Sunflower - Bioenergy Crop Plants - page 722

30.5.1 c Ellulolytic m icroorganiSmS
Cellulases are widely distributed in bacteria, actinomycetes, and fungi (Srinivasan and Seetalaxman
1988; Baldrian and Gabriel 2003; Immanuel et al. 2006). Bacteria such as Cellulomonas ,
Bacteroides , Cellvibrio , Cytophaga , Ruminococcus , Pseudomonas , and Acetivibrio are capable of
degrading cellulose (Srinivasan and Seetalaxman 1988; Chaudhary et al. 1997). The actinomycetes
have also been investigated for their extracellular cellulase, and strains such as Streptomyces
flavogriseus and Thermomonospora have been studied in detail (Srinivasan and Seetalaxman
1988). However, fungi form the most important group for cellulose degradation and formation
of cellulolytic enzymes. Among them the fungi imperfecti and basidiomycetes have received the
most attention and have yielded promising cultures for cellulose biotechnology. One of the most
extensively studied cultures is Trichoderma reesei (Mandels 1982; Kuhls et al. 1997). This strain
has the advantage of a full complement production of cellulase, stability under enzymatic hydrolysis
conditions, and resistance to chemical inhibitors. However, the main disadvantage of Trichoderma
cellulase is its low activity of β-glucosidase. Other fungi that secrete good levels of extracellular
cellulase include Trichoderma harzianum , Penicillium funiculosum, Sclerotium rolfsii , Aspergillus
terreus , A. phoenicis , Talaromyces emersonii , Sporotrichum pulverulentum , Humicola grisea , and
Phanerochaete chrysosporium (Srinivasan and Seetalaxman 1988; Singh et al. 1995; Falih 1998;
De-Paula et al. 1999; Berlin et al. 2006; Kocher et al. 2008). Aspergillus spp. are known to produce
good levels of β-glucosidases and have been supplemented with Trichoderma cellulase to improve
the enzyme performance (Wyman 1996; Itoh et al. 2003; Tabka et al. 2006).
30.5.2 t hE c EllulaSE c omplEx
The enzyme system (i.e., cellulase complex) for the conversion of cellulose to glucose has been
classified on the basis of the mode of catalytic action and now structural properties (Henrissat
et al. 1998) into three enzyme types: (1) endo-l,4-β-glucanase or carboxymethylcellulase
(CMC) (EC3.2.1.4); (2) exo-l,4-β-glucanase, (C 1 ) or cellobiohydrolase (CBH) (EC 3.2.1.91); and
(3) β-glucosidase or cellobiase (EC 3.2..1.21). The cellulolytic enzymes with β-glucosidase act
sequentially and cooperatively to degrade crystalline cellulose to glucose. Endoglucanases act in
a random fashion on the regions of low crystallinity of the cellulosic fiber, whereas exoglucanases
remove cellobiose (β-1,4-glucose dimer) units from the nonreducing ends of the cellulose chains
(Figure 30.2). The presence of carbohydrate binding modules (CBMs) is important for initiation
and processivity of exoglucanases (Teeri 1997). Synergism between these two types of enzymes is
attributed to the endo-exo form of cooperativity and has been studied extensively between cellulases
in the degradation of cellulose by T. reesei (Henrissat et al. 1985; Teeri 1997).
In T. reesei , two cellobiohydrolases (CBH1 and CBHII), five endoglucanases (EGI, EGII, EGIII,
EGIV and EGV), and two β-glucosidases (BGLI and BGL II) have been purified and evaluated
on various substrates. The synergism between CBH1 and endoglucanase I and II depends on the
structural and ultrastructural features of the cellulosic substrate. Synergism is most marked when
highly crystalline substrates were used, low with amorphous cellulose and absent with soluble
derivatives. Four types of synergistic phenomenon have been reported: (1) endo-exo synergy
between endo- and exoglucanases; (2) exo-exo synergy between exoglucanases processing from
the reducing and nonreducing ends of cellulose chains; (3) synergy between exoglucanases and
β-glucosidases that remove cellobiose and cellodextrins as end-products of endo- and exoglucanases,
and (4) intramolecular synergy between catalytic domains and CBMs (Din et al. 1994; Teeri 1997).
30.5.3 c EllulaSE p roduction
As stated earlier, T. reesei has been the most extensively studied strain for cellulase production in
submerged culture so far. The cellulase production cost represents 40-60% of the overall cost of the
process designed to saccharify cellulosic substrates and ferment the products (Ryu and Mandels 1980).