Lignin valorization through efficient microbial production of β-ketoadipate from industrial black liquor

Vanillin and vanillate are the major lignin-derived aromatic compounds produced through the alkaline oxidation of softwood lignin. Because the production of higher-value added chemicals from these compounds is essential for lignin valorization, the microbial production of β-ketoadipate, a promising raw material for the synthesis of novel nylons, from lignin was considered. Pseudomonas putida KT2440 was engineered to convert vanillin and vanillate to β-ketoadipate. By examining the culture conditions with an initial culture volume of 1 L, the engineered strain completely converted 25 g of vanillin and 25 g of vanillate and produced approximately 23 g of β-ketoadipate from each of them with a yield of 93% or higher. Furthermore, this strain showed the ability to efficiently produce β-ketoadipate from softwood lignin extracts in black liquor, a byproduct of pulp production. These results suggest that the production of β-ketoadipate from industrial black liquor is highly feasible for substantial lignin valorization.