Lignocellulosic waste is thestructural material used to make plant cell wall, and is the therefore the maincomponent of plant biomass on Earth. Lignocellulose consist three maincomponents of cellulose 40%-50%, hemicelluloses 25%-30% and lignin 15%-20% (Chandra2015).
Cellulose is primary constituent of lignocellulosic waste, and is apolysaccharide composed of ?-1,4 linked D-glucose units. Cellulose isused for the manufacture of paper and cardboard, and can convert through theaction of cellulase enzymes into glucose monomer, for bioethanol production(Ahmad et al. 2010). Hemicellulose are branched polysaccharide which areassociated with lignin and cellulose in plant cell wall, consist of otherpolysaccharide, principally xylans and mannans, which are closely associatedwith the cellulose filaments, and chemically linked with lignin. The majorhemicelluloses in hardwoods is xylans (15%-30% dry weight), a polysaccharidecomposed of ?-1,4 linked D-xylose units, which can be substituted withother monosaccharide units, whereas softwood hemicelluloses contains mainlyglactoglucomannan (15%-20% dry weight), a polysaccharide composed of ?,1-4linked D-glucose, and D-galactose units (Bugg et al. 2011). Certainly, inaddition to hemicelluloses, hydrothermal pre-treatment of lignocellulosicmaterial also involves solubilization of extract, small portion of celluloseand lignin (Vazquez et al.
2007). Hemicelluloses consist of both linear and branchedheteropolymers. It mainly contains ?ve monomeric sugars, namely D-glucose,D-mannose, D-galactose, D-xylose and L-arabinose linked together by ?-1,4-glycosidicbonds. Covalent bonding between hemicelluloses and lignin provides additionalstrength to the plant.
Cellulose and hemicelluloses are present in the form ofinsoluble crystalline ?bers that are degradable but the processes very complexdue to the involvement of several enzymatic pathways (Aarti et al. 2015). Theterm lignin derives from the Latin word ‘lignum’, which means wood. Lignin isthe most abundant structurally complex aromatic polymerpossessing a high molecular weightand the most recalcitrant, containing of numerous biologically stable linkages. After cellulose, lignin is the second most abundantrenewable biopolymer in nature because of the lowbiodegradability of lignin andlarge lignocellulosic waste generated through various industries such as paperand pulp, timber, distillery etc causes a serious pollution and toxicityproblem in aquatic ecosystem.
For this reason, studies on thebacterial degradation were more preferable for lignocellulosic waste and theproduction of bacterial ligninolytic enzymes has seen increased in recent year(Renugadevi et al. 2011). Lignin degrading enzymes are essentiallyextracellular in nature due to the large and complex structure of lignin whichcannot enter the cell for intracellular action (Sasikumar et al. 2014). Ligninperoxidase enzymes is useful in the treatment of industrial waste and otherxenobiotic as it has bioremediation potential to decolorize the effluents (Shiet al. 2013). Over the years, four enzymatic activities have been reported todepolymerize lignin in decaying plant cell walls like lignin peroxidases(LiPs), manganese peroxidases (MnPs), versatile peroxidases (VPs) and laccases.
These enzymes have gained attention as potential biological catalysts for ligninbiodegradation and other organic pollutants. Of note, ligninolytic enzymes aretoo large to penetrate into undecayed wood cell walls therefore reactive oxygenspecies could be the agents responsible for local lignin decay (Pollegioni etal. 2015). Certainly, the process of lignin degradation is enhanced by severaladditional and accessory microbial enzymes, and some of them are now available.
