Lignin bioconversion into valuable products: fractionation, depolymerization, aromatic compound conversion, and bioproduct formation

Lignin fractionation and depolymerization generates heterogeneous streams of aromatic compounds and conversion of aromatic compounds into valuable products, but it is not efficient. Many microbes in nature have evolved metabolic pathways to convert complex lignin polymers into aromatic compounds and transform these aromatic compounds into central intermediates for bioproduct synthesis. The objective of this paper is to review the recent process development of lignin bioconversion into aromatic compounds and bioproducts. Lignin structural and molecular changes during fractionation and depolymerization are presented. Subsequent lignin conversion into aromatic compounds by upper pathways and further converted into central metabolites and bioproducts via lower pathways are emphasized. In particular, enzymes and mediator systems to enhance lignin conversion and key intermediates in lignin catabolic pathways are discussed. Strategies to enhance bioproduct formation through lignin valorization are summarized.