Insight into Depolymerization Mechanism of Bacterial Laccase for Lignin

Laccase is essential for the biodepolymerization of lignin, but the challenge is that the reaction mechanism has not been fully elucidated. The laccase (Lacc) inactivated mutant of Bacillus ligniniphilus L1 had a sharp decline in its ability to degrade lignin, which proved its indispensable role in lignin depolymerization. The purified Lacc from recombinant Escherichia coli BL21 and its mediator system (LMS) displayed significant lignin degradation capacities as well as remarkable thermotolerance and solvent resistance. The chemical oxygen demand removal rates of LMS for alkaline and milled wood lignin have reached 67.0% and 80.9%, respectively. Comprehensive analyses, including Fourier-transform infrared spectrometry, gas chromatography–mass spectrometry, 2D-HSQC-NMR, and time-of-flight secondary ion mass spectrometry, unveiled that Lacc- and LMS-oxidized lignin include at least 10 or more catalytic reactions. Lacc can effectively degrade G-lignin even without a mediator, and the removal rate of G-lignin is higher than that of S-lignin. In addition, the supplementation of the mediator increased the removal rate of H-lignin by Lacc and the cleavage of interunit linkages such as β-O-4, β-5, β-β, 4-O-5, and 5-5. Moreover, we found that Lacc cannot polymerize some aromatic monomers into dimers or polymers, which is different from fungal and plant laccases. It is by far the most detailed study describing the reaction mechanism of lignin oxidation by bacterial laccase. These results provide new insights into the catalytic mechanism of bacterial laccase and lay the foundation for the application of laccase in lignin valorization.