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

• Published in: PloS one 2015-08, Vol.10 (8), p.e0135080-e0135080
• 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: Adult; Affinity; alpha-Globins - chemistry; alpha-Globins - metabolism; Antifungal agents; beta-Globins - chemistry; beta-Globins - metabolism; Binding; Biochemistry; Circular Dichroism; Coexistence; Cooperativity; Dichroism; Glycine; Glycine - chemistry; Glycine - metabolism; Health sciences; Hemoglobin; Hemoglobins - chemistry; Hemoglobins - metabolism; Histidine; Histidine - chemistry; Histidine - metabolism; Humans; Hydrogen; Hydrogen Bonding; Hydrogen bonds; Imidazole; Imidazoles - chemistry; Imidazoles - metabolism; Interfaces; Iron; Iron - chemistry; Iron - metabolism; Life sciences; Lungs; Magnetic Resonance Spectroscopy; Models, Molecular; Molecular chains; NMR; Nuclear magnetic resonance; Oxygen; Oxygen - chemistry; Oxygen - metabolism; Pharmaceuticals; Protein Binding; Protein Multimerization; Protein structure; Protein Structure, Quaternary; Quaternary; Quaternary structure; Spectrophotometry; Spectrum Analysis, Raman; Tertiary structure
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0135080

Human hemoglobin (Hb), which is an α2β2 tetramer and binds four O2 molecules, changes its O2-affinity from low to high as an increase of bound O2, that is characterized by 'cooperativity'. This property is indispensable for its function of O2 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 α and β subunits has been neglected. To investigate the different roles of the Fe-His(F8) bonds in the α and β subunits, we investigated cavity mutant Hbs in which the Fe-His(F8) in either α or β subunits was replaced by Fe-imidazole and F8-glycine. Thus, in cavity mutant Hbs, the movement of Fe upon O2-binding is detached from the movement of the F-helix, which is supposed to play a role of communication. Recombinant Hb (rHb)(αH87G), in which only the Fe-His in the α subunits is replaced by Fe-imidazole, showed a biphasic O2-binding with no cooperativity, indicating the coexistence of two independent hemes with different O2-affinities. In contrast, rHb(βH92G), in which only the Fe-His in the β subunits is replaced by Fe-imidazole, gave a simple high-affinity O2-binding curve with no cooperativity. Resonance Raman, 1H NMR, and near-UV circular dichroism measurements revealed that the quaternary structure change did not occur upon O2-binding to rHb(αH87G), but it did partially occur with O2-binding to rHb(βH92G). The quaternary structure of rHb(αH87G) appears to be frozen in T while its tertiary structure is changeable. Thus, the absence of the Fe-His bond in the α subunit inhibits the T to R quaternary structure change upon O2-binding, but its absence in the β subunit simply enhances the O2-affinity of α subunit.