32
Surfactants can form emulsions with the hydrophobic lignin and hemicellulose
degradation products promoting lignin removal and allowing enzyme access to the
feedstock’s reaction sites (Tu et al. 2007a; Seo et al. 2011a). The role of surfactants
in reducing the nonproductive adsorption of cellulase to nonproductive sites on biomass has long been recognized (Castanon and Wilke 1981) and has been proven in
several systems (Zheng et al. 2008; Qing et al. 2010). Surfactants have now been
demonstrated to also help the desorption of the nonspecifically bound cellulases,
probably through a competitive mechanism (Li et al. 2016). Structural changes to
lignocellulose by the surfactant Tween 20 contributing to enhanced hydrolysis were
described by Seo et al. (2011b). In another recent study by Eckard et al. (2014), it
was found that there is a reformation of the α-helix substructure of cellulase protein,
which would be another potent contributor to the observed increase in hydrolysis
activity. The same authors have also demonstrated that the surfactants can protect
enzymes from thermal deactivation after extended incubation (Eckard et al. 2013b).
Apparently, surfactants like Tween 80 and Tween 20 or polymeric surfactants like
poly ethylene glycol and even proteins like bovine serum albumin or casein are
probably good choices as additives in enzyme preparations as well as in biomass
hydrolyses since they can enhance the sugars yields through one or all of the mechanisms described above. A more elaborate discussion on the role of amphiphiles may
be found in Eckard et al. (2013a).
Improvement in the enzyme efficacy is yet another method to improve the cost of
cellulase preparations, and changing the enzyme’s hydrolytic turnovers, thermal
stability, or affinities can impact the hydrolytic performance and hence the cost of
hydrolysis. The major advantages projected for thermostable enzymes include (1)
higher specific activity and stability, which allows the enzyme to perform for longer
durations and reduction in the volume/quantity of enzyme to be used per unit
amount of biomass; (2) better compatibility with processes and chemicals used for
biomass (pre)treatments; (3) lower costs of cooling since hydrolysis can proceed at
a higher temperature; (4) reduction in microbial contamination risks due to increased
temperature of hydrolysis; (5) decreased fluid viscosity and hence better mass transfer; (6) ability to store enzyme at room temperature; and (7) greater flexibility for
biorefinery process configurations (Yeoman et al. 2010). More than three times
higher release of sugars has been reported with cellulase cocktails containing engineered thermostable endoglucanase from T. reesei at 60 °C (Trudeau et al. 2014).
Pretreatment of biomass is usually performed at temperatures near or equal to 200
°C and the biomass needs to be cooled down before hydrolysis can be performed
(typically at 50 °C). Having enzymes that perform at higher temperatures can significantly reduce the energy for cooling, thereby reducing the overall cost of ethanol
production (Trudeau et al. 2016).
Reducing the nonspecific binding of enzyme to lignin is another important means
to improve enzyme efficiency, and recently this has been achieved through modification of the linker peptide that connects the catalytic domain and carbohydratebinding module in T. reesei Cel6A (Scott et al. 2016). This was achieved through
modifications of the amino acids in the linker peptide so as to decrease the isoelectric point of the linker peptide and/or increase the ratio of threonine to serine in the
R.K. Sukumaran et al.
Surfactants can form emulsions with the hydrophobic lignin and hemicellulose
degradation products promoting lignin removal and allowing enzyme access to the
feedstock’s reaction sites (Tu et al. 2007a; Seo et al. 2011a). The role of surfactants
in reducing the nonproductive adsorption of cellulase to nonproductive sites on biomass has long been recognized (Castanon and Wilke 1981) and has been proven in
several systems (Zheng et al. 2008; Qing et al. 2010). Surfactants have now been
demonstrated to also help the desorption of the nonspecifically bound cellulases,
probably through a competitive mechanism (Li et al. 2016). Structural changes to
lignocellulose by the surfactant Tween 20 contributing to enhanced hydrolysis were
described by Seo et al. (2011b). In another recent study by Eckard et al. (2014), it
was found that there is a reformation of the α-helix substructure of cellulase protein,
which would be another potent contributor to the observed increase in hydrolysis
activity. The same authors have also demonstrated that the surfactants can protect
enzymes from thermal deactivation after extended incubation (Eckard et al. 2013b).
Apparently, surfactants like Tween 80 and Tween 20 or polymeric surfactants like
poly ethylene glycol and even proteins like bovine serum albumin or casein are
probably good choices as additives in enzyme preparations as well as in biomass
hydrolyses since they can enhance the sugars yields through one or all of the mechanisms described above. A more elaborate discussion on the role of amphiphiles may
be found in Eckard et al. (2013a).
Improvement in the enzyme efficacy is yet another method to improve the cost of
cellulase preparations, and changing the enzyme’s hydrolytic turnovers, thermal
stability, or affinities can impact the hydrolytic performance and hence the cost of
hydrolysis. The major advantages projected for thermostable enzymes include (1)
higher specific activity and stability, which allows the enzyme to perform for longer
durations and reduction in the volume/quantity of enzyme to be used per unit
amount of biomass; (2) better compatibility with processes and chemicals used for
biomass (pre)treatments; (3) lower costs of cooling since hydrolysis can proceed at
a higher temperature; (4) reduction in microbial contamination risks due to increased
temperature of hydrolysis; (5) decreased fluid viscosity and hence better mass transfer; (6) ability to store enzyme at room temperature; and (7) greater flexibility for
biorefinery process configurations (Yeoman et al. 2010). More than three times
higher release of sugars has been reported with cellulase cocktails containing engineered thermostable endoglucanase from T. reesei at 60 °C (Trudeau et al. 2014).
Pretreatment of biomass is usually performed at temperatures near or equal to 200
°C and the biomass needs to be cooled down before hydrolysis can be performed
(typically at 50 °C). Having enzymes that perform at higher temperatures can significantly reduce the energy for cooling, thereby reducing the overall cost of ethanol
production (Trudeau et al. 2016).
Reducing the nonspecific binding of enzyme to lignin is another important means
to improve enzyme efficiency, and recently this has been achieved through modification of the linker peptide that connects the catalytic domain and carbohydratebinding module in T. reesei Cel6A (Scott et al. 2016). This was achieved through
modifications of the amino acids in the linker peptide so as to decrease the isoelectric point of the linker peptide and/or increase the ratio of threonine to serine in the
R.K. Sukumaran et al.