Evidence for heme pi cation radical species in compound I of horseradish peroxidase and catalase.

Magnetic circular dichroism spectra are reported for the compound I species of beef liver catalase (hydrogen-peroxide: hydrogen-peroxide oxidoreductase, EC 1.11.1.6) and horseradish peroxidase (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7) and the pi cation radical derivatives of porphyrins that have been suggested as models of the electronic configuration of the heme in the compound I species of these enzymes. Comparison of the magnetic circular dichroism spectra of the compound I species with the spectra of [Co(octaethylporphyrin)]2Br and [Co(octaethylporphyrin)]2ClO4 indicates that in both the intermediate enzyme species the heme has been oxidized to a pi cation radical. While there is a clear distinction between the magnetic circular dichroism spectra of the 2A2u porphyrin [Co(III)octaethylporphyrin]2ClO4, and the 2A1u porphyrin, [Co(III)octaethylporphyrin]2Br, such specific differences are not observed in the spectra of the two enzymes. Analysis of our data suggests that the ground states in the two enzymes are far more similar than the ground states in the two model porphyrins.