Lignin valorization through biological funneling and chemical catalysis

In production of chemicals from biomass, lignin is typically underutilized as a feedstock and burned for process heat because its inherent heterogeneity and recalcitrance make it difficult to valorize. In nature, however, some organisms evolved metabolic pathways that enable the utilization of lignin-derived aromatic molecules as carbon sources. Aromatic catabolism typically occurs via pathways that act as a “biological funnel” to convert heterogeneous substrates to central intermediates. These intermediates undergo ring cleavage and are further converted via the beta-ketoadipate pathway to central carbon metabolism. Here, we employ a natural aromatic-catabolizing organism, Pseudomonas putida KT2440, to demonstrate that these aromatic metabolic pathways can be employed to convert both aromatic model compounds and heterogeneous, lignin-enriched streams derived from biomass pretreatment into medium chain length polyhydroxyalkanoates (mcl-PHAs). mcl-PHAs were then isolated from the cells, and demonstrated to be similar to conventional carbohydrate-derived mcl-PHAs, which have applications as bioplastics. In a further demonstration of their utility, mcl-PHAs were catalytically converted to both chemical precursors and fuel-range hydrocarbons. In addition, we demonstrate that an engineered version of P. putida can convert lignin-derived aromatics into muconic acid, which is then further converted into adipic acid via catalytic hydrogenation. We also examine the most carbon efficient means to conduct enzymatic ring opening, and evaluate the most viable host organism for maximizing product yield and lignin-derived aromatic species consumption. Overall, this work demonstrates that the use of aromatic catabolic pathways enables an approach to valorize lignin by overcoming its inherent heterogeneity to produce fuels, chemicals, and materials.