Cellulase refers to a group of enzymes which, acting together, hydrolyze cellulose. It has been reviewed by Emert et al. (1974) and Whitaker (1971).
Cellulose is a linear polysaccharide of glucose residues connected by β-1,4 linkages. Like chitin it is not cross-linked. Native crystalline cellulose is insoluble and occurs as fibers of densely packed, hydrogen bonded, anhydroglucose chains of 15 to 10,000 glucose units. Its density and complexity make it very resistant to hydrolysis without preliminary chemical or mechanical degradation or swelling. In nature cellulose is usually associated with other polysaccharides such as xylan or lignin. It is the skeletal basis of plant cell walls. According to Spano et al. (1975) cellulose is the most abundant organic source of food, fuel and chemicals. However, its usefulness is dependent upon its hydrolysis to glucose. Acid and high temperature degradation are unsatisfactory in that the resulting sugars are decomposed; also, waste cellulose contains impurities that generate unwanted by-products under these harsh conditions.
Although cellulases are distributed throughout the biosphere, they are most manifest in fungi and microbial sources. Almin et al. (1975) and Eriksson and Pettersson (1975) report on five endo-β-1,4 glucanases of Chrysosporium lignorum. See also Eriksson and Rzedowski (1969a and b). Cellulase from Penicillim notatum has been characterized by Pettersson (1969) and that from Pseudomonas fluorescens by Yamane et al. (1970). Bucht and Eriksson (1969) and Keilich et al. (1969) report on cellulose digestion by fungi. Eriksson et al. (1975) indicate an oxidation enzyme may play a role in the Sporotrichum pulverulentum system.
The enzymatic mechanism whereby certain microorganisms can quite rapidly and completely degrade cellulose is not yet understood. Reese et al. (1950) proposed that at least two steps are involved: first, a prehydrolytic step wherein anhydroglucose chains are swollen or hydrated and secondly, hydrolytic cleavage of the now susceptible polymers either randomly or endwise. The first step would involve an enzyme designated C1 and the second, hydrolytic enzymes termed Cc. A third type of enzyme is β-glucosidase (cellobiase).
The C1 component attacks highly ordered (crystalline) cellulose, i.e., cotton fibers or Avicel, but has little effect on soluble derivatives such as carboxymethyl cellulose (CMC). According to Spano et al. (1975) C1 "decrystallizes" or hydrates cellulose chains whereas Cc consists of exo and endo β-1,4 glucanases that attack soluble derivatives or cellulose that has been acid or alkali swollen. (Wood and Philips 1969.)
Halliwell and Griffin (1973) and Wood and McCrae (1972) report on isolating the C1 component of Trichoderma koningii. It is a β-1,4-glucan cellobiohydrolase. They have also determined that Cc is not essential for its activity although Halliwell and Riaz (1970) had indicated three components of Trichoderma koningii to be synergistic, having feeble activity by themselves.
The cellulase complex of Trichoderma reesei (formerly named T. viride) has been most thoroughly studied. It is complete in that it can convert native cellulose as well as derived celluloses to glucose (King and Nessal 1969). Howell and Stuck (1975) have described the complex and indicate it to be remarkably resistant to inhibitors except carbohydrates, particularly cellobiose and excess cellulose. Okada (1975) obtained a number of active fractions from Trichoderma reesei differing in carbohydrate content and specificity but otherwise identical.
Berghem and Pettersson (1973) reported on the C1 enzyme isolation from Trichoderma reesei, a β-1,4-glucan cellobiohydrolase which acts upon crystalline cellulose. Cellobiose is the principle product. The enzyme has been further characterized by Berghem, Pettersson and Axio-Fredriksson (1975). They indicate a molecular weight of 42,000 based on amino acid and carbohydrate analysis. The enzyme contained 9.2% carbohydrate covalently bound. The pH optimum is about 4.8 and the reaction rate can be enhanced by addition of cellobiase and additional endo-glucanase.
Characteristics of the Cellulase Complex from Trichoderma reesei:
The Trichoderma reesei complex is a true cellulase in the most rigid sense, being able to convert crystalline, amorphous, and chemically derived celluloses quantitatively to glucose. It has been established that: a) the system is multi-enzymatic; b) that at least three enzyme components are both physically and enzymatically distinct; and c) that all three components play essential roles in the overall process of converting cellulose to glucose (King and Vessal 1969).
Molecular weight: C1, 57,000 (Selby 1969); 42,000 (Berghem et al. 1975).Endoglucanase (E.C.18.104.22.168), 52,000 (Li et al. 1955); 23,500 - 58,000 (Beldman et al. 1985).Exoglucanase (E.C.22.214.171.124), 60,500 - 62,000 (Beldman et al. 1985).β-Glucosidase (E.C.126.96.36.199), 76,000 (Beldman et al. 1985).
Optimum pH: Varies with the substrate in the range 4.2 - 5.2.
Stability: Lyophilized preparations are stable for one year when stored refrigerated.
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