Electromagnetic Properties of Hemoproteins

Abstract Paramagnetic complexes of iron (III), manganese (II), and cobalt (II) protoporphyrins IX and apohemoproteins react with nitric oxide to form spectroscopically distinct compounds. Optical and electron paramagnetic resonance (EPR) studies suggest that a 1-electron transfer between nitric oxides and metal ions which occurs in these reactions results in the formation of spin-paired complexes of these metalloporphyrins with nitric oxide. However, manganese (III) and cobalt (III) protoporphyrin-apohemoprotein complexes do not react with nitric oxide. Ferrous hemoproteins react reversibly with nitric oxide to form paramagnetic complexes, which exhibit an intense EPR absorption centered around g = 2.0. The complexes of nitric oxide with ferrous cytochrome c peroxidase and horseradish peroxidase show well defined EPR absorptions with rhombic symmetry (gx = 2.08, gy = 1.96, and gz = 2.004). The hyperfine structure in the z absorption has been analyzed through isotope substitutions with 15NO and 57Fe-enriched heme. It is concluded that the unpaired electron of nitric oxide is considerably delocalized and that the proximal ligand of both peroxidases is a nitrogen-containing group, probably a histidyl residue. Hemoproteins have been arbitrarily classed into two groups on the basis of the EPR characteristics of their nitric oxide complexes: Type I (gy l 1.96) and Type II (gy g 1.97). Cytochrome c peroxidase, horseradish peroxidase, and lactoperoxidase belong to Type I, whereas cytochrome oxidase, catalase, cytochrome c, hemoglobin, and myoglobin belong to Type II. EPR parameters of nitric oxide complexes of ferrous hemoproteins are not significantly affected by substitutions of two vinyl groups (protoheme) at positions 2 and 4 of the porphyrin ring with two ethyl groups (mesoheme) or hydrogens (deuteroheme), indicating that the nucleophilicity of porphyrin side chains has no appreciable effect on the metal-ligand bond strength.