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

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Abstract
The catalytic decomposition of H2O2 by deuteroferriheme, mesoferriheme, coproferriheme and haematoferriheme was studied as a model for the mechanism of action of catalase (EC 1.11.1.6). For hematoferriheme, anomalous but reproducible results were obtained, which could not be adequately explained. For each of the other ferrihemes studied, both monomeric and dimeric species catalyzed 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 ferriheme monomer. The specific pH-independent rate constants for this reaction were in the order coproferriheme > protoferriheme .gtoreq. mesoferriheme .simeq. deuteroferriheme. The order of magnitude of these rate constants is the same as that for catalysis by Fe(H2O)63+ and the 2nd-order rate constant for decomposition of H2O2 by catalase. The implications on the mechanism of action of catalase are discussed.