The Laccase-Lig Multienzymatic Multistep System in Lignin Valorization

Graphical Abstract

Multistep catalysis: The ratio between phenolic and aliphatic OH groups plays a crucial role in determining the overall reactivity towards the cleavage of non-phenolic aryl glycerol β-O-4 aryl ether bonds of lignin by the Laccase-Lig multistep catalytic system. Lignin samples showing low phenolic/aliphatic OH ratio undergo net depolymerization, while a higher phenolic/aliphatic OH ratio results in the polymerization of the residual lignin irrespective of its origin.

Abstract

A laccase-Lig multienzymatic multistep system for lignin depolymerization was designed and developed. Studies were performed on pristine and fractionated lignins (Kraft and Organosolv) using a specific cascade of enzymes, that is, laccases from Bacillus licheniformis and from Funalia trogii, respectively for Kraft and Organosolv lignin, followed by the Lig system from Sphingobium sp. SYK-6 (β-etherases Lig E and Lig F, glutathione lyase Lig G). Careful elucidation of the structural modifications occurring in the residual lignins associated with the identification and quantification of the generated low-molecular-weight compounds showed that (i) the laccase-Lig system cleaves non-phenolic aryl glycerol β-O-4 aryl ether bonds, and (ii) the overall reactivity is heavily dependent on the individual lignin structure. More specifically, samples with low phenolic/aliphatic OH groups ratio undergo net depolymerization, while an increased phenolic/aliphatic OH ratio results in the polymerization of the residual lignin irrespective of its botanical origin and isolation process.