6
cellulosic ethanol industry (Banerjee et al. 2010), and there have been numerous
efforts toward bringing down the cost of cellulases at the level of both production
strategies and organism engineering. Enzyme majors of the world—Novozymes
and Dupont (Genencor)—have been able to bring down the cost of enzyme per unit
volume of ethanol produced to levels 10–12 folds lower, but a marketable commercial Lignocellulosic (LC) ethanol remains elusive. This is despite the fact that there
are claims from companies on successful running of lignocellulosic ethanol plants.
Several research studies have repeatedly highlighted the contribution of enzyme
cost to the cost of lignocellulosic biofuels, and have shown that the cost of producing enzymes was much higher than that commonly assumed in the literature
(Marcuschamer et al. 2012). A recent report puts the cost of cellulase per liter of
ethanol at US$ 0.72 (48% of the production cost) based on the actual purchase price
of cellulase in the industrial enzyme market (Liu et al. 2016). The above discussions
highlight the importance of cellulase cost reduction in bioethanol production and
active research efforts are now directed toward this cause worldwide. The strategies
range from using cost-effective carbon sources and onsite production of enzymes
(Johnson 2016) to developing genetically modified source organisms (Seiboth et al.
2012a; Fuji et al. 2013). However, reducing the production cost of cellulase alone is
not the solution, and the ways to reduce the cost of cellulase for bioethanol production involve a range of possible solutions including development of efficient pretreatment regimes that allow better access of enzymes to the biomass, preventing
lignin redeposition on biomass after pretreatment, use of surface active agents to aid
hydrolysis, etc. A better understanding of these strategies would require knowledge
on the enzymes and their mechanism of action.
1.2.1 Enzymes for Biomass Hydrolysis – Types and Mode
of Action
While there are a multitude of proteins aiding in biomass hydrolysis, the major
enzymes involved in biomass hydrolysis can be grouped as cellulases, hemicellulases, and lignin-degrading enzymes. Also there are a large number of accessory
enzymes and proteins that are involved in helping the deconstruction, which cannot
be grouped into any general categories. The following discussions will introduce
the biomass-hydrolyzing enzymes and the major emphasis will be on cellulases
since these are the major enzymes which are directly involved in breaking down the
carbohydrate polymers to fermentable sugars and therefore important in biofuel
production. Hemicellulases and lignin-degrading enzymes shall also be
introduced.
1.2.1.1 Cellulases
Cellulases are enzymes, which hydrolyze the β-1,4-D-glucan linkages in cellulose
and produce as primary products glucose, cellobiose, and cello-oligosaccharides.
Cellulases are produced by a number of microorganisms and comprise several different enzyme classifications. Three major types of cellulase enzymes are involved
R.K. Sukumaran et al.
cellulosic ethanol industry (Banerjee et al. 2010), and there have been numerous
efforts toward bringing down the cost of cellulases at the level of both production
strategies and organism engineering. Enzyme majors of the world—Novozymes
and Dupont (Genencor)—have been able to bring down the cost of enzyme per unit
volume of ethanol produced to levels 10–12 folds lower, but a marketable commercial Lignocellulosic (LC) ethanol remains elusive. This is despite the fact that there
are claims from companies on successful running of lignocellulosic ethanol plants.
Several research studies have repeatedly highlighted the contribution of enzyme
cost to the cost of lignocellulosic biofuels, and have shown that the cost of producing enzymes was much higher than that commonly assumed in the literature
(Marcuschamer et al. 2012). A recent report puts the cost of cellulase per liter of
ethanol at US$ 0.72 (48% of the production cost) based on the actual purchase price
of cellulase in the industrial enzyme market (Liu et al. 2016). The above discussions
highlight the importance of cellulase cost reduction in bioethanol production and
active research efforts are now directed toward this cause worldwide. The strategies
range from using cost-effective carbon sources and onsite production of enzymes
(Johnson 2016) to developing genetically modified source organisms (Seiboth et al.
2012a; Fuji et al. 2013). However, reducing the production cost of cellulase alone is
not the solution, and the ways to reduce the cost of cellulase for bioethanol production involve a range of possible solutions including development of efficient pretreatment regimes that allow better access of enzymes to the biomass, preventing
lignin redeposition on biomass after pretreatment, use of surface active agents to aid
hydrolysis, etc. A better understanding of these strategies would require knowledge
on the enzymes and their mechanism of action.
1.2.1 Enzymes for Biomass Hydrolysis – Types and Mode
of Action
While there are a multitude of proteins aiding in biomass hydrolysis, the major
enzymes involved in biomass hydrolysis can be grouped as cellulases, hemicellulases, and lignin-degrading enzymes. Also there are a large number of accessory
enzymes and proteins that are involved in helping the deconstruction, which cannot
be grouped into any general categories. The following discussions will introduce
the biomass-hydrolyzing enzymes and the major emphasis will be on cellulases
since these are the major enzymes which are directly involved in breaking down the
carbohydrate polymers to fermentable sugars and therefore important in biofuel
production. Hemicellulases and lignin-degrading enzymes shall also be
introduced.
1.2.1.1 Cellulases
Cellulases are enzymes, which hydrolyze the β-1,4-D-glucan linkages in cellulose
and produce as primary products glucose, cellobiose, and cello-oligosaccharides.
Cellulases are produced by a number of microorganisms and comprise several different enzyme classifications. Three major types of cellulase enzymes are involved
R.K. Sukumaran et al.