Effect of the tertiary structure alteration by ligation on the interface contacts between subunits of hemoglobin.

In order to obtain knowledge of molecular mechanism underlying the cooperative ligand binding to hemoglobin (Hb), atomic contacts between subunits in the hypothetical intermediate quaternary structures have been investigated using the atomic coordinates of O2-liganded subunits' within the T-state. The atomic coordinates are obtained by energy minimization of hypothetical atomic coordinate sets in which an O2 molecule is placed near the iron atom of a deoxy subunit. In an 'O2-liganded beta-subunit', drastic atomic displacements occur at the distal side amino-acid residues E11Val and E12Leu in order to produce sufficient space for the O2 molecule, while this kind of atomic displacement cannot be seen in an 'O2-liganded alpha-subunit'. Therefore the features of structural alterations at heme surroundings by O2 ligation are markedly different between alpha- and beta-subunits. When an alpha-subunit in deoxy Hb is replaced by the corresponding 'O2-liganded subunit', tight inter-subunit contacts between alpha 1FG4(92)Arg and the two residues beta 2C3(37)Trp and beta 2C6(40)Arg strengthen greatly, while, when a beta-subunit is replaced by a corresponding O2-liganded one, serious steric hindrances occur between beta 2FG4(97)His and alpha 1CD2(44)Pro. The characteristics of structural alteration confirm that the effect of O2-ligation at alpha-subunit is transmitted to the other subunit through the alpha FG4Arg. However, the role of FG4His in beta-subunit is not as clear as that of the alpha FG4Arg.