Sustainable production of aromatic chemicals from lignin using enzymes and engineered microbes

Abstract

Lignin is an aromatic biopolymer found in plant cell walls and is the most abundant source of renewable aromatic carbon in the biosphere. Hence there is considerable interest in the conversion of lignin, either derived from agricultural waste or produced as a byproduct of pulp/paper manufacture, into high-value chemicals. Although lignin is rather inert, due to the presence of ether C-O and C-C linkages, several microbes are able to degrade lignin. This review will introduce these microbes and the enzymes that they use to degrade lignin and will describe recent studies on metabolic engineering that can generate high-value chemicals from lignin bioconversion. Catabolic pathways for degradation of lignin fragments will be introduced, and case studies where these pathways have been engineered by gene knockout/insertion to generate bioproducts that are of interest as monomers for bioplastic synthesis or aroma chemicals will be described. Life cycle analysis of lignin bioconversion processes is discussed.

Fig. 1. (A) Lignin sub-structures. (B) S, G and H units in polymeric lignin. (C) Representative partial lignin structure.

Fig. 2. Condensed units found in Kraft lignin and lignosulfonate.

Fig. 3. β-Ketoadipate pathways for degradation of protocatechuic acid and catechol.

Fig. 4. Pathways for catabolism of p-hydroxycinnamic acids. Route a, cofactor-independent decarboxylation; route b, CoA-dependent β-oxidation; route c, CoA-dependent hydration-retroaldol cleavage.

Fig. 5. 4-Hydroxybenzoylformate catabolic pathway in R. jostii RHA1, responsible for degradation of aryl C₂ lignin fragments, which may be derived from β-5 units in polymeric lignin. R = H or OCH₃.

Fig. 6. Hydroxyquinol degradation pathway.

Fig. 7. Production of vanillin from polymeric lignin using R. jostii RHA1 Δvdh (green) or Arthrobacter C2 ΔxylC (magenta). Gene knockout labelled with blue cross. R = H or OCH₃.

Fig. 8. (A) Structures of PET and PBAT bioplastics. (B) Pathways for production of 2,4-pyridinedicarboxylic acid and 2,5-pyridinedicarboxylic acid in engineered R. jostii RHA1.

Fig. 9. Generation of pyrone-dicarboxylic acid (PDC) in Novosphingobium aromaticivorans. Gene knockout is shown by a blue cross.

Fig. 10. Production of 4-vinylguaiacol in engineered P. putida KT2440 (in orange) and coniferyl alcohol (in green) via enzymatic conversion of ferulic acid, released from grass lignocellulose. The red arrow indicates auto-induction of padC expression by feruloyl CoA.

Fig. 11. Production of cis,cis-muconic acid in engineered bacterial strains, showing the routes engineered in P. putida KT2440 (in orange, also for C. glutamicum and N. aromaticivorans) and Amycolatopsis sp. (in blue).

Fig. 12. Pathways for production of polyhydroxyalkanoates and triacylglycerol lipids from lignin-containing feedstocks, via primary metabolism.

Victoria Sodré

Timothy D. H. Bugg