Keywords

11.1 Introduction

The bio-based economy revolves around the reuse of waste lignocellulosic biomasses (LCB) which are abundantly available on earth and are renewable (Kim et al. 2019). They have proven to be an effective source for the production of various value-added bioproducts. The LCB is available universally and the sources include trees, agro-residues, industrial by-products, grasses, etc. (Anwar et al. 2014; Isikgor and Becer 2015; Watkins et al. 2015). These LCB consist of the lignocelluloses, which comprises three main components i.e., cellulose (40–50%), hemicellulose (25–30%), and lignin (15–20%) (Gray et al. 2006; Singla et al. 2012). These components together form a three-dimensional network complex structure (Sánchez 2009). Cellulose and hemicellulose in the plant cell wall are less accessible to the microorganisms due to their chemical association, tightly bound interaction with lignin, and their ratio of monomers (Ni and Tokuda 2013; Li and Zheng 2017). The deconstruction of the renewable lignocelluloses for the generation of value-added bioproducts can be done by various physical, chemical, and biological methods, however, as the concerns over the environment conservation is gaining impetuous the use of biological methods are more preferred over other methods (Binod et al. 2018; Wagner et al. 2018; Kumar et al. 2020; Kumar and Verma 2020b). As the LCB has gathered attention worldwide among the researcher’s several methodologies for LCB utilization have been developed of which enzymatic hydrolysis is an effective method and environmentally friendly (Chaturvedi and Verma 2013).Thus, keeping in prospect the above points the present chapter focuses on the components of LCB, different enzymes involved in efficient delignification of LCB, mechanism of action of various biocatalyst, methodologies to improve its yield, and its application in the bio-based economy.

11.2 The Lignocellulosic Biomass (LCB) Components

Lignocellulose is a reinforced structure comprising of lignin, cellulose, and hemicelluloses, which are bound together with help of different covalent and noncovalent interactions to form a rigid three-dimensional structure. The three components have been shown Fig. 11.1 and described as follows.

Fig. 11.1
figure 1

Structural makeup of lignocellulosic biomass (LCB)

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11.2.1 Cellulose

Cellulose is a homopolysaccharide consisting of β-d-glucopyranose moieties linked via β-(1, 4) glycosidic bonds. The amount of celluloses in lignocellulosic materials can range from 30 to 50% of the dry biomass weight (Mussatto and Dragone 2016). Celluloses are mainly located in the secondary wall and the degree of polymerization of cellulose chain can be as low as 10,000 glucose residues in wood and as high as 15,000 glucose residues in cotton (Frederick 2012; Guha et al. 2010). Intra and intermolecular hydrogen bonds result in aggregation of cellulose molecules which are called micro-fibrils that can gather together to form fibrils that subsequently bundle together to form cellulose fibers. The cellulose fibers consist of highly ordered crystalline regions and less ordered amorphous regions as a result of intra-and intermolecular hydrogen bonding. These strong hydrogen bonds and the crystalline structure render cellulose relatively stable toward chemical and enzyme attack and render it insoluble in most of the solvents (Rios-Fránquez et al. 2019).

11.2.2 Hemicellulose

Hemicelluloses are the cluster of various homo and heteropolymers (Perkins 2012). These polymers consist of the main chain of xylopyranose, mannopyranose, glucopyranose, and galactopyranose along with various substituents and together they form a complex polymeric structure (Collins et al. 2005; Shallom and Shoham 2003). Hemicelluloses are mostly found in different plants with varying composition and structure. The major component of hemicelluloses is β-1, 4-xylan, which is the second most abundant polysaccharide in nature after cellulose (Collins et al. 2005; Polizeli et al. 2005; Chávez et al. 2006). Hemicelluloses cover one-third of total renewable LCB found on earth and also comprises around 20–30% of total dry weight of annual plants and tropical hardwood (Collins et al. 2005; Singh et al. 2003).

11.2.3 Lignin

Lignin is made up of phenylpropane units (p-coumaryl, coniferyl, guaiacyl, syringyl, and sinapyl) linked with ether and C–C bonds. Phenolic compounds found in lignocellulose are considered as cross-linking agents by ester or ether bonds and by arabinoxylans esterification (Sánchez 2009; Ni and Tokuda 2013). According to Bermek and Eriksson (2009) the 3D structure of lignin is not known, but its composition varies in softwood and hardwood. Softwood comprises of guaiacyl units (14–25% in herbs) and hardwood mainly comprises of syringyl and guaiacyl units (27–32% in woody trees). Higher lignin’s with P-hydroxyphenyl units along with syringyl and guaiacyl units are found on more woody plants, old trees, and annual plants, deposited in their cell wall which provide rigidity, water proof nature, and protection against microbes.

