|
Douglas B. Jordan,
jordanb@ncaur.usda.gov, Fermentation Biotechnology Research Unit,
USDA-Agricultural Research Service, National Center for Agricultural
Utilization Research, 1815 N. University Street, Peoria, IL 61604
|
| β-D-Xylosidase from Selenomonas ruminantium
has been revealed as 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 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
xylosidases indicate that the enzyme active site has only two subsites
for recognition of substrate, the two terminal xylosyl residues that
share the sessile glycosidic bond. The pKa values of the
catalytic acid (ca. 7) and catalytic base (ca. 5) and the two subsites
of the active site are key components of a kinetic model that accounts
for catalytic properties such as substrate specificities, inhibitor
binding, and influences of pH. |