Biochemical Characterization of a GH43 β-Xylosidase from Bacteroides ovatus

Divalent metal-activated glycoside hydrolase family 43 (GH43) β-xylosidases have been found to have high k _cat/K _m for xylooligosaccharides and may demonstrate high efficacy in industrial reactors digesting hemicellulose. By searching an amino acid database, we found a Bacteroides ovatus GH43 β-xylosidase termed BoXA that is 81% identical in overall amino acid sequence to a GH43, divalent metal-activated β-xylosidase with high k _cat/K _m, and also it has 19 of 20 residues in the active site conserved. However, unlike its metal-activated homolog, the B. ovatus enzyme does not lose activity after extensive EDTA treatment nor does it gain activity by addition of divalent metal ions. Thus, either it cannot be activated by divalent metal or it maintains a tightly bound, non-exchangeable metal ion. At 25 °C and pH 6.0, the k _cat is 69 s^-1 for xylobiose and k _cat/K _m is 210 s^-1 mM^-1 for xylotriose, with the latter being 0.7 that of the highest known value. The determined K _i for D-glucose is 4.9 M, which is the highest known for a β-xylosidase. The enzyme has potential utility operating in bioreactors digesting plant biomass.