Substrate Specificity in Glycoside Hydrolase Family 10

The Pseudomonas family 10 xylanase, Xyl10A, hydrolyzes β1,4-linked xylans but exhibits very low activity against aryl-β-cellobiosides. The family 10 enzyme, Cex, fromCellulomonas fimi, hydrolyzes aryl-β-cellobiosides more efficiently than does Xyl10A, and the movements of two residues in the –1 and –2 subsites are implicated in this relaxed substrate specificity (Notenboom, V., Birsan, C., Warren, R. A. J., Withers, S. G., and Rose, D. R. (1998)Biochemistry 37, 4751–4758). The three-dimensional structure of Xyl10A suggests that Tyr-87 reduces the affinity of the enzyme for glucose-derived substrates by steric hindrance with the C6-OH in the –2 subsite of the enzyme. Furthermore, Leu-314 impedes the movement of Trp-313 that is necessary to accommodate glucose-derived substrates in the –1 subsite. We have evaluated the catalytic activities of the mutants Y87A, Y87F, L314A, L314A/Y87F, and W313A of Xyl10A. Mutations to Tyr-87 increased and decreased the catalytic efficiency against 4-nitrophenyl-β-cellobioside and 4-nitrophenyl-β-xylobioside, respectively. The L314A mutation caused a 200-fold decrease in 4-nitrophenyl-β-xylobioside activity but did not significantly reduce 4-nitrophenyl-β-cellobioside hydrolysis. The mutation L314A/Y87A gave a 6500-fold improvement in the hydrolysis of glucose-derived substrates compared with xylose-derived equivalents. These data show that substantial improvements in the ability of Xyl10A to accommodate the C6-OH of glucose-derived substrates are achieved when steric hindrance is removed.

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