Xanthan cellulase

However, in the polysaccharides obtained from some mutant strains, there are deviations from this idealized structure.44 Xanthan is relatively resistant to enzymic hydrolysis, but it has been cleaved by an enzyme preparation from a Bacillus sp. at moderate temperatures and in the presence of buffer salts, yielding mono- and oligo-saccharides 45 A partially purified, enzyme preparation46 hydrolyzed deacetylated or depyruvated xanthan, and also xanthan from several wild-type and mutant strains of Xanthomonas. The release of reducing material varied little with xanthan preparations having differences in O-acetyl and pyruvic acetal contents. Under similar conditions of incubation, cellulase acted only on xanthan from mutant strains that had defective side-chain formation. 

The observation47 that the stability of the secondary-ordered structure of xanthan (and as a consequence, its susceptibility to hydrolysis by cellulase) is a function of temperature, pH, and ionic strength has allowed the application of enzymic techniques to structural analysis. Xanthan is hydrolyzed in salt-free, aqueous solutions at elevated temperatures that is, hydrolysis occurs if the chain is unordered. It was proposed that, at lower temperatures and in the presence of salt, the small side-chains organize around the (1 ->4)-/3-D-glucan backbone and protect it from hydrolysis. 

The action of various cellulase preparations on solutions of commercial xanthans, and those from laboratory strains containing both pyruvate and acetate, pyruvate only, or acetate only, in the unordered state has been... 

Xanthan gum is compatible with most synthetic and natural viscosity-increasing agents. If it is to be combined with cellulose derivatives, then xanthan gum free of cellulase should be used to prevent depolymerization of the cellulose derivative. 

Interaction of xanthan with guar gum in solution leads to enhanced viscosity, whereas elastic thermo-reversible gels are obtained with LEG and konjac mannan sofP (Figure 26.5). Since xanthan may contain cellulases, it cannot be used with cellulose derivatives. 

Cellulase causes a random breakdown ot the main chain of the xanthan from Xanthomonas campestris when the polysaccharide is in the ordered conformation. It is shown that there is no hydrolysis on the ordered helical conformation of the polysaccharide. Quasi-elastic light-scattering studies of aqueous solutions of xanthan reveal the existence of two different structural forms characterized by different values of the translational diffusion coefRcient i>f. Xanthan (mol. wt. 2.16 X 10 ) undergoes a self-association in aqueous solution, an effect that is inhibited in 4 m urea solution... 

As their name implies, hydrolysis is the preferred reaction for hydrolases in water under optimal conditions. Some exanq>les in the polysaccharide area include cellulolytic enzymes for biomass conversion (5), proteases for the degradation of guar gum (21), cellulase for viscosity reduction of xanthan gum (21), and P-D-galactosidase for pectin hydrolysis (21). 

It has been reported previously that cellulase can hydrolyze xanthan in the disordered conformadon in soludon. The reacdon condidons typically involve SO°C in the absence of external salt. A decrease in soludon viscosity is usually observed (7). 

Since cellulases are commercially available, we looked at this reaction further. A number of several commercial cellulases were screened for their ability to reduce the Brookfield viscosity of xanthan solution. Mannosidase and glucosidase were also used in combination with cellulase with the hope of exposing the cellulose backbone by removing the side chains. All of the cellulases tested were found to be somewhat active toward r ucing the molecular weight of xanthan gum (Table 3). 

Case 2. An enzyme has limited reactivity towards the polymer because of the reluctant enzyme/substrate match. An example is xanthan and cellulase. Slight, but real, decrease in molecular weight has been found. In a fortuitous situation an enzyme may show good reactivity towards a non-substrate polymer under a particular set of experimental conditions. The papain-catalyzed reaction of pectin with an amine is such an example. 

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