Redox properties of heme peroxidases

Peroxidases are heme enzymes found in bacteria, fungi, plants and animals, which exploit the reduction 24of hydrogen peroxide to catalyze a number of oxidative reactions, involving a wide variety of organic and 25inorganic substrates. The catalytic cycle of heme peroxidases is based on three consecutive redox steps, 26involving two high-valent intermediates (Compound I and Compound II), which perform the oxidation of 27the substrates. Therefore, the thermodynamics and the kinetics of the catalytic cycle are influenced by the 28reduction potentials of three redox couples, namely Compound I/Fe3+, Compound I/Compound II and 29Compound II/Fe3+. In particular, the oxidative power of heme peroxidases is controlled by the (high) 30reduction potential of the latter two couples. Moreover, the rapid H2O2-mediated two-electron oxidation 31of peroxidases to Compound I requires a stable ferric state in physiological conditions, which depends on 32the reduction potential of the Fe3+/Fe2+ couple. The understanding of the molecular determinants of the 33reduction potentials of the above redox couples is crucial for the comprehension of the molecular deter- 34minants of the catalytic properties of heme peroxidases. 35This review provides an overview of the data available on the redox properties of Fe3+/Fe2+, Compound 36I/Fe3+, Compound I/Compound II and Compound II/Fe3+ couples in native and mutated heme peroxidases. 37The influence of the electron donor properties of the axial histidine and of the polarity of the heme envi- 38ronment is analyzed and the correlation between the redox properties of the heme group with the cat- 39alytic activity of this important class of metallo-enzymes is discussed