Keywords

Introduction

The accumulation of large quantities of lignocellulosic wastes over the years, cause big environmental concerns. However, due to their uniques chemical composition based on sugars and other compounds of commercial interest, they could be utilized for the production of a number of value added products, such as ethanol, food additives, organic acids, enzymes etc. The reuse of carbon obtained from breakdown of lignocellulose can be one of the approach towards decreasing the carbon footprint (Isikgor and Becer 2015). Therefore, along with the associated environmental hazards caused by their accumulation, the failure of the use of these materials encompasses a huge loss of potentially useful sources.

Lignocellulose Composition

Chemical Composition

The Lignocellulosic biomass is an organic material representing the most abundant source of biomass on earth (Lin and Tanaka 2006). The cells of woody and non-woody plant are majorly composed of lignocellulosic material, which includes cellulose, hemicellulose and lignin (Yang et al. 2001). The major component of lignocellulose biomass is cellulose, which is the potential substrate as it is renewable, biocompatible and biodegradable (Ahn et al. 2012). These major polymer constituents are closely associated with each other, constituting the cellular complex of the plant biomass (Fig. 1). Hemicellulose wraps around the cellulose backbone. This core is guarded by the lignin molecule forming the lignocellulosic structure and it act like cellular glue (Isikgor and Becer 2015).

Fig. 1
figure 1

Representation of lignocellulose structure showing cellulose, hemicellulose and lignin fractions

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Cellulose, hemicellulose and lignin consists 30–60%, 20–40% and 15–25% of the lignocellulosic material, respectively (Hamelinck et al. 2005). The properties of each component of lignocellulose is mentioned in Table 1.

Table 1 Various components of the lignocellulosic biomass
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The lignocellulosic biomass is a major component of disposed wastes obtained from industries such as agriculture, forestry and municipal waste treatment (Perez et al. 2002). These wastes are plentiful, renewable and of meagre expense as energy sources, and has been mostly under utilized. The percentage of each component in lignocellulose varies, depending on the source of biomass. The wastes include sponge gourd fibers, bagasse, banana waste, wheat straw, grasses, newspaper, nut shells, rice straw, corn stover, corn cobs, softwood, hardwood, sweet sorghum, sugarcane bagasse (Fig. 2).

Fig. 2
figure 2

Percentage of each component variation in lignocellulose from different sources (Reproduced from Iqbal et al. 2013)

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Tertiary Architecture

The management of the tertiary architecture of lignocellulose structures is done by a diverse covalent and non-covalent linkages between its major components. As cellulose is enmeshed with hemicellulose, lignin, and other components, hence their hydrolysis is a daunting task (Leonowicz et al. 1999; Tomme et al. 1995). The microfibrils of cellulose are stabilized by intra- and intermolecular H-bonds and encircled by hemicellulose (mannans and xylans) which linked to cellulose by covalent and H-bonds (Heredia et al. 1995). The complicated H-bonding network found within and between glycan units merged with high degree of polymerization containing thousands of glucose monomers promote additional limitation of the linkages for the.

hydrolysis. These covalent bonds are intensely resistant to chemical and biological hydrolysis (Ezeilo et al. 2017). On contrary, amorphous regions within the cellulose crystalline structure have a heterogeneous composition characterized by a various types of bonds. Finally, this asymmetrical arrangement, which characterizes amorphous regions, is critical to the biodegradation of cellulose. The accessibility of enzymes towards cell wall polysaccharides is directed by the extent to which they are interconnected with phenolic polymers (Chesson 1981; Kuhad et al. 1997).

Lignocellulose Breakdown

The conversion of lignocellulose to simple sugars requires daunting biological processes i.e.:

  1. 1.

    Delignification—release of free cellulose and hemicellulose from the lignocellulosic biomass;

  2. 2.

    Depolymerization—conversion of cellulose and hemicellulose to generate free sugars.

In the second step, the liberation of free sugar can be done by enzymes which can efficiently carry out the process. But, a complicated and detailed sequence arrangement between polysaccharides (hemicellulose and cellulose) and lignin makes the lignocellulosic material a challenge for carbohydrase and ligninase enzyme systems (de Siqueira et al. 2010).

Delignification

There are many conventional pretreatment techniques reported for the delignification of lignocelluloses which includes physical (grinding and milling), chemical (acid and alkali treatment), physicochemical (use of steam, hot air or ammonia fibres) and biological (fungi) treatments (Sun and Cheng 2002). But conventional methods have several disadvantages: they are time consuming, elaborate and cause damages to the environment by generating secondary pollutants. Of late, lignin modifying enzymes have been used such as laccases, peroxidases and enzymes hydrolyzing lignin-carbohydrate complexes.

