The Topology of the Substrate Binding Clefts of Glycosyl Hydrolase Family 10 Xylanases Are Not Conserved

The crystal structures of family 10 xylanases indicate that the distal regions of their active sites are quite different, suggesting that the topology of the substrate binding clefts of these enzymes may vary. To test this hypothesis, we have investigated the rate and pattern of xylooligosaccharide cleavage by the family 10 enzymes, Pseudomonas fluorescens subsp.cellulosa xylanase A (XYLA) and Cellulomonas fimi exoglucanase, Cex. The data showed that Cex contained three glycone and two aglycone binding sites, while XYLA had three glycone and four aglycone binding sites, supporting the view that the topologies of substrate binding clefts in family 10 glycanases are not highly conserved. The importance of residues in the substrate binding cleft of XYLA in catalysis and ligand binding were evaluated using site-directed mutagenesis. In addition to providing insight into the function of residues in the glycone region of the active site, the data showed that the aromatic residues Phe-181, Tyr-255, and Tyr-220 play important roles in binding xylose moieties, via hydrophobic interactions, at subsites +1, +3, and +4, respectively. Interestingly, the F181A mutation caused a much larger reduction in the activity of the enzyme against xylooligosaccharides compared with xylan. These data, in conjunction with a previous study (Charnock, S. J., Lakey, J. H., Virden, R., Hughes, N., Sinnott, M. L., Hazlewood, G. P., Pickersgill, R., and Gilbert, H. J. (1997)J. Biol. Chem. 272, 2942–2951), suggest that the binding of xylooligosaccharides at the −2 and +1 subsites ensures that the substrates occupy the −1 and +1 subsites and thus preferentially form productive complexes with the enzyme. Loss of ligand binding at either subsite results in small substrates forming nonproductive complexes with XYLA by binding to distal regions of the substrate binding cleft.