11.3 The Role of Lignocelluloytic Enzymes in the Bioconversion of LCB to Value-Added Products

The LCB components have the presence of complex structure which requires the activity of various enzymes for its hydrolysis and efficient conversion into monomeric units such as glucose, xylose, p-coumaryl alcohol, coniferyl alcohol, sinapyl alcohol, and oligomers such as trioses and tetraoses (Annamalai et al. 2009; Irfan et al. 2012). Several microorganisms utilizes LCB as their nutrient source and produce several hydrolytic enzymes collectively called as lignocelluloytic enzymes which include cellulase, xylanase, and laccase, lignin peroxidase (LiP), manganese peroxidase (MnP), and versatile peroxidase (VP). Overviews of these enzymes are described below.

11.3.1 Cellulolytic Enzymes

Cellulases are a multi-enzyme complex which hydrolyzes cellulose. The enzyme complex consists of mainly three enzymes i.e., endoglucanases, cellobiohydrolases, and β-glucosidases. These three enzymes act synergistically on the cellulose polymers to generate glucose monomers (Legodi et al. 2019).

11.3.1.1 Endoglucanases or Endo-1,4-β-d-Glucanase

Endoglucanases (EC 3.2.1.4) hydrolyze internal β-1,4-glucosidic bonds randomly in the amorphous regions of the long cellulose chain resulting in the formation of reducing and nonreducing ends of long-chain oligosaccharides (Singhania et al. 2017; Binod et al. 2018). The crystal structure of endoglucanases (Cel5A) protein of Thermoascus aurantiacus consist of a 335 amino acids and these amino acids mold into an eightfold (β/α)8 barrel architecture. The catalytic subunit of this enzyme consists of a compact structure containing short loops. Secondary structure of the protein is also represented with a short double-stranded anti-parallel β sheet in β/α-loop3 and 3 one turn helices (Leggio and Larsen 2002). Several biocatalysis of cellulose processes require psychrophilic enzymes and thus study of these enzymes has gained impetus (Violot et al. 2005; Kasana and Gulati 2011). Recently, oxidative type of cellulase has been recognized, this oxidative cellulase utilizes free radical reaction mechanism for depolymerization of the cellulose (Violot et al. 2005; Juturu and Wu 2014).

11.3.1.2 Cellobiohydrolases (CBH) or Exoglucanases

Cellobiohydrolases (EC 3.2.1.91) cleaves the long-chain oligosaccharides releasing cellobiose (Singhania et al. 2017; Binod et al. 2018). The crystal structure of Clostridium thermocellum cellobiohydrolase CelS protein folds into an (α/α)6 barrel with tunnel-shaped binding region. At the N-terminal side of the inner α-helices, tunnel cover one-third of the (α/α)6 barrel side and play a crucial role in the substrate binding. Even in the absence of substrate the loops defining the tunnel are stable and thus suggest a dynamic mode of action of cellobiohydrolases (Guimarães et al. 2002).

11.3.1.3 β-Glucosidases or β-Glucoside Glucohydrolases

β-glucosidases (EC 3.2.1.21) hydrolyses the glycosidic bonds of β-d-glucosides or cellobioses and oligosaccharides and generate glucose monomers (Singhania et al. 2017; Binod et al. 2018). Based on the crystal structure of β-glucosidase A (BglA) of Bacillus polymyxa, these proteins belong to clan GH-A and GH family. The BglA protein exists in octameric form, on further screening it was observed that these octamers exist as a tetramer of the dimer in a fourfold axis. These dimmers are strongly stabilized by 14 H-bonds and water molecules. The monomeric units adopt a adopts a single (α/β)8 barrel topology and between the α/β subunits secondary structure is inserted (Sanz-Aparicio et al. 1998; Juturu and Wu 2014).

11.3.2 Xylanolytic Enzymes

Xylanases that are also known as “endo-1, 4-β-xylanase” helps in the hydrolysis of 1, 4-β-d-xylosidic bond of xylan (Kuhad et al. 1997; Bhardwaj et al. 2019b). They are the highly preferred catalyst for the endohydrolysis of xylan because of their high specificity, mild reaction condition, and negligible substrate loss. The xylanolytic enzyme can be of six different types, which are as follows.

11.3.2.1 Endo-1,4-β-d-xylanases

Endo-1,4-β-d-xylanases (EC 3.2.1.8) also known as 1, 4-β-d-xylanxylanohydrolase cleaves the glycosidic bonds of backbone of the xylan by reducing the degree of polymerization of the substrates (Kumar and Shukla 2016, 2018). Different xylanases have difference in the substrate specificity which has an important role in the destruction of xylan (Prajapati et al. 2018). The selection of cleaving sites by these enzymes depends on the presence and arrangement of molecules with a specific degree in the side chain and the chain length (Liab et al. 2000; Polizeli et al. 2005).

