Depolymerization of Straw Lignin by Manganese Peroxidase from Nematoloma frowardii is Accompanied by Release of Carbon Dioxide

SummaryManganese peroxidase preparations (MnP) from the white-rot fungusNematoloma frowardiiwere able to release14CO2directly from14C-labeled milled wheat straw (MWS; total lignin fraction) and milled straw lignin (MSL; dioxane soluble part of MWS). Apart from the formation of14CO2(4–10 %) the treatment of insoluble MWS and MSL with MnP resulted in the formation of water-soluble14C-lignin fragments (lignin solubilization, 14–25%). Analyses with gel permeation chromatography (GPC) demonstrated the formation of lignin fragments with predominant molecular masses around 1 kDa. The extent of MWS mineralization and solubilization was enhanced in the presence of reduced glutathione (GSH) acting as thiol mediator, whereas MSL mineralization was not stimulated by GSH. The principle of direct extracellular mineralization of lignin catalyzed by the MnP system may make a significant contribution to the formation of carbon dioxide in lignincellulose containing habitats.

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Production of Manganese Peroxidase and Organic Acids and Mineralization of 14C-Labelled Lignin (14C-DHP) during Solid-State Fermentation of Wheat Straw with the White Rot Fungus Nematoloma frowardii

Applied and Environmental Microbiology ◽

10.1128/aem.65.5.1864-1870.1999 ◽

1999 ◽

Vol 65 (5) ◽

pp. 1864-1870 ◽

Cited By ~ 99

Author(s):

Martin Hofrichter ◽

Tamara Vares ◽

Mika Kalsi ◽

Sari Galkin ◽

Katrin Scheibner ◽

...

Keyword(s):

Solid State ◽

Organic Acids ◽

Wheat Straw ◽

Solid State Fermentation ◽

Manganese Peroxidase ◽

Ligninolytic Enzyme ◽

Water Soluble ◽

White Rot ◽

White Rot Fungus ◽

Free System

ABSTRACT The basidiomycetous fungus Nematoloma frowardiiproduced manganese peroxidase (MnP) as the predominant ligninolytic enzyme during solid-state fermentation (SSF) of wheat straw. The purified enzyme had a molecular mass of 50 kDa and an isoelectric point of 3.2. In addition to MnP, low levels of laccase and lignin peroxidase were detected. Synthetic 14C-ring-labelled lignin (14C-DHP) was efficiently degraded during SSF. Approximately 75% of the initial radioactivity was released as14CO2, while only 6% was associated with the residual straw material, including the well-developed fungal biomass. On the basis of this finding we concluded that at least partial extracellular mineralization of lignin may have occurred. This conclusion was supported by the fact that we detected high levels of organic acids in the fermented straw (the maximum concentrations in the water phases of the straw cultures were 45 mM malate, 3.5 mM fumarate, and 10 mM oxalate), which rendered MnP effective and therefore made partial direct mineralization of lignin possible. Experiments performed in a cell-free system, which simulated the conditions in the straw cultures, revealed that MnP in fact converted part of the14C-DHP to 14CO2 (which accounted for up to 8% of the initial radioactivity added) and14C-labelled water-soluble products (which accounted for 43% of the initial radioactivity) in the presence of natural levels of organic acids (30 mM malate, 5 mM fumarate).

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