Depolymerization

The hydrolysis of lignocellulose is a major obstruction in the technology of utilization of lignocellulose to valuable products. Presently, there are two major processes known for hydrolysis of lignocellulose into free sugars, includes

  • catalytic hydrolysis and

  • enzymatic hydrolysis.

Catalytic

The catalytic hydrolysis is usually carried out with homogeneous acid or solid acid. The catalysts used are relatively inexpensive. These catalysts are mineral acids such as acetic acid, oxalic acid etc. and organic acids such as H2SO4, HCl for hydrolysis of cellulose (Amiri and Kamiri 2013). However, there are multiple disadvantages of their use such as energy-inefficiency, requires separation, recycling, and treatment of the waste acids. Of late, recyclable solid acid catalysts are being used to replace mineral acids for cellulose hydrolysis (Zhou et al. 2013).

Enzymatic

The enzymatic hydrolysis is a promising method to obtain fermentable sugars from lignocellulosic biomass, which can proceed under mild conditions with high selectivity. But, the high cost and low efficiency of recycling enzymes are a problem for the application of enzymatic hydrolysis (Mascal and Nikitin 2008).

Main Enzymes in Cellulose and Hemicellulose Hydrolysis

The enzymes employed in depolymerization of cellulose and hemicellulose are as follows:

Cellulases

  • Endo-β-1,4-glucanases, cellobiohydrolases, β-glucosidases

  • Fungal cellulases e.g. Trichoderma, Humicola, Acremonium

  • Bacterial cellulases e.g. Clostridium thermocellum

Hemicellulases

  • Backbone degrading enzymes

  • Enzymes removing the side groups

  • β-xylosidases

Other helper enzymes/proteins

  • Swollenin

Inhibitors of Enzymatic Degradation

The inhibitors of enzymatic degradation are as follows:

  • Phenolics

Aromatic compounds show varied effects depending on the functional groups attached and it is related to their interference with the cell membrane composition and function (Larsson et al. 2000).

  • Aliphatic acids

Acids diffuse through the cell membranes and dissociate in the neutral environment of the cytoplasm which causes the pH of the cell to go down and ultimately lead to cell death. It may also compromise on the formation of biomass by producing ethanol (to reverse the lowered pH) (Verduyn et al. 1992).

Glycosyl Hydrolases (GH)

The enzymes are usually classified on the type of the bond that they break. The class of enzymes which hydrolytically break the bonds are grouped under class 3 enzymes and are called hydrolases (Fig. 3) (IUBMB 1992). In this class, the enzymes that hydrolyze glycosidic bonds between two or more sugars and non-sugars within carbohydrate and oligonucleotides are called GH. There are 115 families of these enzymes known, on the basis of mode of action and amino acid sequence (Sathya and Khan 2014). These enzymes hydrolytically breaks the bonds such as ester, glycosidic, ether, and peptide bonds, and also transfers the specific group to water, which acts as an acceptor (IUBMB 1992). GHs are key enzymes involved in the depolymerization of cellulose, which is the most crucial step in formation of fermentable sugars for production of fuels and chemicals. GH catalysed reactions are accelerated by 17 folds compared to spontaneous hydrolysis, which makes them lucrative biocatalysts (Wolfenden et al. 1998).

Fig. 3
figure 3

Hydrolysis reaction of glycosidic bond

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Classification of GH

GH also known as glycosidases or carbohydrases (EC 3.2) comprises of a wide spectrum group of enzymes that has the catalytic properties to cleave N-, S- and O-glycosidic bonds. They either cleave two sugar moieties or a sugar and a non-sugar region of carbohydrate. There are 115 families (based on the amino acid sequence and action) of GH, which have been identified (Lehninger 2004). There are few glycosidic bond, which are most stable covalent bonds found between monomers in biopolymers, especially between two glucose residues in some polysaccharides (Wolfenden et al. 1998). The domain, E.C., CATH, CAZY FAM, and CAZY clan cells, show different entries could be assigned to the given classes (Table 2).

