes were obtained when the allosteric effectors were bound to the fully ligated Hb: the oxygen affinity decreased dramatically, Bohr effect was enhanced, and cooperativity of oxygen ligation was almost absent, emulating a Root effect-like behavior. However, structural analysis, such as Cysβ93 sulfhydryl reactivity and ultraviolet circular dichroism, confirmed that the ligated Hb was in fact in the R state, despite its extremely low affinity state features. These findings provide a new global view for allosteric interactions and invoke for a modern interpretation of the role of allosteric effectors and a reformulation of the Monod–Wyman–Changeaux model for control of allosteric systems, and other complementary models as well.