11.3.2.2 1,4-β-Xylosidases

The1,4-β-xylosidases (EC 3.2.1.37) plays a very important role after endo-1,4-β-xylanases in xylan degradation by hydrolyzing soluble xylooligomers formed by the endoxylanase hydrolysis of insoluble xylan into xylose from the non-reducing ends (Lasrado and Gudipati 2013). Transglycosylation reactions were also catalyzed by 1,4-β-xylosidases in which xylose units release the monosaccharide units or alcohols attached to it (Jordan et al. 2013). Biely et al. (1986) explained the classification of 1,4-β-xylosidases, which can be in two forms i.e., xylobiases for xylobiose and exo-1, 4-β-xylanases for larger xylooligosaccharides. Xylobiose is the most suitable substrate for the 1,4-β-xylosidases and its xylooligosaccharides affinity depends on the degree of polymerization which is inversely proportional to each other (Terrasan et al. 2016). These enzymes can also cleave some artificial substrates e.g., o-nitrophenyl- and p-nitrophenyl- c-d-xylopyranoside (Jordan et al. 2013).

11.3.2.3 Acetyl Xylan Esterase

Acetyl xylanesterase (EC 3.1.1.72) primarily hydrolyze the acetyl xylan esters by removing the O-acetyl groups from 2 and/or 3 positions on the β-d-xylopyranosyl residues. Xylan from different plant origin varies in their arrangement of the side chain. The xylopyranoside units with acetic acid were found in the C2 and C3 position in the hardwood and annual plants, 4-O-methyglucuronic acid at C2 position in hardwood, softwood, and annual plants and arabinose at the C3 position of softwood and annual plants (Adesioye et al. 2016). The enzymatic approach that cleaves the backbone of xylan can be interfered with due to the presence of acetyl side-groups, acetyl xylan esters play an important role by the steric hindrance and elimination of these groups and facilitating the endoxylanase activity (Polizeli et al. 2005).

11.3.2.4 Arabinase

The l-arabinase is a branched polymer made up of (1-5)-, (1-2)-and (1-3)-α-linked l-arabinofuranosyl residues and its component are occupied with covalent cross-linking of polysaccharides of the cell wall. Mode of action of arabinase differentiate it into two types i.e., p-nitrophenyl-α-l-arabinofuranosides and branched arabinans degrading exo-α-l-arabinofuranosyl (EC 3.2.1.55) and linear arabinans hydrolyzing endo-1,5-α-l-arabinase (EC 3.2.1.99) (Semenova et al. 2018).

11.3.2.5 α-Glucuronidase

The α-glucuronidase (EC 3.2.1.139) hydrolyzes the α-1, 2-linkage of glucuronic acid residues, and nonreducing terminal β-d-xylopyranosyl backbone units of glucuronoxylan (aldouronic acids) and releases MeGlcA of GlcA (Yan et al. 2017). Uronic acid and xylose ratio depends on the source of xylan e.g., in softwood the ratio is 1:5 which contains a large amount of 4-O-methyglucuronic acids, whereas in hardwood very less uronic acid is found with the ratio of 1:10. Similarly, in annual plants uronic acids quantity depends upon the plants species and it varies even on their parts (Asmadi et al. 2017; Sporck et al. 2017).

11.3.2.6 Ferulic Acid Esterase and p-Coumaric Acid Esterase

These enzymes cleave the ester bonds present on the xylan. Ferulic acid esterase (EC 3.1.1.73) between arabinose and side group of ferulic acid and p-coumaric acid works with arabinose and p-coumaric acid (EC 3.1.1.) (Hunt et al. 2017; Morris et al. 2017).

11.3.3 Ligninolytic Enzymes

The ligninolytic enzymes consist of laccases, hemeperoxidase which are of two types namely lignin peroxidises (LiP) and manganese peroxidase (MnP), and lastly the versatile peroxidises (VP), which are described as follows.

11.3.3.1 Laccase

Laccases (E.C. 1.10.3.2) also known as benzenediol: oxygen oxidoreductase or p-diphenoloxidase belongs to the oxidoreductase class. The laccase is universally present however the laccase from white-rot fungi are the most commonly studied. The laccase can break the lignocellulosic wall, complex polyphenol structure which consists of lignin. These enzymes have a broad substrate range, though they do not require an inducible component for its production or enhancement, but considering the industrial prospect and utility it can be considerably induced in the presence of copper, dyes, or other recalcitrant compounds (Minussi et al. 2007). The enzyme is capable of degrading the phenolic compounds in the absence of mediator, however, the degradation of the non-phenolic compounds require the mediator to initiate the reaction, which is contributed due to the redox potential of both the enzyme and the substrate which plays an important part in the acting capability of the enzyme on various substrates (Kudanga and Le Roes-Hill 2014; Agrawal et al. 2019).

11.3.3.2 Heme-Peroxidases Consisting of MnP and LiP

The heme-peroxidases are an important part of ligninolytic enzymes consisting of lignin peroxidase (LiP) and manganese peroxidase (MnP). LiP and MnP have three reactions of the catalytic cycle which are as follows a) Hydrogen peroxide oxidizes the enzyme to produce compound I (modified enzyme) and water, (b) The compound I (modified enzyme) catalyzes the production of compound II (second modified form of an enzyme), formed by the electron transfer from the reduced substrate along with a generation of free radical, (c) The compound II reacts with the molecule of a reduced substrate to produce another free radical and water. As the reaction continues the enzymes reduces to its native form, (Piontek et al. 1993; Sundaramoorthy et al. 1994; Choinowski et al. 1999; Martínez 2002; Dias et al. 2007; Piontek et al. 1993) and are represented in Fig. 11.2.

