ESR SPECTROSCOPY OF THE BINUCLEAR CLUSTER OF MANGANESE IONS IN THE ACTIVE-CENTER OF MN-CATALASE FROM THERMUS-THERMOPHILUS

Our previous data of low-temperature ESR spectroscopy indicate that the active center of Mn-catalase from Thermus thermophilus is a binuclear manganese complex. The ESR spectra from initial preparations of Mn-catalase in the temperature range from 8 to 100 K are a superposition of three signals, A, B and C. These were attributed to a binuclear manganese site in the (Mn2+,Mn2+), (Mn2+,Mn3+) and (Mn3+,Mn4+) states, respectively. In the present work we have studied the redox transformations of the manganese cluster and its interaction with exogenous ligands. Hydroxylamine reduces centers with signals B and C to give signal A. The (Mn2+,Mn2+) centers easily form complexes with phosphate buffer, Cl-, N3- and F-. This state autooxidizes in air with the appearance of ESR signals B and C for states (Mn2+,Mn3+) and (Mn3+,Mn4+). Periodate (KIO4) oxidizes all centers to the (Mn3+,Mn4+) state. In the absence of KIO4 this state undergoes spontaneous reduction to form the (Mn2+,Mn2+) state. On the other hand, addition of KI reduces the (Mn3+,Mn4+) state to the (Mn2+,Mn3+). In contrast to the (Mn2+,Mn2+) state, the (Mn3+,Mn4+) state does not undergo ligand exchange with anions. We believe that in the (Mn3+,Mn4+) state the manganese ions are connected by oxo- (or hydroxo-) bridges and that all coordination sites for the binding exogenous ligands are strongly occupied by oxygen atoms.