Laccase-catalyzed conversion of residual agricultural biomass to lignin-derived aromatic compounds

Lignin, one of the three major components of lignocellulosic biomass, is a promising feedstock for value-added bioproducts. However, its utilization is limited by its intrinsic heterogeneity and recalcitrance. Laccases are oxidoreductases that can act on a wide range of phenolic and aromatic compounds. SilA is a small robust thermostable laccase from the actinobacterium Streptomyces ipomoea that is active across a broad range of reaction conditions. In this study, we demonstrated that SilA transforms the lignin of sugar cane straw residue that has been pretreated by steam explosion (SCR_SE), into lignin-derived aromatic compounds (LDACs), specifically 4-hydroxybenzoate, vanillate, syringate, p-coumarate and ferulate, which are in great demand across diverse industries, including chemical, pharmaceutical, food, health, and cosmetic. Additionally, incubation of SCR_SE with SilA also produced acid-precipitable polymeric lignin (APPL), as an indirect indication of delignification. Finally, when the major fraction of SCR_SE polysaccharides was removed by the commercial enzymatic cocktail Cellic^® CTec2, there was a significant increase in the release of LDACs, particularly p-coumarate. This research demonstrates a novel biocatalytic approach to transform lignin from the main residual solid waste stream of lignocellulosic biomass after the hydrolysis of most of the structural polysaccharides, particularly in the context of sugar cane biorefineries, contributing to the sustainable and efficient use of agricultural biomass waste.