Biocatalytic valorisation of lignin via genetic or chemical intervention of bacterial aromatic degradation pathways

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Abstract

Global dependence on fossil fuels is not only finite, but also environmentally unsound. A move from petroleum-based chemicals to an alternative based on renewable sources such as agricultural lignocellulosic waste materials could provide significant environmental and economic gains. Lignin, a component of such feedstocks represents an abundant source of aromatic compounds if an effective method for controlled conversion to fine chemicals can be discovered.

Despite the many potential benefits of chemicals derived from lignin, it still remains an unlocked source of more valuable substances.

Lignin is a complex polymer that gives rigidity and protection to plant cell walls. By its very nature it is inherently very hard to break down. Pre-treatments so far have revolved around removing lignin to access the more exploitable sugars from cellulose and hemicellulose.

If a biocatalytic approach to valorisation could combine the natural microbial breakdown of lignin with advances in genetics, then potentially this may offer a method to derive useful aromatic by-products.

In the present thesis, a combined approach was taken for the valorisation of natural state and industrial sources of lignin. Genetic mutants of lignin-degrading bacteria were prepared by targeting enzymes for deletion based on hypothetical lignin degradation pathways. These mutants were used in incubations with different sources of lignin. A chemical approach using mechanism-based and commercially available enzyme inhibitors is also proposed.

These techniques were investigated and various procedures explored and employed, to varying degrees of success, most notably the production of up to 96 mg/L after 144 h of vanillin using a Δvdh mutant of Rhodococcus jostii RHA1 in incubations with 2.5% wheat straw lignocellulose (w/v).

A chemical approach identified a potent in vitro hydroxamic acid as an inhibitor of protocatechuate 3,4-dioxygenase, with tentative results also suggesting in vivo inhibition.

These results in combination, or as stand-alone technique, may provide a valuable method for the valorisation of lignin from agricultural waste feedstocks. They may also bring the possibility of a commercially viable and environmentally sound concept closer to realisation.

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