The enzyme, originally isolated from the bacterium Bacillus halodurans C-125, releases the xylose unit at the reducing end of oligosaccharides ending with the structure beta-D-xylopyranosyl-(1->4)-beta-D-xylopyranosyl-(1->4)-beta-D-xylopyranose, leaving the new reducing end in the alpha configuration. It is specific for the beta anomers of xylooligosaccharides whose degree of polymerization is equal to or greater than 3.
The penultimate residue must be beta-D-xylopyranose, but replacing either of the flanking residues with glucose merely slows the rate greatly.
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REACTION
REACTION DIAGRAM
COMMENTARY
ORGANISM
UNIPROT
LITERATURE
Hydrolysis of (1->4)-beta-D-xylose residues from the reducing end of oligosaccharides
processivemodeof hydrolysis, overview. The xylooligosaccharides are progressively degraded from Xn to Xn-1 and then subsequently degraded in the same way to xylose and xylobiose
The enzyme, originally isolated from the bacterium Bacillus halodurans C-125, releases the xylose unit at the reducing end of oligosaccharides ending with the structure beta-D-xylopyranosyl-(1->4)-beta-D-xylopyranosyl-(1->4)-beta-D-xylopyranose, leaving the new reducing end in the alpha configuration. It is specific for the beta anomers of xylooligosaccharides whose degree of polymerization is equal to or greater than 3.
The penultimate residue must be beta-D-xylopyranose, but replacing either of the flanking residues with glucose merely slows the rate greatly.
docking analysis of Rex8A with methyl-glucuronic acid branched oligomers. Mixtures containing the aldouronic acids 4-O-methyl-beta-D-glucuronosyl-(1->2)-beta-D-xylopyranosyl-(1->4)-beta-D-xylopyranosyl-(1->4)-D-xylose (MeGlcA3Xyl3) (aldotetraouronic acid) and beta-D-xylopyranosyl-(1->4)-beta-D-xylopyranosyl-(1->4)-[4-O-methyl-alpha-D-glucuronosyl-(1->2)]-beta-D-xylopyranosyl-(1->4)-beta-D-xylopyranosyl-(1->4)-D-xylose (MeGlcA3Xyl5) (aldohexaouronic acid) are prepared from beechwood 4-O-methyl-D-glucuronoxylan by xylanase treatment. Aldotetraouronic acid MeGlcA3Xyl3 consists of a xylotriose with a methyl-glucuronic acid substituent in the third xylose from the reducing end. The cleavage of this substrate to xylose and to an aldouronic acid shortened by one residue indicates that Rex8A accommodates the methyl-glucuronic acid-substituted xylopyranosyl residue in the -2 subsite of the catalytic cleft of the enzyme. Ligand accommodation in the catalytic site, a xylotriose decorated with a 4-O-MeGlcA moiety at O2 of the third xylose from the reducing end (MeGlcA3Xyl3) is modeled into the active-site channel
the enzyme efficiently hydrolyzes xylooligosaccharides and shows minor activity on polymeric xylan. The enzyme shows also catalytic activity on branched xylooligosaccharides, i.e. the release of xylose from the reducing end. Hydrolysis products from oligosaccharides and xylan are analyzed by thin-layer chromatography (TLC) and MALDI TOF/TOF mass spectrometry. No activity with xylobiose, low activity with xylan
the enzyme efficiently hydrolyzes xylooligosaccharides and shows minor activity on polymeric xylan. The enzyme shows also catalytic activity on branched xylooligosaccharides, i.e. the release of xylose from the reducing end. Hydrolysis products from oligosaccharides and xylan are analyzed by thin-layer chromatography (TLC) and MALDI TOF/TOF mass spectrometry. No activity with xylobiose, low activity with xylan
the enzyme is involved in depolymerization of glucuronoxylan, a major component of the lignocellulosic substrates. Rex8A is a reducing-end xylose-releasing exo-oligoxylanase that efficiently hydrolyzes xylose from neutral and acidic xylooligosaccharides generated by the action of other xylanases also secreted by the strain. The hydrolytic ability of Rex8A on branched oligomers can be crucial for the complete depolymerization of highly substituted xylans, which is indispensable to accomplish biomass deconstruction and to generate efficient catalysts
modeling of the three-dimensional structure of Rex8A shows an (alpha/alpha)6 barrel fold where the loops connecting the alpha-helices contour the active site. These loops, which show high sequence diversity among GH8 enzymes, shape a catalytic cleft with a -2 subsite that can accommodate methyl-glucuronic acid decorations. Putative proton donor is Glu70 and catalytic base is Asp265. Residues Leu320, His321, and Pro322 form the loop structure. Structural molecular modeling of Rex8A
modeling of the three-dimensional structure of Rex8A shows an (alpha/alpha)6 barrel fold where the loops connecting the alpha-helices contour the active site. These loops, which show high sequence diversity among GH8 enzymes, shape a catalytic cleft with a -2 subsite that can accommodate methyl-glucuronic acid decorations. Putative proton donor is Glu70 and catalytic base is Asp265. Residues Leu320, His321, and Pro322 form the loop structure. Structural molecular modeling of Rex8A
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CLONED (Commentary)
ORGANISM
UNIPROT
LITERATURE
gene rex8A, DNA and amino acid sequence determination and analysis, sequence comparisons, recombinant expression of N-terminally His6-tagged wild-type and mutant enzymes in Escherichia coli strain BL21 Star (DE3)
The glycoside hydrolase family 8 reducing-end xylose-releasing exo-oligoxylanase Rex8A from Paenibacillus barcinonensis BP-23 is active on branched xylooligosaccharides