Fig. 11.2
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The schematic representation of the three reactions in the catalytic cycle of LiP and MnP

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11.3.3.2.1 Lignin Peroxidise

LiP’s (E.C. 1.11.1.14) were originally discovered in Phanerochaete chrysosporium (Dias et al. 2007). LiP possesses high redox potential and can catalyze the degradation of a broad range of aromatic structures. It oxidizes aromatic rings which are moderately activated by electron-donating substitutes whereas the common peroxidases participate in the catalysis of aromatic substrates. An explanation for this type of catalysis is the production of veratryl alcohol radicals which have higher redox potential than LiP’s compounds I and II and can eventually participate in the degradation of compounds with high redox potential (Khindaria et al. 1996).

11.3.3.2.2 Manganese Peroxidise

MnP (EC 1.11.1.13) are glycoproteins with a molecular weight in the range from 38 and 62.5 kDa (Hofrichter 2002), MnP structure has two domains which are as follows: first heminic group in the middle, second consist ten major helixes, third is a minor helix, and fourth consist of five disulfide bridges. Among those bridges, one bridge participates in the manganese (Mn) binding site, and is this site distinguishes MnP from other peroxidases (Sundaramoorthy et al. 1994). The catalytic reaction has been previously described in Fig. 11.2. However, in case the compound II mandatorily requires the presence of Mn2+ for its reaction to proceed. The compound II then oxidizes Mn2+ to Mn3+ which is responsible to oxidize aromatic compounds. The converted Mn3+ are than stabilized by organic acids which react nonspecifically with organic molecules thereby removing an electron and a proton from the substrates (Hofrichter 2002), The attack of the Mn3+ which is a small size compound having high redox potential diffuses easily in the lignified cell wall thereby attacking inside the plant cell wall facilitating the penetration as well as the action of other enzymes (Martínez 2002; Hammel and Cullen 2008).

11.3.3.2.3 Versatile Peroxidises

The VP (EC 1.11.1.16) can directly oxidize Mn2+, methoxybenzenes, phenolic aromatic substrates similarly to MnP, LiP, and horseradish peroxidase. VP has a very broad substrate specificity range and can oxidize the substrates even in the absence of manganese. It can efficiently oxidize both phenolic and nonphenolic lignin model dimers (Polak and Jarosz-Wilkolazka 2012). Thus considering the broad range of substrate overproduction system is desired for biotechnological and industrial sectors (Plácido et al. 2013; Hoopes and Dean 2004).

11.4 Mode of Action of Various Lignocellulolytic Enzymes

The mode of action of the lignocellulolytic enzymes has been represented in Fig. 11.3 and described as follows:

Fig. 11.3
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Schematic representation of mode of action and application of lignocelluloytic enzymes in deconstruction of LCB and production of value-added products

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11.4.1 Mode of Action of Cellulolytic Enzymes

Cellulolytic enzyme works in synergism and thus cellulose hydrolysis is a stepwise process where the first step is called primary hydrolysis followed by secondary hydrolysis. The primary hydrolysis occurs in the presence of endoglucanases and exoglucanases, these enzymes act on the cellulosic substrate and results in the release of cellobioses. Thus the primary hydrolysis (depolymeriztion) is often regarded as rate-limiting step. The cellobioses are subjected to secondary hydrolysis in presence of β-glucosidase for the generation of glucose monomers (Kubicek 1992; Binod et al. 2018).

Cellulase enzyme has been broadly divided into two types: non-complex system and complex system. The non-complex system i.e., endoglucanases and cellobiohydrolases have a characteristic two-domain structure a catalytic domain and a cellulose-binding domain (CBD) or carbohydrate-binding module (CBM) (Kuhad et al. 2016). The catalytic domain consists of catalytic site and carbohydrate/cellulose-binding domain as the name suggests help in the binding of enzyme to cellulose and these domains are linked by glycosylated flexible linker peptide. The CBD plays a vital role in ensuring the orientation of the substrate with the catalytic domain for perfect binding and subsequent degradation of cellulose. The complex system consists of high-molecular-weight complexes known as “cellulosomes” (Behera et al. 2017). These complex cellulases are characteristics of the anaerobic bacteria but observed in certain anaerobic fungi as well. The cellulosomes are protuberance from the bacterial cell wall, which harbor stable enzyme complex that causes degradation of cellulose by binding to it (Doi and Kosugi 2004).

