Reevaluation of the kinetics of polynuclear mimics for manganese catalases

SIGNORELLA, SANDRA; ROMPEL, ANETTE; BÜLDT-KARENTZOPOULOS, KLAUDIA; KREBS, BERNT; PECORARO, VINCENT L.; TUCHAGUES, JEAN PIERRE

INORGANIC CHEMISTRY

American Chemical Society

Lugar: Washington; Año: 2007 vol. 46 p. 10864 - 10868

0020-1669

Considerable effort has been expended in order to understand the mechanism of manganese catalases and to develop functional mimics for these enzymes.  For many years, the most efficient reactivity mimic was [MnIVsalpn(m-O)]2, -a compound that cycles between the MnIV2 and MnIII2 oxidation levels instead of the MnII2 and MnIII2 oxidation states used by the enzyme- with kcat = 250 s-1 and kcat/KM = 1000 M-1 s-1. Recently, a truly exceptional high value of kcat was reported for the complex [Mn(bpia)(m-OAc)]22+. Based on a calculated kcat value of 1100 s-1 and an efficiency kcat/KM of 34000 M-1 s-1, this complex has been suggested to represent a significant breakthrough in catalytic efficiencies of Mn catalase mimics. However, a plot of ri/[cat]T vs [H2O2]0, where the saturation value approaches 1.5 s-1, is inconsistent with the 1100 s-1 tabulated for kcat. Similar discrepancies are observed for two other families of manganese complexes containing either a Mn2(m-OPh)22+ core and different substituted tripodal ligands or complexes of methyl- and ethyl-salicylimidate, with an Mn2(m-OPh)24+ core. Re-evaluation of the kinetic parameters for these three systems reveals that the originally reported values were overestimated by a factor of ~1000 for both kcat and kcat/KM.  We discuss the origin of the discrepancy between the previously published kinetic parameters and the newly derived values. Furthermore, we provide a short analysis of the existing Mn catalase mimics in an effort to provide sound directions for future investigations in this field.