Crystallographic analysis shows substrate binding at the -3 to +1 active site subsites and at the surface of glycoside hydrolase family 11 endo-1,4-{beta}-xylanases
Résumé
Glycoside hydrolase family 11 xylanases are predominant enzymes in the hydrolysis of heteroxylan, an abundant structural polysaccharide in the plant cell wall. To gain more insight in the protein-ligand interactions of the glycon as well as the aglycon subsites of these enzymes, catalytically incompetent mutants of Bacillus subtilis and Aspergillus niger xylanases were crystallized, soaked with xylo-oligosaccharides and subjected to X-ray analysis. For both xylanases, there was clear density for xylose residues in the -1 and -2 subsites. In addition, for the B. subtilis xylanase, there was also density for xylose residues in the -3 and +1 subsite showing the spanning of the -1/+1 subsites. These results together with the observation that some residues in the aglycon subsites clearly adopt a different conformation upon substrate binding allowed us to identify the residues important for substrate binding in the aglycon subsites. In addition to substrate binding in the active site of the enzymes, the existence of an unproductive second ligand binding site located on the surface of both the B. subtilis and A. niger xylanases was observed. This extra binding site may have a function similar to the separate carbohydrate binding modules of other glycoside hydrolase families.
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