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Title
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Homologous xylanases from Clostridium thermocellum: evidence for bi-functional activity, synergism between xylanase catalytic modules and the presence of xylan-binding domains in enzyme complexes.
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Authors
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A.C.Fernandes,
C.M.Fontes,
H.J.Gilbert,
G.P.Hazlewood,
T.H.Fernandes,
L.M.Ferreira.
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Ref.
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Biochem J, 1999,
342,
105-110.
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PubMed id
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Abstract
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Clostridium thermocellum produces a consortium of plant-cell-wall hydrolases
that form a cell-bound multi-enzyme complex called the cellulosome. In the
present study two similar xylanase genes, xynU and xynV, were cloned from C.
thermocellum strain YS and sequenced. The deduced primary structures of both
xylanases, xylanase U (XylU) and xylanase V (XylV), were homologous with the
previously characterized xylanases from C. thermocellum strain F1. Truncated
derivatives of XylV were produced and their biochemical properties were
characterized. The xylanases were shown to be remarkably thermostable and
resistant to proteolytic inactivation. The catalytic domains hydrolysed xylan by
a typical endo-mode of action. The type VI cellulose-binding domain (CBD)
homologue of XylV bound xylan and, to a smaller extent, Avicel and acid-swollen
cellulose. Deletion of the CBD from XylV abolished the capacity of the enzymes
to bind polysaccharides. The polysaccharide-binding domain was shown to have a
key role in the hydrolysis of insoluble substrates by XylV. The C-terminal
domain of XylV, which is absent from XylU, removed acetyl groups from acetylated
xylan and acted in synergy with the glycosyl hydrolase catalytic domain of the
enzyme to elicit the hydrolysis of acetylated xylan.
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