lignocellulosic material into fermentable sugars. Moreover, recalcitrance nature of
plant cell walls makes them resistant toward hydrolysis by microbes (fungal and
bacterial strains) and their enzymes. These factors are the main issues, which have to
be addressed so as to make bioethanol production inexpensive. Discovery of novel,
potent varieties of thermostable biocatalyst that are able to bear extreme conditions
stably is the current demand and has become the main motto of many researchers
(Bhatia et al. 2012).
3.4.1 Fungal Extracellular Cellulases
There are many lignocellulosic materials that are presently being investigated for
their enzymatic saccharification by cellulases. Main among these materials are agroresidues, forest residues, straw of rice and wheat, bagasse, etc. There are many
microorganisms that can produce cellulases (also known as glucanases). These
organisms can be bacteria or fungi, aerobic or anaerobic, and mesophile or thermophile. Clostridium, Cellulomonas, Bacillus, Thermomonospora, Ruminococcus,
Bacteroides, Erwinia, Acetovibrio, Microbispora, and Streptomyces are the important cellulose-producing bacterial genera. C. phytofermentans, C. thermocellum, C.
hungatei, and C. papyrosolvens are the anaerobic bacteria that potentially generate
cellulases with elevated specific activity. T. reesei is the known producer of commercial glucanases (cellulases). A. niger is producer of β-D-glucosidase. S. rolfsii, P.
chrysosporium, and various species of Trichoderma, Aspergillus, Schizophyllum,
and Penicillium are known to generate cellulases (Bhatia et al. 2012).
Thermostable and acid tolerant enzymes are a choice for hydrolysis presently, and
strategies adopted for pretreatment generally depend on acid and high temperature.
Higher stability and specific activity are two important beneficial points of thermostable enzymes, which enhance their performance of hydrolysis. This would ultimately reduce the operation cost of hydrolysis as lower dose of enzymes is sufficient.
The key producers of these thermostable enzymes are T. emersonii, Chaetomium
thermophilum, and Corynascus thermophilus. The optimal temperature for growth
of these organisms is 30–55
C. Enzymes produced by these organisms can perform
well at high temperature of 60
C and can be used in industrial scales. C.
thermocellum, C. cellulovorans, and T. reesei are among the best organisms that
can proficiently degrade cellulose as well as xylan. Cellulosome complex is produced by C. thermocellum and C. cellulovorans, which is organized on their cell
surface. This complex consists of cellulase and hemicellulase. Interestingly, extracellular secretions of T. reesei consist of three types of cellulases: (1) five
endoglucanases (EG [EC 3.2.1.4]), (2) two cellobiohydrolases (CBH [EC
3.2.1.91]), and (3) two β-glucosidases (BGL [EC 3.2.1.21]) (Bhatia et al. 2012).
Functions of these enzymes are mentioned in Sect. 3.5 of this chapter.
3 Role of Thermophiles in Production of Aviation Biofuels: Fueling the Future
71
plant cell walls makes them resistant toward hydrolysis by microbes (fungal and
bacterial strains) and their enzymes. These factors are the main issues, which have to
be addressed so as to make bioethanol production inexpensive. Discovery of novel,
potent varieties of thermostable biocatalyst that are able to bear extreme conditions
stably is the current demand and has become the main motto of many researchers
(Bhatia et al. 2012).
3.4.1 Fungal Extracellular Cellulases
There are many lignocellulosic materials that are presently being investigated for
their enzymatic saccharification by cellulases. Main among these materials are agroresidues, forest residues, straw of rice and wheat, bagasse, etc. There are many
microorganisms that can produce cellulases (also known as glucanases). These
organisms can be bacteria or fungi, aerobic or anaerobic, and mesophile or thermophile. Clostridium, Cellulomonas, Bacillus, Thermomonospora, Ruminococcus,
Bacteroides, Erwinia, Acetovibrio, Microbispora, and Streptomyces are the important cellulose-producing bacterial genera. C. phytofermentans, C. thermocellum, C.
hungatei, and C. papyrosolvens are the anaerobic bacteria that potentially generate
cellulases with elevated specific activity. T. reesei is the known producer of commercial glucanases (cellulases). A. niger is producer of β-D-glucosidase. S. rolfsii, P.
chrysosporium, and various species of Trichoderma, Aspergillus, Schizophyllum,
and Penicillium are known to generate cellulases (Bhatia et al. 2012).
Thermostable and acid tolerant enzymes are a choice for hydrolysis presently, and
strategies adopted for pretreatment generally depend on acid and high temperature.
Higher stability and specific activity are two important beneficial points of thermostable enzymes, which enhance their performance of hydrolysis. This would ultimately reduce the operation cost of hydrolysis as lower dose of enzymes is sufficient.
The key producers of these thermostable enzymes are T. emersonii, Chaetomium
thermophilum, and Corynascus thermophilus. The optimal temperature for growth
of these organisms is 30–55
C. Enzymes produced by these organisms can perform
well at high temperature of 60
C and can be used in industrial scales. C.
thermocellum, C. cellulovorans, and T. reesei are among the best organisms that
can proficiently degrade cellulose as well as xylan. Cellulosome complex is produced by C. thermocellum and C. cellulovorans, which is organized on their cell
surface. This complex consists of cellulase and hemicellulase. Interestingly, extracellular secretions of T. reesei consist of three types of cellulases: (1) five
endoglucanases (EG [EC 3.2.1.4]), (2) two cellobiohydrolases (CBH [EC
3.2.1.91]), and (3) two β-glucosidases (BGL [EC 3.2.1.21]) (Bhatia et al. 2012).
Functions of these enzymes are mentioned in Sect. 3.5 of this chapter.
3 Role of Thermophiles in Production of Aviation Biofuels: Fueling the Future
71