Microorganisms, cellulase

Marigold petals are rich sources of xanthophyUs, mainly lutein esters. To increase the coloring power, chemical extraction of the colorant from flower meal is performed or a new enzymatic procedure is applied. It was shown that treatment with cellulases or mixed saprophyte microorganisms or solid state fermentation improved the xanthophyll extraction yield. ... 

The anaerobic biological conversion of the major polymeric components of MSW identified require appropriate microorganisms and hydrolytic enzyme systems. Extracellular hydrolytic enzymes, such as cellulases and lipases, have been shown to be effective in the post hydrolysis of anaerobic digester efQuent solids 34) or pretreatment of complex organic polymers before the digestion process 48),... 

R. flavefaciens cells, during growth in pure culture, released cellulase, endoglucanase, and xylanase into the culture fluid. This microorganism hydrolyzed cellulose to yield only cellobiose as a product 49). It had been reported that a ceUobiose phosphorylase and glucokinase were present in R flavefaciens (8). The Ruminococcus cellulase system was repressed by disaccharides such as cellobiose, sucrose, and lactose 50). 

Table II. Cellulase Production by Some Mesophilic and Thermophilic Microorganisms...

Xylanolytic enzymes free of cellulases can be applied in the pulp and paper, textile, and food industries and in basic research. However, most microorganisms grown under natural conditions produce both xylanases and cellulases. Strategies to produce xylanolytic systems free of cellulases are elimination of cellulase activity by separation or inhibition, selection and construction of cellulase-negative strains, and finding conditions for separate production of xylanolytic systems by cellulolytic strains. 

Most microorganisms grown under natural conditions produce both cellulases and xylanases. Biochemistry, microbiology, and molecular biology offer several approaches to obtain xylanase prq)arations largely or completely free of cellulases (77). 

Xylanolytic strains that do not produce cellulases, as well as xylanases that do not attack cellulose, can be found among yeasts and yeast-like microorganisms (Table I). 

Selective Production of Xylanases by Cellulolytic Microorganisms. Until recently there was little information on common or separate genetic control of cellulase and xylanase synthesis in microorganisms (60). Studies on this subject were complicated by the fact that numerous microbial ceUulases and xylanases are non-specific with respect to cellulose and xylan as substrates. As could be expected from a comparison of both polysaccharide structures, non-specificity is more frequently observed with cel-lulases, because their substrate binding sites can easily accommodate substrate using an unsubstituted p-(l 4)-linked chain of D-xylopyranosyl units. 

A given microorganism may produce one or more enzymes of each type. An understanding of the role of each enzyme in cellulose biodegradation requires their purification and characterization, and an analysis of the ways in which they interact with the substrate and with each other. However, it is often quite difficult to determine the number and type of truly different enzymes produced by an organism. Many cellulolytic microorganisms secrete proteases, which may degrade some or all of the cellulases to smaller,... 

Tt is a widely recognized fact that true cellulolytic microorganisms A produce three basic cellulase components IS), and that these enzyme components act in concert to hydrolyze crystalline cellulose to glucose (6). Many research laboratories have undertaken the task to purify cellulose components from various cellulolytic microorganisms and to study the mechanisms of cellulose hydrolysis. Much information has accumulated concerning the mode of action of cellulose hydrolysis since Reese et al. first proposed the Ci-C concept (7). In spite of this, however, conflicting reports still flourish concerning the composition of the "cellulase complex, the multiplicity of cellulase components, the biosynthesis of cellulose, and the mechanisms of cellulose hydrolysis. 

A wide range of prokaryotic and eukaryotic microorganisms have the potential to produce cellulolytic enzymes when cellulose is present in the growth media (20,23,28,30). However, unlike some of the microorganisms that produce an incomplete cellulase system, T. reesei, a true cellulolytic fungus, produces an array of cellulase enzymes, i.e., the cellulase complex, which is able to hydrolyze cellulose to glucose (23). 

Cellulose is a universal inducer of cellulase biosynthesis. Since cellulose is insoluble, the microorganisms are unable to utilize it unless it has been hydrolyzed first to glucose or soluble oligomers of cellulose. But if cellulose cannot be utilized directly without first being solubilized, the question arises as to how it gets into the cells to act as an inducer. There appear to be several possible explanations. 

One possible explanation is that the mere contact of the cell surface on the insoluble inducer is sufficient to trigger cellulase production. Another explanation is that cellololytic microorganisms have trace quantities of constitutive cellulases which are continuously released. A third possibility is that the cells are able to synthesize cellulases under starvation conditions. 

To hydrolyze crystalline cellulose efficiently by enzymatic means, the inaccessibility of crystalline structures must be overcome. T. reesei and some other true cellulolytic microorganisms produce a cellulase complex that is capable of efficiently hydrolyzing crystalline cellulose. One explanation of this capability was first proposed by Mandels and Reese (7). In this model, two factors, Ci and C worked together to disrupt and hydrolyze cellulose. Ci first disrupted the crystalline structure of the cellulose while Cx attacked the available sites formed by Ci. In other words, Ci and C exhibit synergism in hydrolyzing cellulose. Since then, the combined action of cellobiohydrolase ( Ci ) and endoglucanase ( C ) has been identified as the source of the apparent synergism (6,26,55). 

Several microorganisms have been studied with respect to the production of a cellulolytic enzyme system for the saccharification of cellu-losic materials, the most thoroughly investigated organism and best producer of cellulase being Trichoderma viride (I). Recently, good saccharification data have been reported using a strain of Penicillium (2). 

The point to be emphasized in relation to reports of multiple cellulases in plants or microorganisms, is that not all of these are necessarily functional components of an extracellular "cellulase complex that are needed for optimal or complete cellulose breakdown. Though all of the forms may show a capacity for hydrolyzing 3-1,4-linkages in vitro, in vivo they could function in different intra- or extracellular loci on different substrates, and some could represent processed forms of inactive precursors. In general, not enough is known about the mechanisms whereby these enzymes are synthesized and excreted to enable an informed decision to be made on the roles that they perform. 

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