18
1.3.2 Free/Noncomplexed Systems for Cellulose Hydrolysis
Noncomplexed or free cellulase/hemicellulase systems are the most common
among known cellulose digestion paradigms. Soluble cellulases are employed by a
wide array of cellulolytic bacteria and fungi, and this strategy is characteristic of
aerobic microorganisms. Aerobic cellulose-degrading microbes are represented in
several bacterial phyla as well as in several classes of fungi, and these are characterized by elaborate mechanisms of induction and repression control of the cellulase
gene expression. The free enzyme paradigm represents the case where cellulolytic
enzymes diffuse as single catalytic units often containing binding modules covalently attached through linker domains as known in the enzyme suite from the classical model organism for cellulase production – Trichoderma reesei (Hypocrea
jecorina) (Payne et al. 2015). While several fungi can utilize cellulose as an energy
source, only few strains are capable of secreting a complex of cellulases that have
practical application in the hydrolysis of plant biomass, in the context of a lignocellulose biorefinery. Besides T. reesei, the other fungi known to produce high amounts
of secreted cellulases include strains of Aspergillius, Penicillium, Humicola, and
Phanerocheate (Sukumaran 2009). However, to date, most of the commercial cellulase preparations for biomass hydrolysis are produced using Trichoderma reesei
and Aspergillus niger. The noncomplexed systems are characterized by the classical
cellulose hydrolysis paradigm having secreted endo- and exoglucanases acting on
the cellulose, and beta glucosidase cleaving the cellobiose generated by exoglucanase action. Most of the information on the noncomplexed systems has come
through studies on the filamentous fungus Trichoderma reesei, the genetically modified strains of which are used currently for commercial production of cellulases. In
Trichoderma reesei, there are two cellobiohydrolases, at least five endoglucanases,
and seven beta glucosidases which are under tight regulation through induction and
repression mechanisms. These enzymes secreted by the fungus act synergistically
for hydrolysis of cellulose. In most of the fungi, the mechanisms of cellulose degradation and the mode of action of cellulases are similar to that in Trichoderma, while
the regulation of cellulase gene expression might have organism-specific features.
A more detailed discussion on the noncomplexed cellulase action is already presented in Sect. 1.2.
Aerobic bacteria are another important class of microorganisms that uses a free
enzyme strategy for cellulose hydrolysis. These include the aerobic bacteria belonging to the order Actinomycetales (e.g., Streptomyces lividans), aerobic gram- positive
bacteria (e.g., Bacillus, Cellulomonas, Thermobifida), aerobic gliding bacteria (e.g.,
Cytophaga, Sporocytophaga), gram negative (e.g., Cellvibrio), etc. (QuirozCastañeda and Folch-Mallol 2013). While most of the secreted cellulases contain
carbohydrate-binding domains, there are several secreted cellulase which do not
possess CBMs. Secreted cellulases, though a characteristic of aerobic cellulosedegrading organisms, are not unique to them and have been described even in the
R.K. Sukumaran et al.
1.3.2 Free/Noncomplexed Systems for Cellulose Hydrolysis
Noncomplexed or free cellulase/hemicellulase systems are the most common
among known cellulose digestion paradigms. Soluble cellulases are employed by a
wide array of cellulolytic bacteria and fungi, and this strategy is characteristic of
aerobic microorganisms. Aerobic cellulose-degrading microbes are represented in
several bacterial phyla as well as in several classes of fungi, and these are characterized by elaborate mechanisms of induction and repression control of the cellulase
gene expression. The free enzyme paradigm represents the case where cellulolytic
enzymes diffuse as single catalytic units often containing binding modules covalently attached through linker domains as known in the enzyme suite from the classical model organism for cellulase production – Trichoderma reesei (Hypocrea
jecorina) (Payne et al. 2015). While several fungi can utilize cellulose as an energy
source, only few strains are capable of secreting a complex of cellulases that have
practical application in the hydrolysis of plant biomass, in the context of a lignocellulose biorefinery. Besides T. reesei, the other fungi known to produce high amounts
of secreted cellulases include strains of Aspergillius, Penicillium, Humicola, and
Phanerocheate (Sukumaran 2009). However, to date, most of the commercial cellulase preparations for biomass hydrolysis are produced using Trichoderma reesei
and Aspergillus niger. The noncomplexed systems are characterized by the classical
cellulose hydrolysis paradigm having secreted endo- and exoglucanases acting on
the cellulose, and beta glucosidase cleaving the cellobiose generated by exoglucanase action. Most of the information on the noncomplexed systems has come
through studies on the filamentous fungus Trichoderma reesei, the genetically modified strains of which are used currently for commercial production of cellulases. In
Trichoderma reesei, there are two cellobiohydrolases, at least five endoglucanases,
and seven beta glucosidases which are under tight regulation through induction and
repression mechanisms. These enzymes secreted by the fungus act synergistically
for hydrolysis of cellulose. In most of the fungi, the mechanisms of cellulose degradation and the mode of action of cellulases are similar to that in Trichoderma, while
the regulation of cellulase gene expression might have organism-specific features.
A more detailed discussion on the noncomplexed cellulase action is already presented in Sect. 1.2.
Aerobic bacteria are another important class of microorganisms that uses a free
enzyme strategy for cellulose hydrolysis. These include the aerobic bacteria belonging to the order Actinomycetales (e.g., Streptomyces lividans), aerobic gram- positive
bacteria (e.g., Bacillus, Cellulomonas, Thermobifida), aerobic gliding bacteria (e.g.,
Cytophaga, Sporocytophaga), gram negative (e.g., Cellvibrio), etc. (QuirozCastañeda and Folch-Mallol 2013). While most of the secreted cellulases contain
carbohydrate-binding domains, there are several secreted cellulase which do not
possess CBMs. Secreted cellulases, though a characteristic of aerobic cellulosedegrading organisms, are not unique to them and have been described even in the
R.K. Sukumaran et al.