11.4.2 Mode of Action of Xylanolytic Enzymes

Xylan hydrolysis done by xylanolytic enzymes either by inversion or retention of the anomeric centers of monomeric units suggesting the participation of one or two chemical transition state (Moreira et al. 2016). During retention two glutamate residues being employed in the catalytic mechanism, which is double displacement mechanism having an acid catalyst protonating the substrate, a carboxyl group situated with a covalent glycosyl enzyme intermediate with the carboxylate in which substrate is opposite to the sugar anomeric configuration. This can reach both sides through transition state including oxo-carbonium ions (Collins et al. 2005; Uday et al. 2016), whereas in inversion mechanism aspartate along with glutamate is involved (Bhardwaj et al. 2019b). This is a single displacement mechanism, in which only one carboxylate in offers for overall acid-catalyzed group departure. This enzyme also acts as a base for activating a nucleophilic water molecule to attack the anomeric carbon (depending upon the distance between two molecules) for breaking the glycosidic bonds and causing an inversion of anomeric carbon configuration (Subramaniyan and Prema 2002).

11.4.3 Mode of Action of Ligninolytic Enzymes

The action of the enzyme on lignin is an oxidative process where the dissimilar fungal oxidases e.g., glyoxal oxidase, pyranose-2 oxidase, and aryl-alcohol oxidase release extracellular H2O2, which oxidizes the polymer in a reaction which has been catalyzed by LiP, MnP, and laccase (Martínez et al. 2005; Kersten and Cullen 2007; Hammel and Cullen 2008). The high redox potential of LiP and VP has enabled it to act on a wide range of substrates including lignin which oxidizes the benzenic ring irrespective of the degree of methylation and interunit linkages (Ruiz-Dueñas and Martinez 2009), resulting in the formation of unstable aromatic cation radicals, which leads to depolymerization as well as other reactions (Martínez et al. 2005). The released H2O2 activates the heme group and enables it to gain a cofactor via an access channel, thus blocking the ability to oxidize lignin as it is unable to reach the site (heme pocket) where oxidation takes place. Thus these enzymes then oxidize the lignin with the help of the bare tryptophanyl radical (Ruiz-Dueñas and Martinez 2009). As the redox potential of laccase is (≤0.8 V), it can directly attack the phenolic moiety which constitutes 10% of the polymer, but the action of a non-phenolic moiety (redox potential is >1.3 V in comparison to laccase) the direct action would be difficult. However, this problem can be overcome by the use of redox mediators effectively known as laccase mediator system (LMS) (Ralph et al. 2008).

11.5 Microbial Sources for the Production of Lignocellulolytic Enzymes

Naturally, all living beings are capable of degrading carbohydrates; however, capacity to produce cellulase is limited to microorganisms along with some molluscs. Both anaerobic and aerobic bacteria and fungi are capable of producing cellulase. The anaerobic bacteria are mostly found in soil, in cattle rumens, in the termite gut, decomposing plant materials, and isolated from different paper and wood processing industries wastes (Table 11.1) (Rees et al. 2003). As the application of cellulases in various industrial processes requires stability over ambient temperature it has also led to the production of cellulases from thermophilic fungi e.g., Chaetomium thermophilum, Humicola grisea, Myceliopthora thermophila, Sporotrichum thermophile, and Thermoascus aurantiacus (Singhania et al. 2017; Binod et al. 2018).

Table 11.1 Various strain reported for the production of cellulase
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Over the decades many microorganisms such as fungi and bacteria have been reported by many researchers that utilizes hemicelluloses (Table 11.2) (Pokhrel and Yoo 2009; Dong et al. 2012; Liao et al. 2014). Fungi are known as a potential source of xylanases as they produce high titer of extracellular xylanase from various cheap sources such as agricultural residues by degrading their cell wall (Su et al. 2011). Various mesophilic fungi, some white-rot fungi (Schimpf and Schulz 2018), and filamentous fungi (Bhardwaj et al. 2017) are most commonly reported for xylanase production as they produce stable enzymes.

Table 11.2 Various strain reported for the production of xylanase
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The ligninolytic enzymes are universally present and present in different types of organisms as plants, bacteria, insects, and fungi (Table 11.3). The presence of laccase in plants was studied in Japanese lacquer tree, mango, mung bean, peach, tobacco, Zea mays, etc. (Polak and Jarosz-Wilkolazka 2012), bacteria includes actinomycetes, α-proteobacteria, and γ-proteobacteria (Bugg et al. 2011). The presence of laccase has been detected in insects. In the case of fungi, ligninolytic enzymes occur in Ascomycetes, Basidiomycetes, and Deuteromycetes (Dos Santos et al. 2007).

Table 11.3 Various strain reported for the production of laccase, manganese peroxidase, lignin peroxidase and versatile peroxidase
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11.6 Various Approaches Implemented for the Enhanced Production of Lignocellulolytic Enzymes

With advent of biotechnology, several pre-existing harmful chemical mediated processes have been replaced by the enzymatic processes. The applications of enzymes in these industries are limited due to low enzyme yield, temperature, and pH stability and cost of production and suitable downstream processing and reusability. Therefore several researchers are trying to overcome these limitations by adopting various approaches which are as follows:

11.6.1 Fermentation Process Used for the Production of Lignocellulolytic Enzymes

The choice of fermentation technology significantly affects the level of production. Submerged fermentation (SmF) is most accepted for lignocelluloytic enzyme production strategy due to extracellular nature of the enzyme. In SmF method, microbes are cultivated in an aqueous medium rich in essential nutrients for microbial growth. However the cost of the concentration and extraction of enzyme is high. Therefore an alternative i.e., solid state fermentation (SSF) was adapted, it involved the growth of microbes on solid materials (moist), and this method mimicked the natural habitat of the microbes (Hölker et al. 2004). However, the easy handling and monitoring associated with the liquid medium have given an upper hand to the SmF over SSF. Different fermentation method employed for lignocellulolytic enzyme production and their yield have been mentioned in Table 11.4.

