Oxidative cleavage of non-phenolic β-O-4 lignin model dimmers by an extracellular aromatic peroxygenase

The extracellular aromatic peroxygenase of the agaric fungus Agrocybe aegerita catalyzed the H ₂O ₂-dependent cleavage of non-phenolic arylglycerol-β-aryl ethers (β-O-4 ethers). For instance 1-(3,4-dimethoxyphenyl)-2-(2-methoxy-phenoxy)propane- 1,3-diol, a recalcitrant dimeric lignin model compound that represents the major non-phenolic substructure in lignin, was selectively O-demethylated at the para-methoxy group to give formaldehyde and 1-(4-hydroxy-3-methoxyphenyl)- 2-(2-methoxyphenoxy) propane-1,3-diol. The phenol moiety of the latter compound was then enzymatically oxidized into phenoxy radicals and a quinoid cation, which initiated the autocatalytic cleavage of the dimer and the formation of monomers such as 2-methoxy-1,4-benzoquinone and phenoxyl-substituted propionic acid. The introduction of ¹⁸O from H ₂ ¹⁸O ₂ and H ₂ ¹⁸O at different positions into the products provided information about the routes of ether cleavage. Studies with a ¹⁴C-labeled lignin model dimer showed that more than 70% of the intermediates formed were further coupled to form polymers with molecular masses above 10 kDa. The results indicate that fungal aromatic peroxygenases may be involved in the bioconversion of methoxylated plant ingredients originating from lignin or other sources.