Enzymes polysaccharide hydrolysis

In the absence of enzymes, polysaccharide hydrolysis is relatively slow (Table 3.7), with the OC-linked polymers (e.g. amylose) being slightly more stable than their (3-linked counterparts (e.g. cellulose and chitin). It appears that glycosidases can accelerate hydrolysis by a factor of c.1017 (Wolfenden et al. 1998). 

Keith, S. C., and C. Arnosti. 2001. Extracellular enzyme activity in a river-bay-shelf transect Variations in polysaccharide hydrolysis rates with substrate and size class. Aquatic Microbial Ecology 24 243—253... 

Enzyme-catalyzed hydrolysis of polysaccharides containing uronic acids is well established, but, during the past few years, it has become evident that bacterial enzymes may degrade such materials by a process involving eliminations and the production of oligosaccharides containing nonreducing, terminal, 4,5-unsaturated, uronic acid residues. 

Like cytokinins, which were initially isolated from autoclaved DNA, the first oligosaccharins were produced artificially. Indeed, to date, almost all experimental work concerns oligosaccharins that have been generated by the in vitro fragmentation of cell wall polysaccharides, either by acid- or enzyme-catalysed hydrolysis. The evidence for their natural production in vivo will be summarised later. 

The discovery, shortly thereafter, of the enzyme pullulanase from the bacterium Aerobacter aerogenes helped considerably in further structural analysis of the polysaccharide. Hydrolysis of pullulan by diis enzyme showed that essentially no products containing (l->6)-ff-i>-giucosidic linkages were formed, the preponderant product being maltotriose. It was thus clear that the enzyme hydrolyzes (l->6)-a-D-glucosidic link-... 

Such effects are not unique to carboxypeptidase A. The rate of substrate polysaccharide hydrolysis by lysozyme is remarkably dependent on the polysaccharide chain length 153, 154). Both steady-state kinetic studies and X-ray crystallographic studies on enzyme-inhibitor complexes for chymotrypsin 156) trypsin 156), elastase 157), and subtilisin 158) are indicative of the existence of multiple-loci substrate binding sites. Furthermore, the dependence of Acat on substrate chain length for all these enzymes strongly implies that the filling... 

In general, only carbon sources that are rapidly taken up by A. niger lead to massive accumulation of CA. Polysaccharides need to be hydrolyzed in a pretreatment step before CA fermentation, because they are broken down too slowly to match the high rate of sugar cataboHsm required for CA production. The slow speed of polysaccharide hydrolysis is due to the low activity of the hydrolytic enzymes at the low pH required to produce CA. ... 

We can give here an example of topolytic activity of ceUulases (which means the capability of the enzyme to run reactions on the surface of insoluble substrate without deep destmction of cellulose structure), particularly of endo-P-l,4-glucanases (EGs). These enzymes have hydrolytic activities toward polyglucans containing p-l,4-glycosidic bonds, which include cotton and wood cellulose, different soluble cellulose derivatives, p-glucans of oat and barley, and other polysaccharides. Hydrolysis of cellulose by EGs occurs by endo-depolymerization mechanism. 

For the most part, low molecular weight carbohydrates of commerce are made by depolymerization via enzyme- or acid catalyzed hydrolysis of polysaccharides. Only sucrose and, to a very much lesser extent, lactose, both disaccharides, are commercial low molecular weight carbohydrates not made in this way. 

Unlike many of the catalysts that chemists use in the laboratory, enzymes are usually specific in their action. Often, in tact, an enzyme will catalyze only a single reaction of a single compound, called the enzyme s substrate. For example, the enzyme amylase, found in the human digestive tract, catalyzes only the hydrolysis of starch to yield glucose cellulose and other polysaccharides are untouched by amylase. 

Many plants store carbohydrates in their tubers in the form of polysaccharides which upon acid hydrolysis yield D-fructose as the main product. These polysaccharides differ from one another in the size of the molecule as well as in the position of the linkages between the D-fructose residues. Polyfructosans have also been prepared by enzymic action on sucrose. 

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