Valency hybrid hemoglobins with special attention to subunit organization.

To examine the effects of ferric spin state on the allosteric properties and the structure at the alpha 1 beta 2 subunit interface, we measured circular dichroism (CD) and deoxy minus oxy difference spectra in the ultraviolet (UV) region as well as the oxygen equilibrium curves for various ferric ligand complexes of valency hybrid hemoglobins (alpha +2 beta 2 and alpha 2 beta +2 : + signifies ferric heme). Analysis of the oxygen equilibrium curves for the hybrids in terms of the two-state allosteric model showed that the spin state of ferric subunits alters not only the allosteric equilibrium but also the oxygen affinity of the T state. The CD and difference spectral data strongly suggested that in both type of hybrids the alpha 1 beta 2 boundary structure of the high spin derivatives is different from that of the low spin ones. From these results, we concluded that the structure at the alpha 1 beta 2 interface is important for determining the oxygen affinity of the T state.