Manganese(II) oxidation by manganese peroxidase from the basidiomycete Phanerochaete chrysosporium. Kinetic mechanism and role of chelators.

Abstract

Manganese oxidation by manganese peroxidase (MnP) was investigated. Stoichiometric, kinetic, and MnII binding studies demonstrated that MnP has a single manganese binding site near the heme, and two MnIII equivalents are formed at the expense of one H2O2 equivalent. Since each catalytic cycle step is irreversible, the data fit a peroxidase ping-pong mechanism rather than an ordered bi-bi ping-pong mechanism. MnIII-organic acid complexes oxidize terminal phenolic substrates in a second-order reaction. MnIII-lactate and -tartrate also react slowly with H2O2, with third-order kinetics. The latter slow reaction does not interfere with the rapid MnP oxidation of phenols. Oxalate and malonate are the only organic acid chelators secreted by the fungus in significant amounts. No relationship between stimulation of enzyme activity and chelator size was found, suggesting that the substrate is free MnII rather than a MnII-chelator complex. The enzyme competes with chelators for free MnII. Optimal chelators, such as malonate, facilitate MnIII dissociation from the enzyme, stabilize MnIII in aqueous solution, and have a relatively low MnII binding constant.