Symmetry distortion in the human hemoglobin tetramer induced by asymmetric ligation.

To investigate the conformational changes in human tetrameric (αβ)(2) hemoglobin upon binding of the first two ligands, we have measured the kinetics of reactions between 4,4'-dithiodipyridine and β93Cys sulfhydryl groups of four diliganded hemoglobins by using CO-bound Fe(II)-Ni(II) hybrids with and without β-β cross-linking. The data show that all the diliganded intermediates have high sulfhydryl reactivities, which are greater than or equal to that for the fully-liganded end state, especially when containing liganded α subunit(s). The results also reveal that both the asymmetrically (α1β1 and α1β2) diliganded species show similar high rates of sulfhydryl reactivity and biphasic kinetics, suggesting a new conformation but only slight functional distortion caused by asymmetric ligation.