Table 11.4 Reported microbial strains, methodology adapted, substrate used for lignocelluloytic enzymes production and its application in various fields
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11.6.2 Lignocellulose as Substrate Used for Enzyme Production

As the cost associated with the production is very high, thus cheap substrates are essential for the enhanced enzyme production. The lignocellulosic substrates such as different agro and forest residues have been used for cellulase, xylanase, and laccase production and help in lignocelluloytic enzyme synthesis without the supplementation other culture medium component and inducers (Elisashvili and Kachlishvili 2009; Ravindran and Jaiswal 2016; Luo et al. 2019; Novy et al. 2019).

11.6.3 Process Optimization for Enhanced Enzyme Production

The enzyme production by microbes are influenced by various nutrient and physical parameters such as substrates, salts in medium, pH, temperature, and light regulation, agitation/shaking the medium (Zhanga et al. 2019). Considering the high cost of commercially available enzymes and increase in their industrial demands, all the above mentioned factors can be optimized using a well-known conventional method One-Factor-at-a-Time (OFAT). However OFAT is time consuming and it can only give the effect of individual component in the fermentation and does not explain about the interaction among them (Deswal et al. 2011; Kumar et al. 2018). Later Response Surface Methodology (RSM) a statistical tool was designed which helped in analyzing multiple factors at the same time (Abdullah et al. 2018; Neelkant et al. 2019). Thus being economically feasible and time saving in contrast to OFAT, in the present time RSM is much more preferred over OFAT.

11.6.4 Strain Improvement for Enhanced Enzyme Production

Several attempts have been made for the improvement of strains by applying techniques such as (a) mutagenesis and selection, (b) gene cloning, and (c) genome shuffling (Peterson and Nevalainen 2012; Singhania et al. 2017). The strain improvement processes is one of the most exploited techniques for the enhanced production for different enzymes and have evolved greatly with time. The cloning of enzyme encoding genes in homologous and heterologous hosts has been attempted. E.coli is the most favored organism for expression as it has several advantages over other expression system such as yeast or plants. Mandels et al. (1971) had selected a mutant strain Trichoderma viride QM6a, from over 100 Trichoderma wild strains for enhanced cellulase production. In order to utilize xylose directly, d-xylose-utilizing pathway controlling genes was added to the recombinant Saccharomyces cerevisiae HZ3001 (Sun et al. 2012). Similarly Davidi et al. (2016) designed a dockerin-fused variant laccase from the aerobic bacterium Thermobifida fusca. The strain improvement processes is one of the most exploited techniques for the enhanced production for different enzymes and have evolved greatly with time.

11.7 Applications of Lignocelluloytic Enzyme in the Bio-Based Economy

LCB has a wide spectrum of applications in various industries. Traditionally, they are applied in food and brewery production, animal feed processing, detergent production, laundry, textile processing and paper pulp manufacturing. Due to the crisis in sustainable supply of fossil fuel, production of biofuels and chemicals from renewable resources are expected to increase rapidly in the foreseeable future. In biorefinery, generation of biofuels and value-added chemicals from renewable LCB have gained much attention worldwide (Kumar et al. 2020). For utilization of biomass, their depolymerization in order to generate fermentable sugars is one of the basics requirements. Several chemical and physical pre-treatment methods have been suggested; however they are energy intensive and generate several pollutants. Therefore, biological pre-treatment or enzymatic hydrolysis have gained impetuous due to its environmental friendly nature and also has several advantages such as specificity over physic-chemical methods (Agbor et al. 2011). The Table 11.4 represents various strains responsible for cellulase, xylanase and laccase production along with various substrate used, enzyme yield and its application.

11.7.1 Recent Development of the Biocatalyst in the Bio-Based Economy

The recent scenario the conversion of biomass to value-added products, and the focus diverting toward greener methods have gained interest among the researchers. The lignocellulolytic enzymes have been greatly investigated for its ability to produce value-added products as shown in Fig. 11.4 and have been discussed as follows.

