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

Manganese oxidation by manganese peroxidase (MnP) was investigated. Stoichiometric, kinetic, and Mn(II) binding studies demonstrated that MnP has a single manganese binding site near the heme, and two Mn(III) 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. Mn(III)-organic acid complexes oxidize terminal phenolic substrates in a second-order reaction. Mn(III)-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 Mn(II) rather than a Mn(II)-chelator complex. The enzyme competes with chelators for free Mn(II). Optimal chelators, such as malonate, facilitate Mn(III) dissociation from the enzyme, stabilize Mn(III) in aqueous solution, and have a relatively low Mn(II) binding constant.