The Electronic Structure of Protoheme Proteins

Abstract

In this paper we present probable structures for derivatives of horseradish and cytochrome c peroxidases and also propose a simple scheme which encompasses the molecular structures and electronic configurations of the derivatives of these and other protoheme proteins in the oxidation states from II to VI. Four lines of evidence are used to define the structures of the heme groups and their associated ligands. These are: low temperature optical spectra in the visible region, electron paramagnetic resonance spectra taken at 1.5°K, the oxidation state of the compounds, and proof of the identity of the ligands in position 6 of the heme iron atom. Only the ferrous and ferric forms of heme iron are needed to describe all of these compounds, and higher oxidation states of the iron atom need not be invoked. A nomenclature describing the electronic configuration of the heme and its associated ligands is presented, which specifies for each compound, the d electron configuration of the heme, the structure of the sixth ligand of the heme iron atom, and the oxidation state of the compound. The electronic configuration of the iron, ferrous or ferric, is not necessarily related to the oxidation state of the compound. An extension of our reasoning leads us to conclude that the electronic configurations of oxyhemoglobin, oxymyoglobin, and oxyperoxidase are the same, and that the oxygenation reaction involves the migration of an electron from the ferrous heme iron to the oxygen molecule. In consequence, the heme iron atoms of these compounds are formally in the ferric low spin state, and the oxygen molecules in the sixth ligand positions each carry 3 oxidizing eq.