Table 2 Summary of the structurally homologous classes of GH and their sources
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Structure of GH

The 3D structure of Lysozyme was the first to be resolved and it was found that aspartate and glutamate are the major amino acids involved in catalysis. Further studies proved that most glycosyl hydrolases has conserved regions of aspartate or glutamate that were involved in catalysis. Recent examples of neuraminidase show that tyrosine is used in stabilising the intermediates of the reaction (Burmeister et al. 1993; Crennel et al. 1993).

GH catalyse all the hydrolytic reactions of glycosidic bonds and are studied thoroughly. They cleave the C–O exo-cyclic acetal bond.

Study of the spontaneity of the reaction reveal their extremely slow action with the half life a hydrolytic cleavage estimated to be 4.7 million years. Thus GH act as an efficient and powerful catalyst speeding up the reaction by more than 1000× (Stick and Williams 2009).

The most important structural aspect of an enzyme is its active site conformation and GH is found to have three distinct conformation in its active site (Davies and Henrissat 1995).

Active Site Topology

22 families of the protein have been studied and it has been found that the active sites can be classified into three classes:

  • Pocket or crater

    This is mainly when non-reducing saccharides are substrates (example: monosaccharidases like neuraminidase and galactosidase and exopolysaccharidases like amylase). These enzymes act on substrates which have exposed non-reducing ends. They have limited activity on fibrous substrates with no free ends (Davies and Henrissat 1995).

  • Cleft or groove

    These are the binding points for endo-acting polymeric substrates like lysozymes and chitinase (Davies and Henrissat 1995).

  • Tunnel

    These are modified clefts with the protein loops covering the clefts to form a tunnel shaped structure. Cellobiohydrolases are the only class of enzyme with this structure and they pass through the site for the reaction to occur (Rouvinen et al. 1990). Products are released while the polysaccharide chain is still bound to the enzyme enabling better processivity. Although the exact mechanism of action is unknown, it is observed that the direction of enzyme motion changes during the reaction based on the enzyme class (Divne et al. 1994).

Mechanism of Glycosyl Breakdown

During enzyme hydrolysis of the glycosidic bond, two essential components required are proton donor and a nucleophile (Fig. 4).

Fig. 4
figure 4

Mechanism of breakdown of lignocellulose. (a) The retaining method, the acid catalyst AH protonate the oxygen in the glucose and base Q- avails the nucleophilic function to knockout the aglycon group. The glycosyl enzyme is then undergoes hydrolysis. (b) The inverting method, the oxygen of the glucose is protonated and aglycon group knockout occurs simultaneously by the attack of the water molecule which is activated by the base Q- (Reproduced from Davies G. and Henrissat B. 1995)

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GH in Lignocellulose Breakdown

The breakdown of lignocellulosic biomass for the production of valuable products can add to the economics as well as alleviate the detrimental effect on environment caused due to their disposal into soil or landfills (Taherzadeh and Karimi 2008). The cellulose-degrading systems are pertinent for industrial cellulose breakdown as well as to sustain the carbon cycle. The class of enzymes which are crucial in liberation of free sugars after delignification of lignocellulosic biomass is cellulase. Cellulases are the class of enzymes that are associated with the breaking down of cellulose and other related polysaccharides. Although produced by various organisms like bacteria, fungi and yeast, termites and their intestinal symbionts form the major source of cellulase (Watanabe and Tokuda 2001).

Classes of GH in Cellulosic Breakdown

They exist as complexes of various enzymes from the class exocellulase, endocellulase, cellobiolases, cellulose phosphorylases and oxidative cellulases (Tsai et al. 2012). The families of GH with different types of cellulases is mentioned in Table 3.

Table 3 Cellulases, functions and their corresponding GH families
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The major families of Cellulases belong to GH class of 5,9,17 and 48 (Ezeilo et al. 2017). These cellulase of particular class, have the following properties:

  • Enzymes of GH 5—These are characterized by the presence of conserved glutamic acid residues which are the major catalytic groups. They exhibit the retaining mechanism

  • Enzymes of GH 9—These are subdivided as E1 (source: aerobic and anaerobic bacteria) and E11 (Source: mainly non-bacterial). They exhibit the inverting mechanism.

  • Enzymes of GH 17—These exhibits retaining mechanism. It includes 2 major enzyme families: 1,3-β-d-glucan endohydrolases and 1,3;1,4-β-d-glucan endohydrolases

  • Enzymes of GH 48—These have high efficiency for amorphous or crystalline cellulose than carboxymethylcellulose or cellobiose as the latter inhibits its activity. They too exhibit inverting mechanism.

