Glucose-Free cis,cis-Muconic Acid Production via New Metabolic Designs Corresponding to the Heterogeneity of Lignin

Lignin conversion to value-added products is one of the most attractive challenges to develop a complete system for lignocellulosic biomass utilization. The objective of this study was cis,cis-muconic acid (ccMA) production from lignin, in particular, without sugar. Here, two bacterial strains were developed that can synthesize ccMA from lignin without using glucose. One is applicable to softwood lignin (mostly guaiacyl-lignin), and the other is applicable to hardwood lignin (mostly syringyl- and guaiacyl-lignins). The engineered Pseudomonas putida KT2440-based strain produced ccMA from a mixture of vanillic acid (a guaiacyl-lignin model) and 4-hydroxybenzoic acid (a p-hydroxyphenyl-lignin model) with 20% yield (mol/mol) without supplement of glucose. The dissolved oxygen concentration of 5-10% was effective to produce ccMA with a higher yield. The Sphingobium sp. SYK-6-based strain produced ccMA from vanillic acid with 45% yield (mol/mol) while growing on syringic acid (a syringyl-lignin model). These ccMA-producing strains can synthesize ccMA from lignin extracts of Japanese cedar and birch without using glucose.