Agaricales lignin peroxidase: structure-function studies and evaluation of its degradative capabilities

Abstract

Understanding lignin biodegradation by saprotrophic fungi has attached an increasing interest during the last decades, as they secrete an array of oxidative enzymes that may be exploited for this polymer revalorization in biorefineries [1]. Among these enzymes, lignin peroxidases (LiPs) stand as the most efficient oxidizing lignin, being capable of acting towards the major non-p-her:¡olic moiety of its structure [2]. Until now, LiPs had been only identified and studied in white-rot Polyporales, and nothing was known about their presence in Agaricales and about the contribution of these fungi to lignin degradation in nature. Genomic studies of the grass-litter fungus Agrocybepediades revealed the existence of a lignin peroxidase in this Agaricales species, being the first LiP identified out of wood-rotting polypores [3]. Crystal structure and biochemical characterization of this enzyme (ApeLiP hereafter) revealed several features similar to other well-known ligninolytic peroxidases from Polyporales. Thus, ApeLiP contains a conserved heme-pocket and a solvent exposed tryptophan as catalytic sites. Characterization of a variant lacking the surface tryptophan showed that this is the oxidation site for non-phenolic lignin model compounds and other high redox potential substrates, also playing a role in phenolic substrate oxidation. Moreover, using stopped-flow rapid spectrophotometry and 2D-NMR, it was d'emonstrated that ApeLiP can also act on real lignin. The reduction potentials of its catalytic-cycle intermediates, estimated by stopped-flow in equilibrium reactions, showed similar activation by H20 2, but a lower potential for the rate-limiting step (compound-II reduction) compared to other LiPs. Finally, ApeLiP is stable from acidic to basic pH, a relevant feature for its biotechnological application considering its different optima for oxidation of phenolic and non phenolic compounds. These studies prove that not only wood-rottingPolyporales, but al so Agaricales mushrooms contain enzymes with high relevance both for carbon recycling in nature and for biotechnological modification of lignin.

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