Microbial Ligninolysis: Toward a Bottom-Up Approach for Lignin Upgrading

Lignin, an underutilized source of biomass, is a challenging target for decomposition by both biological and nonbiological approaches. As lignin accounts for 15-40% of the dry weight of lignocellulose, its conversion to valued chemicals is considered promising. Although depolymerization of the complicated lignin structures is still not fully understood, biochemical reactions for various lignin-derived compounds, especially dimeric and monomeric aromatics, have been characterized in some fungi and bacteria known as natural lignin degraders. Synthetic biology, as a useful tool for exploring and engineering biosystems, could be used for protein design and constructing biosynthetic pathways for lignin utilization and upgrading. Understanding mechanisms of ligninolysis will facilitate synthetic biology implementation. In this Perspective, fungal and bacterial biochemical mechanisms for breaking lignin linkages, degrading monomers, and cleaving aromatic rings are examined, whereas enabling valued chemical biosynthesis from lignin-derived compounds via synthetic biology approaches is emphasized. On the basis of the advances in enzyme discovery and metabolic reconstruction, we propose a bottom-up approach to developing microbial platforms for producing valued chemicals from lignin sources.