Catalase model systems. Decomposition of hydrogen peroxide catalysed by mesoferrihaem, deuteroferrihaem, coproferrihaem and haematoferrihaem

The catalytic decomposition of H2O2 by deuteroferrihaem, mesoferrihaem, coproferrihaem and haematoferrihaem was studied as a model for the mechanism of action of catalase. For haematoferrihaem, anomalous but reproducible results were obtained, which could not be adequately explained. For each of the other ferrihaems studied, both monomeric and dimeric species catalysed decomposition, although the activity of monomer (aM) was much greater than that of dimer (aD). The pH variation of aD in the range 6.5–11 was consistent with an inverse dependence on [H+]1/2. The molecular mechanism whereby such a dependence could be achieved is not apparent. A study of the pH-dependence of aM in the range 6.5–11 revealed a linear inverse relationship with [H+]. This is interpreted in terms of attack by HO2- on ferrihaem monomer. The specific pH-independent rate constants for this reaction were in the order coproferrihaem greater than protoferrihaem greater than or equal to mesoferrihaem congruent to deuteroferrihaem. The order of magnitude of these rate constants is the same as that for catalysis by Fe(H2O)63+ and the second-order rate constant for decomposition of H2O2 by catalase. The implications on the mechanism of action of catalase are discussed.