Laccases, EC 1.10.3.2, CAZY family AA1, are blue multicopper oxidases that catalyze oxidation of mono- and diphenolics and other electron rich substrates during concomitant reduction of O2 to H2O. Laccases produced by white-rot fungi (Basidiomycetes) currently receive high research attention due to their apparent ability to catalyze modification of lignin and lignin-derived substances and in this way assist in biorefining of plant biomass. The T1 copper site in the enzyme and the redox potential define the ability of the enzyme to catalyze the abstraction of electrons from phenolic substrates, and the trinuclear cluster allows for the remarkable copper-catalyzed oxidoreduction reaction which can be exploited for modification and functionalization of lignin. The range of substrates that can be oxidized by laccases can be expanded by mediators that are believed to function as electron shuttles. However, recently, we discovered that fungal laccases could catalyse grafting of –N-OH type mediators, HPI (N-hydroxyacetanilide) and HBT (1-hydroxybenzotriazole), to lignin. HPI grafted better than HBT. We suggest that this laccase catalyzed grafting is a result of radical-radical coupling mechanisms involving laccase catalyzed oxidation of the mediator followed by either direct or indirect catalytic oxidation of lignin moieties. These data open up for new biorefining uses for lignin by laccase-assisted modification.

Enzymatic modification of lignin by fungal laccases

255th National Meeting (2018)

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