The Electronic Structure of Protoheme Proteins: I. AN ELECTRON PARAMAGNETIC RESONANCE AND OPTICAL STUDY OF HORSERADISH PEROXIDASE AND ITS DERIVATIVES

Abstract

Abstract This study establishes facts required to deduce the structures of the several higher oxidation states of protoheme proteins. Low temperature optical spectra (15–77°K) are correlated with electron paramagnetic resonance (EPR) spectra (1.4°K) of horseradish peroxidase and its derivatives. These include the high spin and low spin forms of ferri-horseradish peroxidase, the ferro- and ferri-horseradish peroxidase cyanides, ferro-horseradish peroxidase, oxyperoxidase, carbon monoxide-ferro-horseradish peroxidase, Compound I, and Compound II of horseradish peroxidase. Solutions of ferri-horseradish peroxidase, prepared at pH 7.0 to 8.4, exhibit optical and magnetic spectra which can be ascribed to a single paramagnetic species, high spin heme iron. The EPR signal indicates that the heme of peroxidase exists in an environment which has lower symmetry than axial. Solutions of ferri-peroxidase prepared at pH 11.4 exhibit optical and EPR spectra which can be ascribed to low spin heme iron. The optical spectrum of oxyperoxidase shows little change with temperature, indicating that it is not an equilibrium mixture of species whose relative abundance is affected by temperature. EPR spectra of oxyperoxidase show no signal attributable to oxyperoxidase and therefore it is probably diamagnetic, as is oxyhemoglobin. The wave length maxima of the low temperature optical spectrum of oxyperoxidase are the same as those of the low temperature spectrum of the low spin (alkaline) form of ferri-peroxidase. Similarly, the wave length maxima of the low temperature optical spectrum of oxymyoglobin coincide with those of the low temperature spectrum of low spin (alkaline) ferri-myoglobin. Optical spectra of both Compound I and Compound II of horseradish peroxidase change little with temperature, indicating that neither of these compounds is an equilibrium mixture of species whose relative abundance is affected by temperature. EPR spectra at 1.4°K of Compounds I and II of peroxidase show no signals attributable to the compounds.