Cleavage of nonphenolic β‐1 diarylpropane lignin model dimers by manganese peroxidase from Phanerochaete chrysosporium

Abstract

Purified manganese peroxidase (MnP) from Phanerochaete chrysosporium oxidizes nonphenolic b-1 diarylpropane lignin model compounds in the presence of Tween 80, and in three-to fourfold lower yield in its absence. In the presence of Tween 80, 1-(3¢,4¢-diethoxyphenyl)-1-hydroxy-2-(4¢-methoxyphenyl)propane (I) was oxidized to 3,4-diethoxybenzaldehyde (II), 4-methoxyacetophenone (III) and 1-(3¢,4¢-diethoxyphenyl)-1-oxo-2-(4¢-methoxyphenyl)propane (IV), while only 3,4-diethoxybenzaldehyde (II) and 4-methoxyacetophenone (III) were detected when the reaction was conducted in the absence of Tween 80. In contrast to the oxidation of this substrate by lignin peroxidase (LiP), oxidation of substrates by MnP did not proceed under anaerobic conditions. When the dimer (I) was deuterated at the a position and subsequently oxidized by MnP in the presence of Tween 80, yields of 3,4-diethoxybenzaldehyde, 4-methoxyacetophenone remained constant, while the yield of the a-keto dimeric product (IV) decreased by approximately sixfold, suggesting the involvement of a hydrogen abstraction mechanism. MnP also oxidized the a-keto dimeric product (IV) to yield 3,4-diethoxybenzoic acid (V) and 4-methoxyacetophenone (III), in the presence and, in lower yield, in the absence of Tween 80. When the reaction was performed in the presence of 18 O 2 , both products, 3,4-diethoxybenzoic acid and 4-methoxyacetophenone, contained one atom of 18 O. Finally, MnP oxidized the substrate 1-(3¢,5¢-dimethoxyphenyl)-1-hydroxy-2-(4¢-methoxyphenyl) propane (IX) to yield 3,5-dimethoxybenzaldehyde (XI), 4-methoxyacetophenone (III) and 1-(3¢,5¢-dimethoxyphenyl)-1-oxo-2-(4¢-methoxyphenyl)propane (X). In sharp contrast, LiP was not able to oxidize IX. Based on these results, we propose a mechanism for the MnP-catalyzed oxidation of these dimers, involving hydrogen abstraction at a benzylic carbon, rather than electron abstraction from an aromatic ring.

Structure and Action Mechanism of Ligninolytic Enzymes

“…For non-phenolic substrates, the oxidation by Mn(III) involves the formation of reactive radicals in the presence of a second mediator [133]. This is in contrast to that of LiPcatalyzed reaction, which involves electron abstraction from the aromatic ring forming a radical cation.…”

“…This is in contrast to that of LiPcatalyzed reaction, which involves electron abstraction from the aromatic ring forming a radical cation. Mn(III) in the presence of thiols, such as glutathione, mediates the oxidation of substituted benzyl alcohols and diarylpropane structures to their respective aldehydes [133,134]. In these reactions, Mn(III) oxidizes thiols to thiyl radicals, which subsequently Fig.…”

“…13) [133,[135][136][137][138]. The mechanism involves hydrogen abstraction from the benzylic carbon (C a ) via lipid peroxy radicals, followed by O 2 addition to form a peroxy radical, and subsequent oxidative cleavage and nonenzymatic degradation.…”

Lignin Degradation Processes and the Purification of Valuable Products

“…In contrast to lignin peroxidases, manganese peroxidases under normal conditions cannot catalyze the oxidation of more recalcitrant non-phenolic structures [52]. However, a purified manganese peroxidase from P. chrysosporium can oxidize even non-phenolic lignin model compounds in the presence of the detergent Tween-80, which acts as radical mediator [63].…”

Novel Promoter Sequence Required for Manganese Regulation of Manganese Peroxidase Isozyme 1 Gene Expression in Phanerochaete chrysosporium

“…MnPs are heme-containing, H 2 O 2 -dependent, extracellular glycoproteins with molecular masses ranging from 45 to 49 kDa (15,20,39). MnP oxidizes Mn 2ϩ to Mn 3ϩ and the latter, chelated with oxalate or another dicarboxylic acid, acts as a diffusible mediator to oxidize polymeric lignin, lignin model compounds, and aromatic pollutants (14,15,35,41,42,44,48). Virtually all white-rot basidiomycetous fungi produce MnP (22,37,38).…”