On the state of the iron and the nature of the ligand in oxyhemoglobin

Abstract

The structure of the oxygenated heme group of oxyhemoglobin may be formulated as [Hb(Heme d(1/2) (5)).OO(-)]. The heme iron atom is formally ferric, and the ligand is bound superoxide anion. When deoxyhemoglobin combines reversibly with oxygen a partial transfer of an electron occurs from the ferrous iron atom to the oxygen molecule. By surrendering its electron the iron atom has become ferric; in accepting an electron the ligated oxygen molecule has become a new species, the bound superoxide anion (.OO(-)). The configuration of the heme iron atom is deduced from comparison of the optical spectrum in the visible region of oxyhemoglobin with that of alkaline ferric hemoglobin whose configuration is established by electron paramagnetic resonance spectroscopy. The configuration of both species is low spin ferric heme iron (Heme d(1/2) (5)). The configuration of the ligated oxygen molecule of oxyhemoglobin is not accessible to study by magnetic or optical probes. However it may be known by analogy with the configuration of the ligated oxygen molecule of reversibly oxygenated cobalt complexes whose structure has been proved by both electron paramagnetic resonance and x-ray diffraction analysis. It is bound superoxide anion (.OO(-)). Other physical studies bearing on the structure of the oxygenated heme group are discussed. Reasons are given for believing that the proposed formulation of the oxyhemoglobin structure is consistent with the known stability of oxyhemoglobin.