Depolymerization and Conversion of Lignin to Value-Added Bioproducts

Weng et al. Biotechnol Biofuels (2021) 14:84 https://doi.org/10.1186/s13068-021-01934-w Biotechnology for Biofuels REVIEW Open Access Depolymerization and conversion of lignin to value-added bioproducts by microbial and enzymatic catalysis Caihong Weng1,2, Xiaowei Peng1,2 and Yejun Han1,2* Abstract Lignin, the most abundant renewable aromatic compound in nature, is an excellent feedstock for value-added bioproducts manufacturing; while the intrinsic heterogeneity and recalcitrance of which hindered the efcient lignin biorefnery and utilization. Compared with chemical processing, bioprocessing with microbial and enzymatic catalysis is a clean and efcient method for lignin depolymerization and conversion. Generally, lignin bioprocessing involves lignin decomposition to lignin-based aromatics via extracellular microbial enzymes and further converted to value-added bioproducts through microbial metabolism. In the review, the most recent advances in degradation and conversion of lignin to value-added bioproducts catalyzed by microbes and enzymes were summarized. The lignin-degrading microorganisms of white-rot fungi, brown-rot fungi, soft-rot fungi, and bacteria under aerobic and anaerobic conditions were comparatively analyzed. The catalytic metabolism of the microbial lignin-degrading enzymes of laccase, lignin peroxidase, manganese peroxidase, biphenyl bond cleavage enzyme, versatile peroxidase, and β-etherize was discussed. The microbial metabolic process of H-lignin, G-lignin, S-lignin based derivatives, protocatechuic acid, and catechol was reviewed. Lignin was depolymerized to lignin-derived aromatic compounds by the secreted enzymes of fungi and bacteria, and the aromatics were converted to value-added compounds through microbial catalysis and metabolic engineering. The review also proposes new insights for future work to overcome the recalcitrance of lignin and convert it to value-added bioproducts by microbial and enzymatic catalysis. Keywords: Lignin, Depolymerization, Enzymatic degradation, Lignin-derived aromatics, Metabolic pathways, Value- added bioproducts, Biosynthesis Background bioproducts [6, 7], while most lignin cannot be utilized Converting the renewable biomass to chemicals and efciently. Large amounts of lignin have been formed 8 fuels is an attractive and green method for the sustain- and estimated to be in the range 5–36 × 10 tons annu- able environment development. Lignocellulose, the most ally [8]. Among them, the biomass refnery and pulp/ 7 7 abundant renewable resource in nature, is mainly com- paper industries contribute about 6.2 × 10 and 5 × 10 posed of cellulose, hemicellulose, and lignin [1]. Cellulose tons of lignin per year, respectively, including kraft lignin, and hemicellulose can be degraded into monosaccharides lignosulfonate, and soda lignin [9]. In most cases, lignin by enzymatic hydrolysis [2–5] and fermented to various is currently used for energy supply or discarded as waste. Lignin is a promising feedstock to produce biofuels and biochemicals owing to its high carbon-to-oxygen ratio *Correspondence: [email protected] and rich aromatic skeleton. To exploit lignin valoriza- 1 National Key Laboratory of Biochemical Engineering, Institute of Process tion, it is an urgent need to understand the degradation Engineering, Chinese Academy of Sciences, Beijing 100190, China Full list of author information is available at the end of the article © The Author(s) 2021. 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The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Weng et al. Biotechnol Biofuels (2021) 14:84 Page 2 of 22 process and develop efcient metabolic pathway for structure and recalcitrance of lignin are the main chal- conversion. lenges for its efcient depolymerization and utilization. Lignin is an amorphous heteropolymer consisting of Currently, thermochemical and biological approaches are three phenylpropanoid units of guaiacyl alcohol, p-cou- the main methods for lignin depolymerization. Termo- maryl alcohol, and syringyl alcohol, which are connected chemical processes including pyrolysis (thermolysis), gas- by the chemical bonds of aryl ether (β-O-4), phenyl- ifcation, hydrogenolysis, and chemical oxidation require coumaran (β-5), resinol (β-β), biphenyl ether (5-O-4), stringent