Whole-Cell Bioconversion of Renewable Biomasses-Related Aromatics to cis,cis-Muconic Acid

Lignin and wheat bran represent renewable feedstocks for generation of useful and value-added compounds such as vanillin (a popular flavoring agent) and cis,cis-muconic acid (ccMA, a building block for the synthesis of plastic materials). In the present work, we report on the setup of an efficient and green process for producing such valuable compounds based on (a) the optimization of the extraction procedures for vanillin from lignin and ferulic acid from wheat bran and (b) the genetic engineering of an Escherichia coli strain with up to three plasmids differing in copy numbers to modulate the expression of up to seven recombinant enzymes. In detail, we used two sequential reactions catalyzed by the decarboxylase Fdc and the dioxygenase Ado to convert wheat bran-derived ferulic acid into vanillin: nature-identical vanillin was produced in one pot with a >85% yield in 20 h. Next, the dehydrogenase LigV, the demethylase VanAB, the decarboxylase AroY, and the dioxygenase C12O converted lignin-derived vanillin into ccMA with a >95% conversion yield and a productivity of 4.2 mg of ccMA/g of Kraft lignin in 30 min. Finally, when the optimized E. coli strain expressing all the abovementioned enzymes was used, ccMA was produced with a >95% conversion yield starting from ferulic acid in 10 h following product isolation, corresponding to 0.73 g of ccMA/g of ferulic acid, 1.4 g of ccMA/L, and 2.2 g of ccMA/g of wheat bran biomass. The optimized whole-cell system represents a sustainable and cost-competitive process for producing high value-added products from renewable resources.