Do the extracellular enzymes cellobiose dehydrogenase and manganese peroxidase form a pathway in lignin biodegradation?

The extracellular enzyme manganese peroxidase is believed to degrade lignin by a hydrogen peroxide-dependent oxidation of Mn(II) to the reactive species Mn(III) that attacks the lignin. However, Mn(III) is not able to directly oxidise the non-phenolic lignin structures that predominate in native lignin. We show here that pretreatment of a non-phenolic lignin model compound with another extracellular fungal enzyme, cellobiose dehydrogenase, allows the manganese peroxidase system to oxidise this molecule. The mechanism behind this effect is demethoxylation and/or hydroxylation, i.e. conversion of a non-phenolic structure to a phenolic one, mediated by hydroxyl radicals generated by cellobiose dehydrogenase. This suggests that cellobiose dehydrogenase and manganese peroxidase may act in an extracellular pathway in fungal lignin biodegradation. Analytical techniques used in this paper are reverse-phase high-pressure liquid chromatography, gas chromatography connected to mass spectroscopy and UV-visible spectroscopy.