Bacterial catabolism of lignin‐derived aromatics: New findings in a recent decade: Update on bacterial lignin catabolism

Abstract

Summary Lignin is the most abundant phenolic polymer; thus, its decomposition by microorganisms is fundamental to carbon cycling on earth. Lignin breakdown is initiated by depolymerization catalysed by extracellular oxidoreductases secreted by white‐rot basidiomycetous fungi. On the other hand, bacteria play a predominant role in the mineralization of lignin‐derived heterogeneous low‐molecular‐weight aromatic compounds. The outline of bacterial catabolic pathways for lignin‐derived bi‐ and monoaryls are typically composed of the following sequential steps: (i) funnelling of a wide variety of lignin‐derived aromatics into vanillate and syringate, (ii) O demethylation of vanillate and syringate to form catecholic derivatives and (iii) aromatic ring‐cleavage of the catecholic derivatives to produce tricarboxylic acid cycle intermediates. Knowledge regarding bacterial catabolic systems for lignin‐derived aromatic compounds is not only important for understanding the terrestrial carbon cycle but also valuable for promoting the shift to a low‐carbon economy via biological lignin valorisation. This review summarizes recent progress in bacterial catabolic systems for lignin‐derived aromatic compounds, including newly identified catabolic pathways and genes for decomposition of lignin‐derived biaryls, transcriptional regulation and substrate uptake systems. Recent omics approaches on catabolism of lignin‐derived aromatic compounds are also described.

High value‐added monomer chemicals and functional bio‐based materials derived from polymeric components of lignocellulose by organosolv fractionation

“…Catabolic pathways of lignin‐derived aromatic precursors in bacteria (DDVA: dehydrodivanillic acid; PCA: protocatechuic acid; CHMS: 4‐carboxy‐2‐hydroxymuconate‐6‐semialdehyde; PDC: 2‐pyrone‐4,6‐dicarboxylate; OMA: 4‐oxalomesaconate; CHA: 4‐carboxy‐4‐hydroxy‐2‐oxoadipate; CHMOD: 4‐carboxy‐2‐hydroxy‐6‐methoxy‐6‐oxohexa‐2,4‐dienoate). Adapted from Kamimura et al ., Nakajima et al ., Masai et al ., Mori et al ., and Vardon et al ., …”

“…). The monolignols are connected by different C▬O▬C bonds and C▬C bonds formed by free radical polymerization during biosynthesis . The β ‐ O ‐4 ether motif is the main linkage in lignin structure.…”

“…Second, the aromatic rings of catechols derivatives are cleaved by ring‐opening dioxygenases through lower pathways. Finally, the ring‐opened aromatic derivatives are further metabolized and converted to tricarboxylic acid cycle intermediates . Recently, numerous studies have been conducted to identify new catabolic pathways and genes for the decomposition and subsequent bioconversion of lignin‐derived aryl compounds.…”

“…Genetic modification have been made to intensify the production of precursor chemicals for biomaterials . Pyruvate and l ‐lactic acid yields from lignin could be improved significantly after replacing the endogenous ortho catechol degradation pathway of Pseudomonas putida KT 2440 with an exogenous meta‐cleavage pathway .…”

Promising Techniques for Depolymerization of Lignin into Value‐added Chemicals

“…From a viewpoint of mechanism, the bacteria can funnel various of lignin derived aromatics into vanillate and syringate intermediates which are successively converted into catechol‐like derivatives. And the derivatives decompose by ring‐opening to form intermediates for tricarboxylic acid cycle and, finally, form CO 2 and H 2 O . Thus, on the other hand, valuable intermediates in the funnel process can be accumulated by blocking the pathway to subsequent intermediates and building up new pathway to target products.…”