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.