Cellulolytic enzymes cellulose

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). 

This chapter deals with three aspects of the cellulolytic enzyme system of Thermoactinomyces sp. the location of the CM-cellulase, Avicelase, and / -glucosidase (cellobiase) activities in the culture, the multiplicity of the extracellular enzyme system, and the stability of the different activities as a function of pH, temperature, and time. The results are discussed with reference to saccharification of cellulosic materials. 

Because of its ability to produce and secrete the complete set of cellulolytic enzymes, thus making it particularly potent in hydrolyzing the cellulose polymer to glucose monomers, the soft-rot fungus Trichoderma, in particular T. reesei has been the focus of cellulase research for decades (8). The preferred substrates used by most researchers for cellulase production are pure celluloses such as Avicel, Solka-floc, and cotton (9). Cellulase production by Trichoderma is controlled by a complex metabolic regulation (10-12). Cellulose acts (indirectly) as an inducer for the production of cellulases. Expression of cellulases is furthermore subject to repres-... 

The production of fuel ethanol from renewable lignocellulosic material ("bioethanol") has the potential to reduce world dependence on petroleum and to decrease net emissions of carbon dioxide. The lignin-hemicellulose network of biomass retards cellulose biodegradationby cellulolytic enzymes. To remove the protecting shield of lignin-hemicellulose and make the cellulose more readily available for enzymatic hydrolysis, biomass must be pretreated (1). 

The filamentous fungi investigated showed coinduction of cellulolytic and xylanolytic enzymes. During growth on cellulose, products from the hydrolysis of cellulose also induced production of xylanolytic enzymes, and during growth on xylan, products from the hydrolysis of xylan also induced the production of cellulolytic enzymes. 

ENZYMATIC LIQUEFACTION OF THE RAW MATERIAL. Mixtures of pectinolytic and cellulolytic enzymes are used. Together they bring about a stepwise degradation, with initial loosening and separation of the cell aggregates, then breakdown of the cell walls and finally conversion of cellulose to sugar. After liquefaction of the mash, the remaining solids are usually separated off in a decanter. 

This initial oxidative depolymerization of cellulose evidently opens up the wood cell wall structure so that cellulolytic and hemi-cellulolytic enzymes can reach their substrates despite the presence of lignin. Solubility of wood in 1% NaOH increases markedly on brown-rot attack IS) and reflects cellulose depolymerization and the opening up of the wood structure. 

Enzymatic action can be defined on three levels operational kinetics, molecular architecture, and chemical mechanism. Operational kinetic data have given indirect information about cellulolytic enzyme mode of action along with important information useful for modeling cellulose hydrolysis by specific cellulolytic enzyme systems. These data are based on measurement of initial rates of enzyme hydrolysis with respect to purified celluloses and their water soluble derivatives over a range of concentrations of both substrate and products. The resulting kinetic patterns facilitate definition of the enzyme s mode of action, kinetic equations, and concentration based binding constants. Since these enable the enzymes action to be defined with little direct knowledge of its mechanistic basis, the rate equations obtained are referred to as operational kinetics. The rate patterns have enabled mechanisms to be inferred, and these have often coincided with more direct observations of the enzyme s action on a molecular level [2-4]. 

Improved protein separation techniques utilizing hquid chromatography and electrophoresis coupled with X-ray diffraction and NMR studies have given insights into the three-dimensional structures of cellulolytic enzymes. This molecular architecture data coupled with DNA sequence information has given clues to the chemical mechanisms of enzymatic hydrolysis and molecular interaction between cellulose and the enzymes. 

Cellulolytic enzymes are classified into three main groups cellobiohydrolases, endoglucanases, and P-glucosidases. According to the prevailing hypothesis, cellobiohydrolases (CBHs) attack the chain ends of cellulose polymers to re-... 

Cellulose is a heterogeneous substrate that makes modeling cellulolytic enzyme hydrolysis difficult. Cellulose is composed of chains of glucose connected by P 1-4 glycosidic bonds. One chain end is termed the reducing end because the hemiacetal is able to open to expose the reducing aldehyde. The other chain end is called the non-reducing end because the 1 carbon in the hemiacetal is involv-... 

Work needs to be done to find the specific activity of purified enzymes on the cellulose structure, especially the synergism between different cellulolytic enzymes. With this understood, one can then undertake the task of studying the endoglucanase-cellobiohydrolase system. This requires the identification of binding sites, reaction sites, and changes in cellulose structure as a result of the enzyme action. 

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