Fig. 11.4
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Application of lignocelluloytic enzymes in various fields fitting into the concept of “reduce-reuse-recycle” and contributing toward “circular economy”

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11.7.2 Bioethanol and Bio-Based Chemicals

Bioethanol generation from amorphous cellulose and cellulose soaked in ionic liquid were analyzed separately using the recombinant strain S. cerevisiae and highest ethanol yield was observed (Yanase et al. 2010; Nakashima et al. 2011). Kumar et al. (2018) demonstrated the application of in-house produced cellulase from Schizophyllum commune and xylanase from A oryzae LC1 in saccharification of wheat straw and rice straw alone and in combination. The in-house produced cellulase showed results in comparable with the commercial cellulase enzyme Onozuka R-10 from the Trichoderma viridae. Bhardwaj et al. (2018) used partially purified xylanase for generating xylooligosaccharides which included xylobiose, xylotriose and xylotetraose by the direct enzymatic hydrolysis of untreated sugarcane bagasse, wheat straw, and wheat bran. Xylan fermenting thermophilic Clostridium strain was used for direct conversion of LCB to bioethanol production. The strain showed preferential bioethanol production efficiency when wide ranges of substrates were tested with mixture of cellulose and xylan and untreated rice straw. This strain also presented broad range of primary metabolic end products such as acetate, lactate, and hydrogen substrate spectrum utilizing wide range of substrate spectrum (Singh et al. 2018b). Similarly laccase from Lentinus squarrosulus MR13 resulted in lignin removal from Saccharum spontaneum (karn grass) and subsequently resulting enhanced sugar yield (Rajak and Banerjee 2016).

Bhardwaj et al. (2020) demonstrated the synergistic action of laccase, cellulase and xylanase in generation of ethanol using single pot culture system performing simultaneous delignification, saccharification and fermentation. LCB is a sustainable platform for the production of the bio-based chemicals and polymer (Isikgor and Becer 2015; Kumar and Verma 2020b; Agrawal and Verma 2020b). The C5 and C6 carbon generating as a result of hydrolysis of xylanase and cellulase can be used for production of1,4-diacid, 5-HMF, 2,5-FDCA, 3-HPA, aspartic acid, glutamic acid, glucaric acid, itaconic acid, Glycerol, sorbitol, 3-hydroxybutyrolactone, Lactic acid, ABE, xylose-furfural-arabinitol, and levulinic acid platform chemicals.

11.7.3 Textile Industry

The specificity of enzyme has led to development of enzyme as important biochemical in textile industry. The enzymes are highly specific, nontoxic and therefore enzymes have replaced the intensive conventional chemical processes in textile processing. The cellulases and xylanases can effectively remove the hairiness of the cellulose thread with less weight loss, impart brightness and help in giving textile a smooth texture and glossy appearance. Acidic cellulases obtained from the T. reesei (Kuhad et al. 2011), neutral cellulases H. insolens (Anish et al. 2007) xylanases from Bacillus pumilus (Singh et al. 2018a; Battan et al. 2012), and laccase from Pleurotus ostreatus and Trametes versicolor (Anish et al. 2007; Mojsov 2014) are widely used in the textile processing industries.

11.7.4 Detergent Industry

The detergents have several applications such as laundering, dishwashing and cleaning (Schäfer et al. 2005). Traditional chemical detergents have strong bleaching chemicals and phosphates and it adversely affect to the environment and the user (Olsen and Falholt 1998). Due to this drawback, development of detergents for industrial application using enzymes is one of the major research areas today. The enzymes help in removal of proteins starch, fats oil stains and due to its hydrolytic properties increases effectiveness of detergents (Kirk et al. 2002; Hasan et al. 2010). Different hydrolyzing enzymes such as lipases, proteases, cellulases, pectinase, xylanases and laccases are used in different detergent composition for efficient cleaning and fabric care (Kuhad et al. 2011; Li et al. 2012). Apart from removing stains these enzymes cleaves off damaged cotton fabric and maintain color, whiteness and fabric smoothness. Different alkaline cellulases enzymes are used in detergents which are capable of passing through the inter fibril spaces of the cotton and help in efficient stain removal (Juturu and Wu 2014).

11.7.5 Food and Feed

At present, these so called wastes are judiciously utilized to produce valuable products such as enzymes, sugars, biofuels, chemicals, cheap energy sources for fermentation, improved animal feeds, human nutrients and intermediates compounds for generating value-added chemicals (Kuhad et al. 2011). The enzymes have huge applicability and food in food industry as well α-amylases and xylanases have been used for maintaining the softness and elasticity of bread. Cellulase and laccase has been effectively used for the clarification of juice where it catalyzes the cross-linking of polyphenols, thereby resulting in an easy removal of polyphenols by the help of filtration. It has also been used for flavor enhancement in beer. It is possible that better functional understanding of these enzyme classes will broaden its applications within the food industry. The use of enzyme mixtures containing pectinases, cellulases, and hemicellulases can also be used for improved extraction of olive oil as well.

The enzymes have been effectively used as feed additives as well; xylanases and β-glucanases have been used in cereal-based feed for monogastric animal’s (Kirk et al. 2002). The use of pre-treatment of agricultural silage and grain feed by the combination of cellulases or xylanases has helped improve its nutritional value (Godfrey and West 1996) eliminate anti-nutritional factors; degrade certain feed constituents which would help improve its overall nutritional value for its effective use (Choct 2006; Juturu and Wu 2014; Vasconcellos et al. 2015).

