Elucidation of microbial lignin degradation pathways using synthetic isotope-labelled lignin

Pathways by which the biopolymer lignin is broken down by soil microbes could be used to engineer new biocatalytic routes from lignin to renewable chemicals, but are currently not fully understood. In order to probe these pathways, we have prepared synthetic lignins containing C-13 at the sidechain beta-carbon. Feeding of [beta-C-13]-labelled DHP lignin to Rhodococcus jostii RHA1 has led to the incorporation of C-13 label into metabolites oxalic acid, 4-hydroxyphenylacetic acid, and 4-hydroxy-3-methoxyphenylacetic acid, confirming that they are derived from lignin breakdown. We have identified a glycolate oxidase enzyme in Rhodococcus jostii RHA1 which is able to oxidise glycolaldehyde via glycolic acid to oxalic acid, thereby identifying a pathway for the formation of oxalic acid. R. jostii glycolate oxidase also catalyses the conversion of 4-hydroxyphenylacetic acid to 4-hydroxybenzoylformic acid, identifying another possible pathway to 4-hydroxybenzoylformic acid. Formation of labelled oxalic acid was also observed from [beta-C-13]-polyferulic acid, which provides experimental evidence in favour of a radical mechanism for alpha,beta-bond cleavage of beta-aryl ether units.

Automated summary

The breakdown pathways of lignin by soil microbes have the potential to be used for developing new routes for the production of renewable chemicals, but remain incompletely understood. By feeding β-C-13-labelled DHP lignin to Rhodococcus jostii RHA1, it was found that C-13 label was incorporated into metabolites such as oxalic acid, 4-hydroxyphenylacetic acid, and 4-hydroxy-3-methoxyphenylacetic acid, confirming their origin from lignin breakdown. The identification of glycolate oxidase enzyme and its role in the production of oxalic acid and 4-hydroxybenzoylformic acid provides insights into the pathways involved in lignin degradation.