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Abstract

Manganese peroxidase generally mediates the oxidation of Mn2+ to Mn3+
with H2O2 as an oxidant. Several manganese peroxidases purified from
different lignin-degrading fungi were found to mediate a
fluoride-dependent conversion of organic substrates such as
monochlorodimedone or 2,6-dimethoxyphenol in the absence of manganese
ions. Using the manganese peroxidase MnP-1 from Bjerkandera adusta
strain lid I, these fluoride-dependent reactions were studied with
respect to different substrates converted, reaction products, and
kinetic properties to shed some light on the reaction mechanism of
manganese peroxidase. The analysis of the reaction products formed from
monochlorodimedone and 2,6-dimethoxyphenol showed that the substrates
were oxidized rather than fluorinated. The addition of fluoride to MnP-1
resulted in altered absorption spectra, indicating a coordinative
binding of fluoride or HF to the heme iron; the fluoride:heme
stoichiometry was determined to be 1:1 and the K-D value to be similar
to 2.5 in M at pH 3.4. The high K-D value indicates weak binding of
fluoride to the heme. Fluoride appeared to act as a partially
competitive inhibitor with respect to hydrogen peroxide for binding to
the heme as the sixth ligand. From the findings, a putative model for
the fluoride-dependent reaction was developed. The data were interpreted
to indicate that changes of the reaction center of manganese peroxidase
as, for example, caused by fluoride binding may lead to the oxidation of
organic compounds in the absence of manganese by opening a long-range
electron transfer pathway.