Sunday, April 29, 2007

b-D-Xylosidase from Selenomonas ruminantium: catalyzed reactions with natural and artificial substrates

Douglas Jordan, 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 saccharification processes. Kinetic parameters, kcat and kcat/Km, are greater 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 xylosidases indicate that the enzyme active site has  two subsites for recognition of substrate, the two terminal xylosyl residues that share the scissile glycosidic bond. In addition to xylosidase activity, the enzyme efficiently catalyzes hydrolysis of 4-nitrophenyl-α-L-arabinofuranoside. Interestingly, 1,4-β-D-xylobiose is a better substrate than 4-nitrophenyl-β-D-xylopyranoside, indicating that subsite +1 offers considerably more transition state stabilization to the natural substrate than to the artificial substrate. Dependence of catalyzed reactions on pH and inhibition by reaction products were determined.