Heterogeneity between Two α Subunits of α2β2 Human Hemoglobin and O2 Binding Properties: Raman, 1H Nuclear Magnetic Resonance, and Terahertz Spectra.

Following a previous detailed investigation of the β subunit of α2β2 human adult hemoglobin (Hb A), this study focuses on the α subunit by using three natural valency hybrid α(Fe2+-deoxy/O2)β(Fe3+) hemoglobin M (Hb M) in which O2 cannot bind to the β subunit: Hb M Hyde Park (β92His → Tyr), Hb M Saskatoon (β63His → Tyr), and Hb M Milwaukee (β67Val → Glu). In contrast with the β subunit that exhibited a clear correlation between O2 affinity and Fe2+-His stretching frequencies, the Fe2+-His stretching mode of the α subunit gave two Raman bands only in the T quaternary structure. This means the presence of two tertiary structures in α subunits of the α2β2 tetramer with T structure, and the two structures seemed to be nondynamical as judged from terahertz absorption spectra in the 5-30 cm-1 region of Hb M Milwaukee, α(Fe2+-deoxy)β(Fe3+). This kind of heterogeneity of α subunits was noticed in the reported spectra of a metal hybrid Hb A like α(Fe2+-deoxy)β(Co2+) and, therefore, seems to be universal among α subunits of Hb A. Unexpectedly, the two Fe-His frequencies were hardly changed with a large alteration of O2 affinity by pH change, suggesting no correlation of frequency with O2 affinity for the α subunit. Instead, a new Fe2+-His band corresponding to the R quaternary structure appeared at a higher frequency and was intensified as the O2 affinity increased. The high-frequency counterpart was also observed for a partially O2-bound form, α(Fe2+-deoxy)α(Fe2+-O2)β(Fe3+)β(Fe3+), of the present Hb M, consistent with our previous finding that binding of O2 to one α subunit of T structure α2β2 tetramer changes the other α subunit to the R structure.