Xyloglucanases

Xyloglucan is one of the major hemicellulose polymers in growing primary cell walls of various plant species. Due to analytical problems, it has been difficult to differentiate it from cellulose and xylan. Xyloglucan is closely associated with cellulose microfibrils via hydrogen bonding, thus providing the load-bearing network of the cell wall, which protects the cell wall from collapsing due to the osmotic stress. Most research efforts have been focused on determination of plant enzymes responsible for control and modification of the expanding cell wall (42-60). Although not as heavily branched as xylans, the xylose and other substituents on xyloglucan can make enzymatic digestion more complicated than that of cellulose and p-glucan.

Xyloglucan has cellulose-like backbone of p-1,4-D-glucopyranose residues to which a-D — xylose residues are attached at C-6 position. Generally, 60-75% of the glucose residues are branched with xylose, except in grasses, which have a lower degree ofsubstitution. Xylose can form side chains with D-galactopyranose and L-fucopyranose saccharides, and rarely with L-arabinofuranose. Plant cell wall degrading enzymes, such as endoglucanases, xyloglucan endotransglycosylases, and exoglycosidases, like a-fucosidases or p-galactosidases have been reported to digest the xyloglucan. Also, some cellulases of Trichoderma reesei are able to hydrolyze the xyloglucan backbone (61). A new class of polysaccharide-degrading enzymes comprise the specific xyloglucanases, or xyloglucan-specific endoglucanases, which can attack the backbone also at substituted glucose residues (62).

Some xyloglucanases require a specific xylose substitution pattern while others are more general (43). This determination seems to be dependent on the binding sub-sites in specific endoglucanases (61). A xyloglucanase from Aspergillus niger has been shown to be active against several p-glucans, but having the highest activity against tamarind xyloglucan (63). This, combined with its lack of synergy with cellulases, indicates specificity different from traditional endoglucanases. A plant-specific enzyme believed to be responsible for modifica­tion of xyloglucan in the cell wall through endo-hydrolysis and glycosyl transferase activities has also been characterized (59, 60).

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