An Origin of Cooperative Oxygen Binding of Human Adult Hemoglobin: Different Roles of the [alpha] and [beta] Subunits in the [alpha].sub.2[beta].sub.2 Tetramer

• Published in: PloS one 2015-08, Vol.10 (8)
• Main Authors: Nagatomo, Shigenori, Nagai, Yukifumi, Aki, Yayoi, Sakurai, Hiroshi, Imai, Kiyohiro, Mizusawa, Naoki, Ogura, Takashi, Kitagawa, Teizo, Nagai, Masako
• Format: Article
• Language: English
• Subjects: Chemical properties; Hemoglobins; Histidine; Hydrogen bonds; Physiological aspects
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0135080

Human hemoglobin (Hb), which is an [alpha].sub.2 [beta].sub.2 tetramer and binds four O.sub.2 molecules, changes its O.sub.2 -affinity from low to high as an increase of bound O.sub.2, that is characterized by 'cooperativity'. This property is indispensable for its function of O.sub.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.sub.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 [alpha] subunits is replaced by Fe-imidazole, showed a biphasic O.sub.2 -binding with no cooperativity, indicating the coexistence of two independent hemes with different O.sub.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.sub.2 -binding curve with no cooperativity. Resonance Raman, .sup.1 H NMR, and near-UV circular dichroism measurements revealed that the quaternary structure change did not occur upon O.sub.2 -binding to rHb([alpha]H87G), but it did partially occur with O.sub.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.sub.2 -binding, but its absence in the [beta] subunit simply enhances the O.sub.2 -affinity of [alpha] subunit.