Hemoglobin A: an electron paramagnetic resonance study of the effects of interchain contacts on the heme symmetry of high-spin and low-spin derivatives of ferric alpha chains.

Hemoglobin, the oxygen-binding heme protein of blood is composed of four subunits of two types, alpha and beta. Although the protein when isolated is diamagnetic, by oxidation it can be converted to various paramagnetic ferric forms exhibiting specific electron paramagnetic resonance (EPR) spectra. Using electron paramagnetic resonance, we have shown that the symmetry of the heme of isolated ferric alpha chains is different from that of the heme of these same subunits when associated with ferric beta chains in the parent tetramer. The high-spin form of the isolated ferric alpha chains shows a clearly resolved departure from tetragonal symmetry in contrast to ferrihemoglobin A, where the environment of the iron of all four chains is nearly tetragonal. This difference in heme symmetry suggests that the maintenance of tetragonal symmetry in the hemoglobin tetramer must be brought about by the configuration the alpha chains assume due to their contact with the beta chains. When isolated from the tetramer, some parts of the alpha chain must undergo physical displacement from a configuration that had been stabilized by the beta chains.Although differences in symmetry between monomer and tetramer are seen with high-spin forms, no such difference exists with two low-spin ferric forms. Here the symmetries of iron are primarily governed by the immediate ligand environment of the iron and not by the state of association of the protein.