conditions, intensive energy input, and expen- and dibenzodioxocin (5–5) [10] (Fig. 1). Te complex sive facilities [11]. In contrast, the bioprocessing of lignin Fig. 1 Major backbone units and representative linkages in lignin molecule. a The building blocks of lignin consist of three primary types of monolignols, namely p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The alcohols form the corresponding phenylpropanoid units like p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) in lignin polymer, respectively. b Backbone units are conjugated via diferent chemical bonds (e.g., β-O-4, β-β, 5–5, and β-5) resulting in high resistance to lignin depolymerization Weng et al. Biotechnol Biofuels (2021) 14:84 Page 3 of 22 has the advantage of high specifcity, low-energy input, Lignin depolymerization by microorganisms and cost-efectiveness [12]. Te bioprocessing treatment and enzymes with microorganism normally include two steps: native Lignin depolymerization by fungi lignin is frstly degraded to heterogeneous aromatics, Te depolymerization of lignin is critical for lignin uti- which then enter the central carbon metabolism [13]. So lization, and diverse lignin-degrading enzymes and far, a large number of microorganisms including fungi metabolic system of microorganism have been evolved and bacteria have been found to be able to degrade and for lignin degradation and conversion [24]. Fungi are metabolize lignin. the most efective lignin-degrading microorganisms, Lignin is mainly depolymerized by extracellular oxi- which can secrete a variety of lignin-degrading enzymes. dases secreted by microorganisms such as lignin per- According to the degradation mechanism of lignin, the oxidase (LiP), manganese peroxidase (MnP), versatile lignin-degrading fungi mainly include three types: white- peroxidase (VP), dye-decolorizing peroxidase (DyP), and rot, brown-rot, and soft-rot fungi [25]. Among the three laccase [14]. During the oxidation process, the unstable lignin-degrading fungi, only white-rot fungi can com- free radicals produced by the oxidase can attack lignin, pletely degrade lignin to CO2 and H2O [26], and the typi- and cleave the chemical bonds [15]. Te resulted aro- cal fungi for lignin degradation are shown in Table 1. matic compounds were further metabolized by microbes White-rot fungi White-rot fungi are the main lignin via the enzymatic reactions [16] and β-ketoadipate path- degradation microorganism in nature, and its degrada- way, and fnally converted to valuable products. To now, tion ability is better than brown-rot and soft-rot fungi lignin has been successfully manufactured into value- [25]. Te lignin-degrading white-rot fungi include most added products of polyhydroxyalkanoates (bioplastic) strains of basidiomycetes and a few species of ascomy- [17], lipids (often used as biofuel) [18], animal feed addi- cetes [27]. Among white-rot fungi, the species of Ceri- tive [19], pesticides [20], compost (generally as bioferti- poriopsis subvermispora, Phellinus pini, Ganoderma lizers) [21], vanillin [22], and muconic acid [23]. australe, and Phlebia tremellosa specifcally degrade In the present review, we focused on the bioprocessing lignin and hemicellulose but not cellulose. However, of lignin and bioconverting it to value-added bioprod- other strains such as Phanerochaete chrysosporium, ucts. Te most recent development on lignin depolym- Trametes versicolor, Heterobasidion annosum, and Irpex erization by microorganisms, the microbial secreted lacteusare can simultaneously degrade cellulose, hemi- oxidases, and the decomposition mechanism were sum- cellulose, and lignin [28, 29]. Te main extracellular marized. Additionally, the metabolism of lignin-derived enzymes secreted by white-rot fungi for lignin-degrading aromatics in diferent microorganisms was illustrated were oxidases and peroxidases. Te oxidative reactions and the production of value-added products through catalyzed by oxidoreductase for lignin decomposition microbial metabolic engineering was proposed. include the cleavage of carbon–carbon bonds and ether linkages, and the removal of side chain and aromatic rings [30]. P. chrysosporium is a model white rot fungus for lignin degradation, which has been applied for biological pretreatment of lignocellulosic biomass [31, 32].

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