15
1.3
Microbial Production of Cellulases and the Systems
for Cellulose Hydrolysis
While cellulases, hemicellulases, ligninolytic enzymes, and a myriad of different
accessory enzymes and proteins are involved in the hydrolysis of lignocellulosic
biomass in nature, not all of them are used in the preparations for commercial
hydrolysis of biomass for biorefinery applications. In commercial preparations of
biomass-hydrolyzing enzymes, enzymes are only used in their crude form, the only
processing steps employed being concentration, stabilization, and formulation.
Major attention is often only given to cellulases, though it is also implied that other
enzymes are present in the preparations, since there are no elaborate purification
steps involved. Often cellulase preparations contain hemicellulases, LPMOs, ligninolytic enzymes, etc. depending on the source organisms and techniques employed
for production. It may be noted that the most commonly employed microorganism
for cellulase production is Trichoderma reesei, which is often genetically modified
for enhanced cellulase expression and is derepressed for carbon catabolite repression. T. reesei is limited in its ability for synthesis of beta glucosidase, and, often in
biomass hydrolyzing blends BGL and xylanase from heterogeneous sources are
added to make the enzyme more effective. Since cellulases are the major determinants of the efficacy of the biomass-hydrolyzing enzyme cocktails, the current discussion is limited to cellulases. Detailed discussion on hemicellulases and other
enzymes may be found in Shalom and Shoham (2003), Juturu and Wu (2013), and
Placido and Capareda (2015).
Ability to degrade cellulose is not a common trait among microorganisms and
only a few specialized microorganisms – mostly bacteria and filamentous fungi –
are capable of cellulose depolymerization (Quiroz-Castañeda and Folch-Mallol
2013). The machinery for cellulose degradation is radically different in the anaerobic bacteria and the rest of the organisms, and these involve cell wall-bound complex structures known as cellulosomes. Aerobic bacteria and filamentous fungi
normally secrete a complex array of free enzymes that act synergistically to convert
cellulose like the T. reesei cellulases. In yet another mechanism recently hypothesized, certain bacteria (e.g., Fibrobacter succinogenes) found in the rumen of herbivores use a mechanism involving both cell wall-bound and free enzymes for
cellulose hydrolysis (Burnet et al. 2015). The following discussion will describe the
free and bound systems of cellulose hydrolysis in the context of microbial degradation of cellulose, and the third mechanism shall be introduced separately.
1.3.1 Cellulose Hydrolysis Through Cell Wall-Bound Enzyme
Complexes
Cell wall-bound cellulase-degrading enzyme complexes called cellulosomes are
employed by several anaerobic bacteria for breaking down cellulose in nature.
These include several Clostridia including the typical strain Clostridium thermocellum. The other common anaerobic bacteria include C. cellulovorans, C.
1 Enzymes for Bioenergy
1.3
Microbial Production of Cellulases and the Systems
for Cellulose Hydrolysis
While cellulases, hemicellulases, ligninolytic enzymes, and a myriad of different
accessory enzymes and proteins are involved in the hydrolysis of lignocellulosic
biomass in nature, not all of them are used in the preparations for commercial
hydrolysis of biomass for biorefinery applications. In commercial preparations of
biomass-hydrolyzing enzymes, enzymes are only used in their crude form, the only
processing steps employed being concentration, stabilization, and formulation.
Major attention is often only given to cellulases, though it is also implied that other
enzymes are present in the preparations, since there are no elaborate purification
steps involved. Often cellulase preparations contain hemicellulases, LPMOs, ligninolytic enzymes, etc. depending on the source organisms and techniques employed
for production. It may be noted that the most commonly employed microorganism
for cellulase production is Trichoderma reesei, which is often genetically modified
for enhanced cellulase expression and is derepressed for carbon catabolite repression. T. reesei is limited in its ability for synthesis of beta glucosidase, and, often in
biomass hydrolyzing blends BGL and xylanase from heterogeneous sources are
added to make the enzyme more effective. Since cellulases are the major determinants of the efficacy of the biomass-hydrolyzing enzyme cocktails, the current discussion is limited to cellulases. Detailed discussion on hemicellulases and other
enzymes may be found in Shalom and Shoham (2003), Juturu and Wu (2013), and
Placido and Capareda (2015).
Ability to degrade cellulose is not a common trait among microorganisms and
only a few specialized microorganisms – mostly bacteria and filamentous fungi –
are capable of cellulose depolymerization (Quiroz-Castañeda and Folch-Mallol
2013). The machinery for cellulose degradation is radically different in the anaerobic bacteria and the rest of the organisms, and these involve cell wall-bound complex structures known as cellulosomes. Aerobic bacteria and filamentous fungi
normally secrete a complex array of free enzymes that act synergistically to convert
cellulose like the T. reesei cellulases. In yet another mechanism recently hypothesized, certain bacteria (e.g., Fibrobacter succinogenes) found in the rumen of herbivores use a mechanism involving both cell wall-bound and free enzymes for
cellulose hydrolysis (Burnet et al. 2015). The following discussion will describe the
free and bound systems of cellulose hydrolysis in the context of microbial degradation of cellulose, and the third mechanism shall be introduced separately.
1.3.1 Cellulose Hydrolysis Through Cell Wall-Bound Enzyme
Complexes
Cell wall-bound cellulase-degrading enzyme complexes called cellulosomes are
employed by several anaerobic bacteria for breaking down cellulose in nature.
These include several Clostridia including the typical strain Clostridium thermocellum. The other common anaerobic bacteria include C. cellulovorans, C.
1 Enzymes for Bioenergy