## 6 Bioconversion of Lignin Derivative into Bio-Based Chemicals and Materials  
The ligninolytic enzyme-based bioconversion of lignin and its derivatives would be an attractive green technology for reducing wastes and energy consumption. Most ligninolytic enzymes for lignin degradation could be useful biocatalysts for biotechnological applications, due to their oxidation capacity toward both aromatic and non-aromatic compounds [37; 38; 39; 40]. Among these ligninolytic oxidases, lacases are most broadly applicable due to their powerful oxidizing capability in the fields of high demand for highly oxidized agents such as the degradation of xenobiotics, hormones, biocides, and drug chemicals; decolorization of dyes; and detection of redox potential as biosensors [37; 38; 39; 40] (Table 1). Lacase can oxidize lignin through the abstraction of a single electron from a monopolinol subunit, which causes activated radicals to react to functionalization on the lignin surface (Figure 4a). Activation of functionalized structures in lignin could propagate C-C bond and alkyl-aryl linkage cleavages, functional group modification, and radical coupling (Figure 4b). These oxidation mechanisms of lacase are applicable for biomass pretreatment as well as the detoxification of biomass hydrolysate in biorefinery processes [37; 38; 39; 40]. In the biomass pretreatment, physicochemical processes generally use harsh reaction conditions with intensive energy consumption for the breakdown of biomass [1; 2; 3; 4; 5]. On the other hand, the biological process using white-rot fungi to produce a ligninolytic enzymes cocktail with lactose could reduce energy consumption and decrease reduce toxic residual compounds [38; 39]. Moreover, the enzymatic process with laccase has already been used for typical delignification and bio-bleaching in the pulp and paper industries instead of chemical oxidation agents [146]. The oxidation of laccase was also used to degrade inhibitory compounds, phenolic and furanic substances, and carboxylic acids, generated in biomass pretreatment processes due to thermal lignin or sugar decompositions [147]. This detoxification could enhance the fermentation productivity or enzymatic conversion yield of biomass hydrolysate for the biorefinery process. However, the synthetic mediators in LMS-based pretreatment increase the overall process cost. Moreover, laccase-based biomass pretreatment combined with enzymatic hydrolysis showed low sugar generation yields due to laccase mediators  
Figure 4: Summary of the potential applications of various lignin derivatives and other biomolecules by laccase catalytic activity for lignin valorization. (**a**) Schematic representation of the laccase catalytic mechanism (**b**) Biotechnological applications for depolymerization, grafting, and polymerization based on laccase catalytic activities. inhibiting cellulases and lytic polysaccharide monooxygenases [37; 148; 149]. Thus, these two processes should be separated to enhance their benefits to biomass utilization. Another unique application of lactose is grafting, which is the covalent bond formation between a macromolecule, e.g., lignin, and a small molecule, soluble functional groups, carboxylic and sulfonic acid, and hydrophilic N-OH [144; 150]. Laccase-mediated grafting would be advantageous to converting hydrophobic lignin into soluble lignin by changing the functional groups on the lignin surface to enhance their reactivity to make enzymes easily accessible in the aqueous reaction phase. Grafting of sulfanilic acid or \(p\)-aminobenzoic acid by laccase showed that lignin is soluble in water under acidic or neutral pH conditions [151]. The bioconversion of lignin or lignin derivative directly to valuable chemicals and materials is still challenging with the existing techniques due to lignin's properties for monomerization via depolymerization and the limitation of suitable enzymes for enzymatic processes. A recent review summarized lignin depolymerization for renewable chemicals based on chemical catalyst processes [29].