Abstract
Lignocellulosic biomass comprises three components – cellulose, hemicellulose and lignin. Biorefining is defined as the separation, isolation and conversion of cellulose, hemicellulose and lignin from lignocellulosic biomass into fuels, chemicals, materials and energy. While a number of processes have been developed over the years to produce valuable products from cellulose and hemicellulose, processes utilizing lignin have been few and far between, largely owing to the severe recalcitrance of lignin. Our inability to utilize lignin is a lost opportunity for the green economy. Lignin is abundant and can provide a myriad of chemicals that could be used as building blocks for life-saving pharmaceuticals or even as flavours and food ingredients. The development of greener conversion platforms that can efficiently convert lignin into valuable products is highly desired. This paper summarizes recent progress made towards the development of a family of biocatalytic processes that convert lignin to pharmaceutical building blocks, flavouring agents and drug delivery platforms. Biocatalytic processes are greener, emit lesser carbon dioxide and are more energy efficient than other alternatives. The first family of biocatalysts selectively degrades lignin to its monoaromatic constituents and the second family of biocatalysts then modifies the monoaromatic constituents to desired target molecules. In particular, this paper describes the novel use of functional metagenomics and whole-cell biosensors for the discovery of 147 new biocatalysts that convert lignin into vanillin and syringaldehyde, and the identification of a novel transaminase that catalyzes the asymmetric synthesis of chiral amines from vanillin, syringaldehyde and 12 other monoaromatic aldehyde and ketone degradation products of lignin. Recent progress on the assembly of a metabolic pathway for the biosynthesis of the spice molecule capsaicin from vanillylamine, and the integration of the entire pathway that valorizes lignin to capsaicin into the metabolic network of E. coli has also been discussed. This work will eventually lead to the development of consolidated bioprocesses that convert lignin into value-added chemicals.
