Monday, May 5, 2008
6-37

β-D-Xylosidase from Selenomonas ruminantium:  Thermodynamics of Enzyme-Catalyzed and Noncatalyzed Reactions

Douglas B. Jordan and Jay D. Braker. Fermentation Biotechnology, USDA-ARS, 1815 N. University Street, Peoria, IL 61604

β-D-Xylosidase from Selenomonas ruminantium is the best catalyst known for promoting hydrolysis of 1,4-β-D-xylooligosaccharides and it has potential utility in industrial saccharification processes.  Kinetic parameters, kcat and kcat/Km, are more than 10-fold larger than those reported for the enzyme isolated from other organisms.  In cleaving 1,4 glycosidic bonds, the family 43 glycoside hydrolase acts through an inversion mechanism and cleaves a single xylose residue from the nonreducing end of xylooligosaccharides per catalytic cycle without processivity.  Three-dimensional structures of homologous GH43 β-xylosidases indicate that the enzyme active site has only two subsites for recognition of substrate, the two terminal xylosyl residues that share the scissile glycosidic bond.  In addition to its β-xylosidase activity, the enzyme efficiently catalyzes hydrolysis of 4‑nitrophenyl-α-L-arabinofuranoside using the same active site as for its b-xylosidase activity.  Temperature dependence of kinetic parameters of enzyme-catalyzed reactions and noncatalyzed reactions were determined.

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