Lignin is a major component of lignocellulose,accounting for 15％-30％on a dry weight basis,with an annual yield estimated to be 20 billion tonnes.Lignin is a heterogenous aromatic polymer of phenylpropanoids linked by various C—C and C—O bonds.It is an integral component of the secondary cell wall from terrestrial plants,providing plants with rigidness and fending off microbial pathogens.The abundance and renewability of lignin has recently attracted ample interest in valorizing this readily available polymer.However,the complex nature of lignin presents a significant challenge for lignin breakdown and utilization,and at present the majority of lignin is simply burned as a fuel.Among the different methods,biological utilization of lignin has emerged as a highly attractive approach,since it proceeds under mild conditions and is generally considered environmentally friendly,especially considering that environmental sustainability is trending worldwide.This review comprises three major sections.First,we will summarize key enzymes that nature has created to break down lignin,including laccase,manganese peroxidase,lignin peroxidase,dye-decolorizing peroxidase,and versatile peroxidase etc.Relevant enzymes and their catalytic mechanisms will also be briefly discussed.Second,we will review key reactions in priming and processing lignin derived aromatics before they enter microbial metabolic pathways:O-demethylation,hydroxylation,decarboxylation,and ring opening,as well as representative enzymes involved and their catalytic mechanisms.Finally,we will present engineering efforts toward biological valorization of lignin and lignin derived aromatics,which is largely driven by synthetic biology approaches.Biological valorization of lignin is undoubtedly a field full of potential,however its realization still faces several major hurdles,such as low conversion efficiency and long processing time.Nevertheless,as synthetic biology is developing rapidly,harnessing the power of genetic and metabolic engineering to improve the efficiency of lignin breakdown and utilization,microbial tolerance to toxic aromatics,and redox balance will certainly be a promising path forward,paving the way for industrial application in the near future.
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