Direct oxidation of polymeric substrates by multifunctional manganese peroxidase isoenzyme from Pleurotus ostreatus without redox mediators

VPs (versatile peroxidases) sharing the functions of LiP (lignin peroxidase) and MnP (manganese peroxidase) have been described in basidiomycetous fungi Pleurotus and Bjerkandera. Despite the importance of this enzyme in polymer degradation, its reactivity with polymeric substrates remains poorly understood. In the present study, we first report that, unlike LiP, VP from Pleurotus ostreatus directly oxidized two polymeric substrates, bovine pancreatic RNase and Poly R-478, through a long-range electron pathway without redox mediators. P. ostreatus produces several MnP isoenzymes, including the multifunctional enzyme MnP2 (VP) and a typical MnP isoenzyme MnP3. MnP2 (VP) depolymerized a polymeric azo dye, Poly R-478, to complete its catalytic cycle. Reduction of the oxidized intermediates of MnP2 (VP) to its resting state was also observed for RNase. RNase inhibited the oxidation of VA (veratryl alcohol) in a competitive manner. Blocking of the exposed tryptophan by N-bromosuccinimide inhibited the oxidation of RNase and VA by MnP2 (VP), but its Mn2+-oxidizing activity was retained, suggesting that Trp-170 exposed on an enzyme surface is a substrate-binding site both for VA and the polymeric substrates. The direct oxidation of RNase and Poly R by MnP2 (VP) is in sharp contrast with redox mediator-dependent oxidation of these polymers by LiP from Phanerochaete chrysosporium. Molecular modelling of MnP2 (VP) revealed that the differences in the dependence on redox mediators in polymer oxidation by MnP2 (VP) and LiP were explained by the anionic microenvironment surrounding the exposed tryptophan.

Stimulation of aryl metabolite production in the basidiomycete Bjerkandera sp. strain BOS55 with biosynthetic precursors and lignin degradation products

Aryl metabolites are known to have an important role in the ligninolytic system of white rot fungi. The addition of known precursors and aromatic acids representing lignin degradation products stimulated the production of aryl metabolites (veratryl alcohol, veratraldehyde, p-anisaldehyde, and 3-chloro-p-anisaldehyde) in the white rot fungus Bjerkandera sp. strain BOS55. The presence of manganese (Mn) is known to inhibit the biosynthesis of veratryl alcohol (T. Mester, E. de Jong, and J.A. Field, Appl. Environ. Microbiol. 61:1881-1887, 1995). A new finding of this study was that the production of the other aryl metabolites, p-anisaldehyde and 3-chloro-p-anisaldehyde, was also inhibited by Mn. We attempted to bypass the Mn-inhibited step in the biosynthesis of aryl metabolites by the addition of known and suspected precursors. Most of these compounds were not able to bypass the inhibiting effect of Mn. Only the fully methylated precursors (veratrate, p-anisate, and 3-chloro-p-anisate) provided similar concentrations of aryl metabolites in the presence and absence of Mn, indicating that Mn does not influence the reduction of the benzylic acid group. The addition of deuterated benzoate and 4-hydroxybenzoate resulted in the formation of deuterated aryl metabolites, indicating that these aromatic acids entered into the biosynthetic pathway and were common intermediates to all aryl metabolites. Only deuterated chlorinated anisyl metabolites were produced when the cultures were supplemented with deuterated 3-chloro-4-hydroxybenzoate. This observation combined with the fact that 3-chloro-4-hydroxybenzoate is a natural product of Bjerkandera spp. (H. J. Swarts, F. J. M. Verhagen, J. A. Field, and J. B. P. A. Wijnberg, Phytochemistry 42:1699-1701, 1996) suggest that it is a possible intermediate in chlorinated anisyl metabolite biosynthesis.

Oxidation of ferrocytochrome c by lignin peroxidase

We demonstrate direct oxidation of ferrocytochrome c by lignin peroxidase (LiP) from the lignin-degrading basidiomycete, Phanerochaete chrysosporium. Steady-state kinetic data fit a peroxidase ping-pong mechanism rather than an ordered bi-bi ping-pong mechanism, suggesting that the reductions of LiP compounds I and II by ferrocytochrome c are irreversible. The pH dependence of the overall reaction apparently is controlled by two factors, the pH dependence of the electron-transfer rate and the pH dependence of enzyme inactivation in the presence of H2O2. In the presence of 100 microM H2O2, veratryl alcohol (VA) significantly enhanced cytochrome c oxidation at pH 3.0 but had little effect above pH 4.5. In the presence of < 10 microM H2O2, the stimulating effect of VA on the reaction is greatly diminished. As with cytochrome c peroxidase reactions, LiP oxidation of ferrocytochrome c decreased as the ionic strength increased, implying the involvement of electrostatic interactions between the polymeric substrate and enzyme. The reaction product ferricytochrome c inhibited VA oxidation by LiP in a noncompetitive manner, suggesting that cytochrome c binds to LiP at a site different from the small aromatic substrate binding site. Recent crystallographic studies show that the heme is buried in the LiP protein and unavailable for direct interaction with polymeric substrates, suggesting that electron transfer from ferrocytochrome c to LiP occurs over a relatively long range. The role of VA in this electron-transfer reaction is discussed.(ABSTRACT TRUNCATED AT 250 WORDS)