Lignin depolymerization with laccase and its mediators system: an opportunity and technological challenge that needs further development and optimization

Lignocellulosic pretreatment technologies for ethanol production (e.g organosolv, hydrothermal and dilute acid) often produce a large amount of lignin as a major residue. Because of its unique characteristics, lignin is considered as the untapped biopolymer in biomass conversion technologies. The efficient utilization of isolated lignin involves its depolymerization to low-molecular weight phenolic compounds and aromatics that can serve as the building blocks for chemical synthesis of high-value products. The ability of laccase – a copper containing oxidase (EC 1.10.3.2) to attack and modify lignin along with mediators (LMS - laccase mediator system) is currently viewed as one of the potential applications for lignin valorization. It is believed that the action of laccase catalyzes the lignin polymerization, while LMS catalyze its depolymerization. However, a few studies in recent years showing effective lignin depolymerization by LMS open gaps for a question: does LMS catalyze lignin depolymerization or polymerization? We have investigated the effect of commercial laccases combined with mediators for lignin oxidation by different mechanisms on lignins derived from lignocellulosic pretreatment technologies. The changes in the lignins and the compounds released after different incubation systems were analyzed by different techniques as UHPLC, GPC and NMR. The oxygen and mediator consumption during the reactions were also monitored. The results provide new insights about how reaction conditions favor a specific course of laccase action.