Enhanced delignification of steam-pretreated poplar by a bacterial laccase

Woody biomass represents a sustainable source of fuel and biomaterials. However, its efficient transformation is hindered in part due to the recalcitrance of its components, particularly lignin – an abundant but heterogeneous aromatic polymer. The recent discovery of several bacterial lignolytic enzymes facilitates the development of novel biocatalysts. However, these enzymes have largely been characterized using model substrates; direct evidence for their action on biomass is lacking. Herein, we report the delignification of woody biomass by a small laccase (sLac) from Amycolatopsis sp. 75iv3. Incubation of steam-pretreated poplar (SPP) for 48 h with sLac in a buffered aqueous solution released ~6-fold more lignin than the control. NMR spectroscopy of the released lignins, recovered as acid-precipitable polymeric lignins (APPL), revealed that both were significantly syringyl-enriched (or guaiacyl-depleted) relative to the original material, and that significantly more oxidation of syringyl units was seen in the sLac material. sLac treatment also released lignin-derived mono-aryls, such as vanillate, syringate and syringaldehyde. Moreover, the S:G ratio of the recovered lignin was significantly higher than that of the SPP (~16:1 vs. ~3:1). Gel-permeation chromatography coupled with light scattering revealed that sLac treatment reduced the molar mass of the APPL from 11400 ± 300 g·mol^-1 to 5070 ± 10 g·mol^-1. Finally, sLac acted synergistically with a commercial cellulase cocktail to increase glucose production from SPP ~5%. Overall, this study provides unambiguous evidence for the lignolytic activity of a bacterial enzyme on woody biomass and has important implications for the development of effective lignocellulolytic biocatalysts.