Probing the catalytically essential residues of the α-L-arabinofuranosidase from Thermobacillus xylanilyticus

The alpha-L-arabinofuranosidase D3 from Thermobacillus xylanilyticus is an arabinoxylan-debranching enzyme which belongs to family 51 of the glycosyl hydrolase classification. Previous studies have indicated that members of this family are retaining enzymes and may form part of the 4/7 superfamily of glycosyl hydrolases. To investigate the active site of alpha-L-arabinofuranosidase D3, we have used sequence alignment, site-directed mutagenesis and kinetic analyses. Likewise, we have shown that Glu(28), Glu(176) and Glu(298) are important for catalytic activity. Kinetic data obtained for the mutant Glu(176)-->Gln, combined with the results of chemical rescue using the mutant Glu(176) -->Ala, have shown that Glu(176) is the acid-base residue. Moreover, NMR analysis of the arabinosyl-azide adduct, which was produced by chemical rescue of the mutant Glu(176)-->Ala, indicated that alpha-L-arabinofuranosidase D3 hydrolyses glycosidic bonds with retention of the anomeric configuration. The results of similar chemical rescue studies using other mutant enzymes suggest that Glu(298) might be the catalytic nucleophile and that Glu(28) is a third member of a catalytic triad which may be responsible for modulating the ionization state of the acid-base and implicated in substrate fixation. Overall, these findings support the hypothesis that alpha-L-arabinofuranosidase D3 belongs to the 4/7 superfamily and provide the first experimental evidence concerning the catalytic apparatus of a family 51 arabinofuranosidase.