An Origin of Cooperative Oxygen Binding of Human Adult Hemoglobin: Different Roles of the α and β Subunits in the α2β2 Tetramer

Human hemoglobin (Hb), which is an alpha(2)beta(2) tetramer and binds four O-2 molecules, changes its O-2-affinity from low to high as an increase of bound O-2, that is characterized by 'cooperativity'. This property is indispensable for its function of O-2 transfer from a lung to tissues and is accounted for in terms of T/R quaternary structure change, assuming the presence of a strain on the Fe-histidine (His) bond in the T state caused by the formation of hydrogen bonds at the subunit interfaces. However, the difference between the alpha and beta subunits has been neglected. To investigate the different roles of the Fe-His(F8) bonds in the alpha and beta subunits, we investigated cavity mutant Hbs in which the Fe-His(F8) in either alpha or beta subunits was replaced by Fe-imidazole and F8-glycine. Thus, in cavity mutant Hbs, the movement of Fe upon O-2-binding is detached from the movement of the F-helix, which is supposed to play a role of communication. Recombinant Hb (rHb)(alpha H87G), in which only the Fe-His in the a subunits is replaced by Fe-imidazole, showed a biphasic O-2-binding with no cooperativity, indicating the coexistence of two independent hemes with different O-2-affinities. In contrast, rHb(beta H92G), in which only the Fe-His in the beta subunits is replaced by Fe-imidazole, gave a simple high-affinity O-2-binding curve with no cooperativity. Resonance Raman, H-1 NMR, and near-UV circular dichroism measurements revealed that the quaternary structure change did not occur upon O-2-binding to rHb(alpha H87G), but it did partially occur with O-2-binding to rHb(beta H92G). The quaternary structure of rHb(alpha H87G) appears to be frozen in T while its tertiary structure is changeable. Thus, the absence of the Fe-His bond in the alpha subunit inhibits the T to R quaternary structure change upon O-2-binding, but its absence in the beta subunit simply enhances the O-2-affinity of a subunit.