35
are now available, especially the enzymes from Novozymes and Genencor (Dupont).
Still it is considered that the cost of enzymes is much larger than what is normally
assumed and reported in the literature, and can be as high as 48% of the production
cost of bioethanol. This highlights the importance of further active R&D on the
biomass hydrolyzing enzymes, covering various aspects like improvement of
enzyme performance and stability, reducing the production cost of enzymes, development of enzymes that do not exist naturally (e.g., enzyme chimeras, artificial
cellulosomes), reducing inhibitions by lignin and glucose, reducing non-specific
binding to lignin and improvements in the strategies for efficient usage of enzymes
that include recovery and reuse of enzymes, and usage of minimal enzyme dosage
achieved through predicting optimal cocktails. Several of the current studies show
that the strategy cannot be successful by just addressing enzyme production costs,
but needs an integrated approach which addresses issues like making the target biomass more susceptible to hydrolysis by adopting appropriate pretreatment strategies
and the onsite production of enzyme for reduction of logistic and storage costs. One
of the important conclusions that emerge is that the enzymes themselves need to be
made more efficient and, for this, the use of modern biotechnological tools can be
employed. Thus there could be enzymes with reduced binding to lignin, higher turnovers, and less inhibition by products. All these are possible only through a thorough understanding of the enzyme action and its synergies. It is important to note
that there are still possibilities of as yet undiscovered novel activities in nature
whose potential may be tapped. Knowledge is being gained on the structure- function
relationship of biomass-hydrolyzing enzymes and this information can go a long
way in deriving novel activities by engineering the existing enzymes for new features or creating entirely new functions like in the case of enzyme chimeras.
This chapter has tried to address biomass-hydrolyzing enzymes in the context of
bioenergy and more specifically bioethanol and has tried to highlight the importance of these enzymes in future biorefineries. An overview is presented about the
mode of action of these enzymes, especially the latest knowledge in this domain and
also on the aspects of production of the enzymes. It also provides an overview on
the regulation of expression of cellulases, the knowledge on which is critical for
designing production strategies. From the foregoing discussion on these aspects and
about the strategies on cost reduction of enzymatic hydrolysis, it emerges that there
are still several gaps in the technologies for enzymatic biomass conversion, especially regarding the technical developments needed for bringing effective cost
reduction. There is no single effective solution, and it is apparent that only an integrated approach covering various areas like engineering of enzymes to onsite production would make the enzymatic hydrolysis cost effective.
References
Akel E, Metz B, Seiboth B, Kubicek CP (2009) Molecular regulation of arabinan and L-arabinose
metabolism in Hypocrea jecorina (Trichoderma reesei). Eukaryot Cell 8:1837–1844
Akin DE, Borneman WS, Lyon CE (1990) Degradation of leaf blades and stems by monocentric
and polycentric isolates of ruminal fungi. Anim Feed Sci Technol 31(3–4):205–221
1 Enzymes for Bioenergy
are now available, especially the enzymes from Novozymes and Genencor (Dupont).
Still it is considered that the cost of enzymes is much larger than what is normally
assumed and reported in the literature, and can be as high as 48% of the production
cost of bioethanol. This highlights the importance of further active R&D on the
biomass hydrolyzing enzymes, covering various aspects like improvement of
enzyme performance and stability, reducing the production cost of enzymes, development of enzymes that do not exist naturally (e.g., enzyme chimeras, artificial
cellulosomes), reducing inhibitions by lignin and glucose, reducing non-specific
binding to lignin and improvements in the strategies for efficient usage of enzymes
that include recovery and reuse of enzymes, and usage of minimal enzyme dosage
achieved through predicting optimal cocktails. Several of the current studies show
that the strategy cannot be successful by just addressing enzyme production costs,
but needs an integrated approach which addresses issues like making the target biomass more susceptible to hydrolysis by adopting appropriate pretreatment strategies
and the onsite production of enzyme for reduction of logistic and storage costs. One
of the important conclusions that emerge is that the enzymes themselves need to be
made more efficient and, for this, the use of modern biotechnological tools can be
employed. Thus there could be enzymes with reduced binding to lignin, higher turnovers, and less inhibition by products. All these are possible only through a thorough understanding of the enzyme action and its synergies. It is important to note
that there are still possibilities of as yet undiscovered novel activities in nature
whose potential may be tapped. Knowledge is being gained on the structure- function
relationship of biomass-hydrolyzing enzymes and this information can go a long
way in deriving novel activities by engineering the existing enzymes for new features or creating entirely new functions like in the case of enzyme chimeras.
This chapter has tried to address biomass-hydrolyzing enzymes in the context of
bioenergy and more specifically bioethanol and has tried to highlight the importance of these enzymes in future biorefineries. An overview is presented about the
mode of action of these enzymes, especially the latest knowledge in this domain and
also on the aspects of production of the enzymes. It also provides an overview on
the regulation of expression of cellulases, the knowledge on which is critical for
designing production strategies. From the foregoing discussion on these aspects and
about the strategies on cost reduction of enzymatic hydrolysis, it emerges that there
are still several gaps in the technologies for enzymatic biomass conversion, especially regarding the technical developments needed for bringing effective cost
reduction. There is no single effective solution, and it is apparent that only an integrated approach covering various areas like engineering of enzymes to onsite production would make the enzymatic hydrolysis cost effective.
References
Akel E, Metz B, Seiboth B, Kubicek CP (2009) Molecular regulation of arabinan and L-arabinose
metabolism in Hypocrea jecorina (Trichoderma reesei). Eukaryot Cell 8:1837–1844
Akin DE, Borneman WS, Lyon CE (1990) Degradation of leaf blades and stems by monocentric
and polycentric isolates of ruminal fungi. Anim Feed Sci Technol 31(3–4):205–221
1 Enzymes for Bioenergy