## 6 Bioconversion of Lignin Derivative into Bio-Based Chemicals and Materials
Interestingly, at least 37 chemicals could potentially be produced as lignin-based monomers obtained via different homogeneous and heterogeneous catalytic processes. Although the monomer content in lignin is heterogeneous in the origin of lignocellulosic biomass, the procedure follows two steps of fractionation and catalytic depolymerization or a single step with reductive catalytic fractionation for platform chemicals. Bulky monomers are then converted into value-added chemicals by functionalization or fuel additives by defunctionalization. Ideally, these fractionation and depolymerization processes are performed by a chemical catalyst under high temperatures (100-300 \({}^{\circ}\)C) in acidic, alkaline, or solvent conditions [29]. Most chemical catalyst strategies are not adaptable for bioconversion processes due to the strict thermal and chemical reaction conditions. Only two monomers among the 37 putative chemicals derived from lignin are biologically producible in temperature below 50 \({}^{\circ}\)C and under mild reaction conditions: hexa-2,4-dieneioic acid (muconic acid) and medium-chain-length polyhydroxyalkanoate. These two chemicals were biosynthesized by a wild type or engineered _Pseudomonas_ strain, which is the most dominant soil bacteria capable of mineralizing a broad range of recalcitrant xenobiotics [152; 153]. Various _Pseudomonas_ strains and other bacteria can degrade aromatic compounds to either catechol or protocatechuric acid [154; 155]. Through subsequent oxidative reactions, these compounds are converted to acetyl-CoA and succinate or pyruvate and acetaldehyde, which are common metabolites in the main metabolisms of most organisms (Figure 5) Depolymerized heterogeneous monomers from lignin consisting of syringyl-, gualaxyl-, and hydroxyphenyl-aromatic polymers can also be metabolized to reconstruct various bio-building blocks by microorganisms engineered through metabolic engineering or synthetic biology technology [150; 155; 156; 157]. Since the reconstruction of a bio-building block through bacterial metabolisms of lignin monomers has been proposed, various microorganisms have been engineered to convert lignin and lignin-derived aromatics into a variety of value-added compounds: _cis,cis_-muconic acid, catechol, protocathuate, benzoate diol, \(p\)-hyroxybenzoate, guaiaicol, pyrogallol, syringe vaiillin, vanillil, vanilllyl alcohol, coniferyl aldehyde, pyruvate, lactate, succinate, pyridine-related organic acids, polyhydroxyalkanoates (PHAs), and lipids [150; 155; 156; 157; 158]. Recently, itaconic acid, a chemical building block listed among the top 12 value-added chemicals, was also produced from \(p\)-coumaric acid by the engineered _Pseudomonas_ strain [159; 160]. In addition, the engineered _Pseudomonas_ strain accumulated and converted \(p\)-coumaric/ ferulic acid into 4-vinylphenol/4-vinylguaiaicol, which were further applied to polymerize 4-vinylguaicol and 4-vinylcatechol styrene by the enzymatic reaction of _T. versicolor_ laccase [161]. These results showed that bioconversion processes for lignin valorization could be potential cell factories for the production of a variety of renewable chemicals and materials through metabolic engineering and synthetic biology technology. However, the biological processes still have lower productivities and yields than chemical processes due to the physiochemical properties of lignin and its derivatives and their toxicities. The alternative utilization of lignin as a functional material source without depolymerization has been intensively studied. Although the physicochemical properties of lignin are relatively reactive, immiscibility, strong inter- and intra-hydrogen bonding in complex aromatic compounds, and lactose-mediated grafting can modify functional groups and convert their characteristics by introducing functional molecules (Figure 4b).