11.7.6 Bioremediation

In the wastewater processing sector cellulase enzyme is employed in the bioremediation of the ink released during recycling of paper in pulp and paper industry (Karigar and Rao 2011). Thermo-alkaline tolerant cellulases are new favorites of the scientist working in the area biofuel production and waste management’s (Annamalai et al. 2016; Khan et al. 2016) Humicola species cellulase is highly adaptive to the harsh environment so it is used in bioremediation of wastewater generated from detergents and washing powder industries (Imran et al. 2016). Laccase also play crucial role in decolorization of azo dyes by oxidizing the aromatic ring and transform these azo dyes into less harmful by-products (Verma and Madamwar 2005). Agrawal et al. (2020a) and Agrawal and Verma (2020a) demonstrated the white laccase obtained from Myrothecium verrucaria ITCC-8447 has resulted in complete removal of hazardous chemical i.e., phenol and resorcinol. Similarly the same group has also reported that yellow laccase obtained from Stropharia sp. ITCC 8422 was efficient in degrading anthraquinone violet R and alizarin cyanine green dye (Agrawal and Verma 2019a,b; Agrawal et al. 2020b).

The Commercial laccase obtained from genetically modified Aspergillus oryzae effectively treated meat industry wastewater resulting in reduction of COD and color under optimum conditions (Thirugnanasambandham and Sivakumar 2015). Peroxidases has shown capability to treat wide spectra of aromatic compounds such as aromatic compounds (Chen et al. 2014), anilines (Nakamoto and Machida 1992), aromatic dyes (Bhunia et al. 2001), poly-carbonated biphenyls (Köller et al. 2000), poly-aromatic hydrocarbons (Baborová et al. 2006), and phenols (Bayramouglu and Arica 2008), along with different pollutant of industrial contamination (Cheng et al. 2006). These enzymatic biotransformation serve dual purpose in waste management i.e., treatment of waste and generating the several essential chemicals and intermediates.

11.7.7 Pulp and Paper Industry

The utilization of chemicals in the paper and pulp industry is very high, thus the environmental concern has now replaced the chemical based method to the bio-based methods. The use of enzymes has been an effective and promising tool in the paper and pulp industry. Cellulases individually, or in combination with xylanases, can be used for the deinking of various types of paper wastes (Kumar et al. 2018). Maximum applications involving the use of cellulases and hemicellulases leads to the release of ink from the fiber surface by the partial hydrolysis of carbohydrate molecules (Kuhad et al. 2011; Juturu and Wu 2014; Kumar et al. 2018). Similarly, laccase in the paper and pulp industry plays a role in the delignification and brightening of the pulp but can also remove the lipophilic extractives responsible for pitch deposition from both wood and non-wood paper pulps. It can also improve the physical, chemical, as well as mechanical properties of pulp either by forming reactive radicals with lignin or by functionalizing lignocellulosic fibers. Further it also targets the colored and toxic compounds which are released as effluents from various paper and pulp industries rendering them nontoxic by the polymerization and depolymerization reactions (Virk et al. 2012).

11.8 Overcoming the Limitations and Future Prospects Toward Making “Circular Bio-Economy” a Reality

The major challenges toward making circular based bio-economy a reality through feasible solutions are mentioned below:

  1. 1.

    Selection of suitable strains for the lignocellulolytic enzyme production which can be used in synchronization for biomass deconstruction and development of low cost enzyme production process and effective downstream production technologies.

  2. 2.

    Development of biological and environmental friendly pre-treatment technology for minimizing the natural recalcitrance of LCB.

  3. 3.

    Selection of promising strains or development of suitable technology which can be employed for conversion of lignin, xylan, and cellulose to several reactions intermediated for economically important compounds.

  4. 4.

    Supply chain mechanisms of the wastes collection from field to industries and the development of decentralized system for continuous supply of the biomass to the bio-based industries.

These are the major points may be taken into consideration in order to make concept of circular bio-economy a reality in the future (Kumar and Verma 2020a). In this participation of governments, different stack holders, scientist, economist and engineers will play a pivotal role. The cooperation and synergy among these institutions will lead to a sustainable system of the bio-based economy where recycling and waste reduction is reality.

11.9 Conclusion

The application of lignocellulolytic in biomass deconstruction for generation of sugars and value-added products have to a parallel economy where waste materials such as agricultural residues which were earlier considered as waste are now part of the bio-based economy also known as circular bio-economy. The conversion of hemicelluloytic content of LCB by cellulase and xylanase to reducing sugar which can further be converted to bioethanol can act as green substitute to fossil fuels. Xylanase can help in direct conversion of xylan to different xylooligosaccharides which can be used as dietary supplements and essential pharmaceutical The removal of lignin using laccase can enable the better utilization of the agro-residues and contribute toward green environment and circular economy as well. This parallel bio-based economy where the concept of reduce-reuse-recycle is applied on reutilization of waste material, the concept of circular bio-economy evolves mostly around the LCB and these hydrolyzing lignocellulolytic enzymes. The combination of both the concepts by efficient utilization of wastes and technologies can help achieve the goal of “reduce-reuse-recycle” and contribute toward “circular economy” as well.