Myoglobin (Mb) and horseradish peroxidase (HRP) were both reconstituted with a meso-unsubstituted iron corrole and their electronic configurations and peroxidase activities were investigated. The appearance of the 540 nm band upon incorporation of the iron corrole into apoMb indicates axial coordination by the proximal histidine imidazole in the Mb heme pocket. Based on (1)H NMR measurements using the Evans method, the total magnetic susceptibility of the iron corrole reconstituted Mb was evaluated to be S = 3/2. In contrast, although a band does not appear in the vicinity of 540 nm during reconstitution of the iron corrole into the matrix of HRP, a spectrum similar to that of the iron corrole reconstituted Mb is observed upon the addition of dithionite. This observation suggests that the oxidation state of the corrole iron in the reconstituted HRP can be assigned as +4. The catalytic activities of both proteins toward guaiacol oxidation are quite different; the iron corrole reconstituted HRP decelerates H(2)O(2)-dependent oxidation of guaiacol, while the same reaction catalyzed by iron corrole reconstituted Mb has the opposite effect and accelerates the reaction. This finding can be attributed to the difference in the oxidation states of the corrole iron when these proteins are in the resting state.