Novel insights into fungal degradation of lignin: role of a flavoenzyme in assisting laccase-mediated oxidations

Lignin is the third most abundant biopolymer on earth, after cellulose and chitin, and accounts for up to 30% of plant biomass. It could be an inestimable source of aromatics of interest for organic syntheses, and probably the primary one after petrol runs out. However, its heterogeneous, randomly assembled and extremely complex chemical structure (fundamental for its protective role in the plant cell wall) makes its valorisation and exploitation scarce. A solution to this problem can be found in nature. In fact, filamentous fungi stood out as efficient degraders of lignin owing to a synergistic action of redox proteins, secreted by the fungus, that perform an enzymatic combustion. These enzymes are mostly annotated within the “Auxiliary Activities” (AA) class defined in the Carbohydrate Active enZymes (CAZy) database 1 . AA encompass enzymes with different cofactors, such as heme (peroxidases, CAZy family AA2), copper (laccases, AA1) and many others. Attempts to simulate laccase-mediated lignin degradation in vitro showed that highly reactive radicals produced by laccase oxidation are prone to repolymerization into higher molecular weight intermediates, that are even more recalcitrant to further enzymatic treatment. During in vivo degradation of lignin, fungi can prevent the massive repolymerization observed in vitro, leading to the hypothesis that they possess mechanisms to control or redirect the pool of radicals produced during oxidative attack of lignin. A few studies have pointed out the possible role of a few enzymes and their interplay in this event.