Catalatic activity of iron(III)-centred catalysts. Role of dimerization in the catalytic action of ferrihaems

1. The specific stoicheiometric catalatic activity of deuteroferrihaem is 10–100-fold greater than that for protoferrihaem, depending on pH. It is suggested that the difference in activity may be related to quantitative differences in the extent of dimerization in aqueous solutions of proto- and deutero-ferrihaem (Brown, Dean & Jones, 1970 b ). 2. A quantitative comparison of the kinetic and equilibrium data implies that the catalytic activities of ferrihaems are determined by the proportion of monomer present. The specific activity of ferrihaem monomer calculated varies inversely with H + ion concentration and attains a value equal to the maximal activity of catalase at pH>p K a (H 2 O 2 ). 3. A comparison of catalatic behaviour in the series of iron(III)-centred catalysts aqua-iron(III) ion, ferrihaem monomer and catalase suggests that the unique feature of catalase action resides in the pH-independence of the reaction.