Structural changes of lignin in biorefinery pretreatments and consequences to enzyme-lignin interactions

The main target of a biorefinery pretreatment process is to break down the lignin-reinforced plant cell wall structure prior to enzymatic hydrolysis of polysaccharides to fermentable sugars. Various physico-chemical alterations occur in lignin during the biomass pretreatment, but effects of those structural changes on subsequent enzymatic hydrolysis have remained ambiguous. We review the reinforcing and detrimental lignin-enzyme interactions and their underlying mechanisms, and use this structure-function information to assess critical features of current and emerging pretreatment technologies. Our perspective is that truly multidisciplinary research is needed to develop pretreatments that render lignin non-inhibiting to enzymes and with high potential for further valorisation.