Catalytic mechanisms and reactivity of peroxidases, catalases and iron–porphyrin model systems

Abstract

Water-soluble iron(III)–porphyrins display both peroxidase and catalase-like properties [1, 2]. In both enzymic and model systems the basic catalytic mechanism is: Catalyst + Oxidising Substrate → Oxidised Catalyst Intermediate Oxidised Catalyst + Reducing Substrate → Catalyst + Oxidised Product The kinetics of both phases of the catalytic cycles have been studied with the objective of elucidating the factors (steric, electronic, electrostatic) which influences reactivity, the natures of the intermediates and the mechanistic origins of enzymic reactivity and selectivity. The reactions of peroxidases (different proteins with the same iron-porphyrin prosthetic group) have been studied with a range of oxidising substrates (hydroperoxides) [3–5] and reducing substrates [6, 7]. Parallel studies have been made with protein-free, water-soluble iron(III)–porphyrins [3, 8–10] where, in addition, the influence of variation in the porphyrin ligand on reactivity has been examined [11]. The stoichiometry of formation, spectra and reactivity of intermediates formed with a variety of oxidising substrates (hydroperoxides, chlorite) have also been studied [12, 13]. The results reveal some remarkably simple relationships between the reactivities of enzymic and model species and these afford an approach to assessment of the co-catalytic roles played by protein in the expression of enzymic function.