Polysaccharides glycosidases

Faijes, M. and Planas, A. (2007) In vitro synthesis of artificial polysaccharides by glycosidases and glyco-synthases. Carbohydrate Research, 342, 1581-1594. 

The structure of the leaving group is a decisive factor for donor properties. Natural substrates for glycosidases are polysaccharidic chains and the corresponding disaccharides are still used in transglycosylation reactions as... 

Due to the extreme variety of xylan structures, it is obvious that many kinds of enzymes are needed for their complete hydrolysis in nature. Xylanases (EC 3.2.1.8.) are the polysaccharide hydrolases responsible for the attack of the polymer backbone itself. The total hydrolysis or modification of heteroxylans requires in addition several different exo-glycosidases and esterases. The present knowledge of these enzymes is reviewed in this paper. 

Glycoside hydrolases (glycosidases) are essential and consequently widely abundant enzymes in all living systems that rely on the processing of carbohydrates. From the degradation of such polysaccharides as starch, cellulose, or chitin to the highly... 

There is thus some evidence for the tight binding of enzymes, especially glycosidases, to cell walls in both dicots and monocots. The nature and localization of these enzymes suggest that they may, perhaps, play a role in wall breakdown and such other processes as elongation growth. The membrane systems of plant cells are known to be involved in the transport, and introduction, of polysaccharides into the cell wall247-249 enzymes localized in the wall may also play a part in the metabolism of these polymers when they are transferred from the membrane system to the wall. 

The properties and action patterns of glycosidases and polysaccharide depolymerases have been reviewed,2 5 as also have the enzymes involved in biosynthetic pathways.6,7 An understanding of biosynthesis can provide insights into the structures of polysaccharides. Reviews concerning particular polysaccharides have appeared, and references to these will be given in relevant sections. 

Lysozyme and /3-galactosidase, which are both glycosidases, catalyze very similar reactions. Both enzymes are found to catalyze the alcoholysis of their polysaccharide substrates with retention of configuration at the C-l carbon (equation 8.25).14-17 This is consistent with the evidence presented in Chapter 7, section C3, that there is at least one (but probably only one) intermediate on the reaction pathway. However, kinetic isotope data are consistent with the interpretation that the intermediate in the reaction of /3-galactosidase is covalent and that there are two successive SN2 displacements, whereas the intermediate with lysozyme is a bound carbonium ion formed in an SN1 reaction (Chapter 16). The carbonium ion, unlike an analogous one in solution, reacts stereospecifically on the enzyme. Thus, the stereochemical evidence by itself has given no indication of the nature of the intermediate. 

Starch molecules are digested in the stomach by enzymes called a-glycosidases, which break down the polysaccharide chain and release individual glucose molecules. As is usually the case with enzyme-catalyzed reactions, a-glycosidases are... 

In contrast to cellulases, the hemicellulases encompass a much broader suite of activities. In addition to analogue versions of endo-, exo-, and glycosidase cellulase activities, multiple debranching activities are needed to handle the high complexity of the heterogeneous hemicelluloses (Table 33.3). The varied backbone composition of hemicelluloses also adds complexity. Xylans, xyloglucans, man-nans, and numerous other minor polysaccharide chains form the backbone for different hemicelluloses. 

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