The mechanisms stated above for each class were obtained from CAZYPedia.

Model of Cellulose Degradation

The model known for enzymatic degradation of cellulose is depicted (Fig. 5).

Fig. 5
figure 5

Enzymatic cellulose degradation model (Reproduced from Lynd et al. 2002)

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The diffusion of enzymes into the substrate has been simulated and the biological conditions understood. The model is based on (Luterbacher et al. 2013a, b):

  1. 1.

    The dependency of the hydrolysis on the particle size (it can be modified by a suitable pretreatment)

  2. 2.

    Sudden decrease in the reaction rate after the initial rush of the enzyme into the substrate.

Cellulosome Complex

Cellulosomes have the capability to breakdown cellulose. The understanding of structure, mechanism and functions of this multi-enzyme complex began from Clostridium thermocellum (Lamed et al. 1983). The key features of cellulosome are stability and flexibility, composed of many subunits, its organization promotes synergistic action, and contains catalytic and non catalytic units. The components of cellulosome are the scaffoldin subunit (1800 amino acids), cohesin modules, dockerin modules, and catalytic subunits (Bayer et al. 2004). The catalytic subunits of cellulosomes consist of enzymes such as endoglucanases, xylanases and cellobiohydrolases. These enzymes portray array of substrate specificities and catalytic mechanisms Apart from the catalytic module, the dockerin module modulates enzyme interaction alongside the scaffoldin protein, responsible for assembly of the cellulosome complex (Bayer et al. 2004). The synergism of different subunits ensures a well-harmonized enzymatic coordinations, thus making cellulosomes the most efficient system for the breakdown of cellulose (Xiros et al. 2016). Recently, artificial cellulosomes are being built for the dedicated aim of enhancing the catalytic efficiency for the depolymerization of cellulose (Gefen et al. 2012).

Factors Influencing the Enzymatic Hydrolysis of Lignocellulose

Complexity of the Substrate

Cellulose is a complex and heterogeneous molecule which makes its breakdown difficult to achieve. Thus achieving a fool-proof model for its breakdown is an issue. Cellulose structure has both intra and inter-chain bondings forming a tightly packed crystalline structure interspaced with some amorphous regions each of which are at different complexity and availability (Yang et al. 2011; Bubner et al. 2013).

The enzymatic breakdown of the biomass is affected by various factors like (Fox et al. 2013; Luterbacher et al. 2013a, b):

  • Biomass content

  • Available surface area

  • Bond accessibility

  • Pretreatment used (minimal inhibitors produced)

  • Degree of polymerization

  • Organization of cellulose

Heterogeneity of the Enzymes

Microbial cellulases have been in use since the 1990s. The demand for the enzymes has only increased with time making it imperative to find other sources for the bulk production of the enzymes. The search led to a finding that both bacteria and fungi are efficient cellulase producers with organisms like Clostridium, Bacillus, Erwinia and Streptomyces being some of them (Yang et al. 2011; Bubner et al. 2013).

Based on the enzyme source they can be generally classified as (Coughlan and Ljungdahl 1988; Sun and Cheng 2002; Elkins et al. 2010; Bayer et al. 1998):

  1. 1.

    Non-complexed system: Produced by fungi and aerobic bacteria.

  2. 2.

    Complexed system: Produced by anaerobic organisms.

Recently, the study on cellulases produced by Trichoderma reesei has gained momentum due to the large amount of enzyme released which is beneficial economically. It is the only non-complexed system studied as the degradation of the biomass by the organism occurs simultaneously (Coughlan and Ljungdahl 1988); Sun and Cheng 2002; Elkins et al. 2010; Bayer et al. 1998). On the other hand complexed system has various advantages like:

  • A coordinated process

  • Only specific interaction

Applications of Cellulase

The addition of novel cellulases to the existing list of GH in lignocellulosic breakdown is increasing the applications of these enzymes in various industries (Fig. 6).

Fig. 6
figure 6

Diverse applications of cellulases

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Conclusion

The requirement of huge energy and chemicals demand is compelling the human race to find alternative to renewable clean technologies. In this context, one of the most lucrative substrate which promises to partially fulfil our demand is lignocellulosic biomass. The usage of lignocellulose at industrial scale is limited due to the huge expenses incurred in enzymes used for their hydrolysis. Thus, in present scenario the focus is being targeted to improve enzymes with respect to increased stability, wider substrate affinity, and magnus increase in catalytic activity at wide range of pH and temperature.