Fungal cellulases

Histamine HCl (10 mg/ml) Negative control Coca-glycerol control Cellulase Fungal 7 mm (=3+) 0 0 5 mm (2+)... 

Figure 2. Advanced stage of barley leaf penetration by C. sativus. The pathogen has penetrated the anticlinal cell wall junction between two host epidermal cells (e). The fungal appressorium (a) is visible above the cell comer. The host cell comer matrix has been displaced by an enlarged hyphal element (h) situated between the thin cell walls of the host epidermal cells. The host epidermal cell walls have been densely labeled with the cellulase-gold probe. An intercellullar hyphal element (ih) is present within the penetrated host cell. Bar = 1 pM.

Cellulase is not produced in the human digestive system. Cellulolytic enzyme preparations obtained from A. niger or other fungal sources are available, and it is thought that their ingestion may improve overall digestion, particularly in relation to high-fibre diets. 

In the treatment of cellulose pulps one essential criterion for a suitable enzyme preparation is that its cellulase activity should be as low as possible, or preferably absent completely. As even extremely low cellulase activities may ruin pulp quality, Trichoderma enzyme preparations are unlikely to be suitable for these applications. Many bacterial and fungal enzymes with low cellulase activity have been shown to be suitable for treatment of pulps 14, 15, 16,17), Regulation of the often synchronous production of cellulolytic and hemicellulolytic enzymes in micro-organisms is not well understood, and is further complicated by substrate cross-specificity of these enzymes. Enzymes with both endoglucanase and xylanase activity have been reported for bacteria 18, 19) and fungi 20, 21, 22), In addition to selection of strain and... 

Cost sensitivity studies have shown that the successful commercialization of cellulase-based processes, such as the conversion of cellulose to fermentable sugars, is highly dependent on the cost of enzyme production (i). Because fungal -D-glucosidase (EC 3.2.1.21) is the most labile enzyme in this system under process conditions (2), and k to efficient saccharification of cellulose, this enzyme was targeted for application of stabilization technology, both through chemical modification and immobilization to solid supports. 

The catalytic domain sequences do not seem to be strongly, or at least solely influenced by phylogeny. For example, we see in Family 1 the presence of both bacterial and fungal cellulase sequences. This family has a different relationship between its members than do Families 4 and 5, which are comprised of members of a single species. However, we do notice that cellulase sequences from a particular organism can be distributed into several different families. 

CBDs also occur at the amino or carboxyl termini of fungal cellulases (19) and the structure of the CBD for T. reesei CBH I has been resolved by 2-dimensional NMR (20). There is little sequence similarity between the bacterial and fungal CBDs although hydrophobic and... 

It has been proposed that the production of xylanases and cellulases is under separate regulatory control in some filamentous fungi (1). Hrmova et al. (42) reached a similar conclusion after monitoring the daily production of these enzymes in Trichoderma reesei QM 9414. Xylanase and cellulase activities followed independent production profiles during fungal growth. The same effect has been observed in batch cultures of T. harzianum. We have observed peak xylanase activity on the third day of growth whereas the cellulase activity peaked after day five or six (unpublished). 

Selective inactivation of cellulase activities is another method which has been applied to fungal culture filtrates to produce cellulase-free xylanase preparations. Cellulase inactivation of a crude enzymatic complex was achieved by Barnoud et al. (15) using a 1 mM mercuric chloride solution. In the presence of this sulfhydryl binding metal, complete inactivation of endocellulases was observed whereas the xylanases retained 80% of their activity. 

Another method which may become a useful technique for selective inactivation of cellulases in enzyme mixtures is the use of selective heat inactivation. While establishing the thermostability properties of crude xylanases from a fungal strain Y-94, Mitsuishi et al. (80) observed differential heat labilities of the cellulase and xylanase activities in the culture filtrate. After an incubation period of 20 minutes at 65°C, the xylanase activity was reduced by 5-10% whereas the Avicelase and /3-glucosidase activities were reduced by 100% and 60%, respectively. We have observed a similar temperature dependency of xylanase and cellulase activities in T. auranti-acus. As indicated in Figure 2, treatment of the culture filtrate at 70°C for 20 minutes resulted in less than a 5% loss in xylanase activity whereas cellulase activities were reduced by 40-50%. A similar effect has also been observed for the xylanases and cellulase enzymes produced in culture filtrates from T. harzianum (93). Further work in the area of heat treatments may improve the effectiveness of cellulase inactivation. Since the cellulase activities of some enzyme preparations can be more rapidly inactivated on... 

Jones, D.I.H. and Hayward, M.V. (1975) The effect of pepsin pre-treatment of herbage on the prediction of dry matter digestibility from solubility in fungal cellulase solutions. Journal of the Science of Food and Agriculture 26, 711-718. 

Cellulose, the most abundant of all biopolymers, is extremely stable but is attacked by a host of bacterial and fungal (3-glycanases.96 Animals do not ordinarily produce cellulases but some termites do.97 Cellulase structures are varied, being represented by 10 of 57 different glycosylhydrolase families.98 Most, like lysozyme, retain the P configuration in their products but some invert.98 100... 

Table V. Synergistic Effects on Cellulase Activity Shown by Combination of Ci and Cx from Various Fungal Sources ...

Table VI. Synergistic Effects on Cellulase (Cotton-Solubilizing) Activity Shown by Some Fungal Culture Filtrates, when Supplemented with Ci from T. koningii or P. funiculosum Cellulasesa...

All naturally occurring fungal strains of Trichoderma require an inducer for cellulase synthesis. In the absence of an inducer such as cellulose, cellobiose (21,22), or sophorose (12,13,14,23), Trichoderma does not make any detectable cellulase complex enzymes. The true physiological inducer of cellulase is currently unknown. Insoluble cellulose is presumably not such an inducer since there is no way for the internal cell machinery to sense the presence of this insoluble material. However, a small transglycosylation product such as sophorose, 2-0-/ -glucopyranosyl-D-glucose, may well be the natural inducer. 

The possibility that BS and BI cellulase could act synergistically, as has been recorded for many components of fungal cellulolytic complexes (1,2), was tested in several assay systems by adding the enzymes separately or together at the same total activity levels (CMCase units). The assays included the hydrolysis of CMC, cellohexaose, and cellulose powder. The results (not shown here) indicated that the pea cellulases were no more or less effective when added together than when added singly, i.e., there is no indication of any interaction between the enzymes, or any preference by one for the products generated by the other. 

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