5
organized into microfibrils and there is cross-linking between adjacent chains
through hydrogen bonds leading to crystalline and amorphous domains (Pu et al.
2013). Cellulose can make up to 15–30% of primary cell walls and up to 40% of
secondary cell walls (Sticklen 2008). Hemicellulose on the other hand is a diverse
group of short chain, branched, substituted polymer of sugars with a DP of ~70–200
(Zhao et al. 2012). The sugar monomers in hemicelluloses can be xylose, mannose,
galactose, rhamnose, and arabinose, with xylose being the most abundant one. They
can also contain the sugar acids like glucuronic or galacturonic acid and the hydroxyl
groups of sugars can be partially substituted with acetyl groups (Gírio et al. 2010).
Lignin is a class of complex cross-linked phenyl propane units, primarily comprising the monomeric units—p-coumaryl, coniferyl, and sinapyl alcohols. Typical
plant cell wall structure consists of cellulose microfibrils embedded in a matrix of
lignin interspersed with hemicellulose fibers forming a very rigid and organized
structure, which is rather difficult to break (Fig. 1.1).
Nature’s arsenal for breaking plant cell wall structures includes enzymes that can
hydrolyze all of these compounds, and these catalysts play a very important role in
the recycling of organic carbon on the globe. Different microorganisms are capable
of producing enzymes that can degrade cellulose, hemicellulose, or lignin, or a
combination of all. Recently, cellulases have taken the center stage in enzyme
research, primarily because of their important role in second-generation ethanol
(bioethanol) from lignocellulosic biomass. Sugar polymers in the lignocellulosic
biomass are linked through β-1,4 glycosidic linkages that can be hydrolytically
cleaved to release monomeric sugars. Enzymatic hydrolysis primarily employs cellulases derived from filamentous fungi – especially strains of Trichoderma,
Penicillium, and Aspergillus, mostly in a crude concentrated form. While cellulases
have been around for several decades, the enzymes tailored for efficient biomass
hydrolysis are a recent development and involve deliberate blending of multiple
enzymes from different sources so as to achieve maximum hydrolytic efficiency.
Natural cellulases are slow acting and are affected by several parameters from the
reaction environment. While recent research has been successful in improving the
efficiencies of biomass-hydrolyzing enzymes and their reaction rates, the same cannot be said for their cost of production. It has been realized that the cost of biomasshydrolyzing enzymes is a major hurdle for developing an economically viable
Fig. 1.1 Schematic representation of the lignocellulose structure
1 Enzymes for Bioenergy
organized into microfibrils and there is cross-linking between adjacent chains
through hydrogen bonds leading to crystalline and amorphous domains (Pu et al.
2013). Cellulose can make up to 15–30% of primary cell walls and up to 40% of
secondary cell walls (Sticklen 2008). Hemicellulose on the other hand is a diverse
group of short chain, branched, substituted polymer of sugars with a DP of ~70–200
(Zhao et al. 2012). The sugar monomers in hemicelluloses can be xylose, mannose,
galactose, rhamnose, and arabinose, with xylose being the most abundant one. They
can also contain the sugar acids like glucuronic or galacturonic acid and the hydroxyl
groups of sugars can be partially substituted with acetyl groups (Gírio et al. 2010).
Lignin is a class of complex cross-linked phenyl propane units, primarily comprising the monomeric units—p-coumaryl, coniferyl, and sinapyl alcohols. Typical
plant cell wall structure consists of cellulose microfibrils embedded in a matrix of
lignin interspersed with hemicellulose fibers forming a very rigid and organized
structure, which is rather difficult to break (Fig. 1.1).
Nature’s arsenal for breaking plant cell wall structures includes enzymes that can
hydrolyze all of these compounds, and these catalysts play a very important role in
the recycling of organic carbon on the globe. Different microorganisms are capable
of producing enzymes that can degrade cellulose, hemicellulose, or lignin, or a
combination of all. Recently, cellulases have taken the center stage in enzyme
research, primarily because of their important role in second-generation ethanol
(bioethanol) from lignocellulosic biomass. Sugar polymers in the lignocellulosic
biomass are linked through β-1,4 glycosidic linkages that can be hydrolytically
cleaved to release monomeric sugars. Enzymatic hydrolysis primarily employs cellulases derived from filamentous fungi – especially strains of Trichoderma,
Penicillium, and Aspergillus, mostly in a crude concentrated form. While cellulases
have been around for several decades, the enzymes tailored for efficient biomass
hydrolysis are a recent development and involve deliberate blending of multiple
enzymes from different sources so as to achieve maximum hydrolytic efficiency.
Natural cellulases are slow acting and are affected by several parameters from the
reaction environment. While recent research has been successful in improving the
efficiencies of biomass-hydrolyzing enzymes and their reaction rates, the same cannot be said for their cost of production. It has been realized that the cost of biomasshydrolyzing enzymes is a major hurdle for developing an economically viable
Fig. 1.1 Schematic representation of the lignocellulose structure
1 Enzymes for Bioenergy