Inactivation and reactivation of manganese catalase: oxidation-state assignments using X-ray absorption spectroscopy

The oxidation states of the Mn atoms in three derivatives of Mn catalase have been characterized using a combination of X-ray absorption near-edge structure (XANES) and EPR spectroscopies. The as-isolated enzyme has an average oxidation state of Mn(III) and contains a Mn(III) form, together with a reduced Mn(II) form and a variable amount (10-25%) of a Mn(III)/Mn(IV) mixed-valence derivative. Treatment with NH2OH rapidly reduces the majority of the enzyme to a Mn(II) derivative with no loss of activity. Inactivation by treatment with NH2OH + H2O2 converts all of the enzyme to a mixed-valence Mn(III)/Mn(IV) form. The inactive, mixed-valence derivative can be completely reactivated by long-term (greater than 1 h) anaerobic incubation with NH2OH, giving a reduced Mn(II)/Mn(II) derivative. These data suggest a catalytic model in which the enzyme cycles between a reduced Mn(II)/Mn(II) state and an oxidized Mn(III)/Mn(III) state.