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PDBsum entry 2cc0
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References listed in PDB file
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Key reference
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Title
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Structure and activity of two metal ion-Dependent acetylxylan esterases involved in plant cell wall degradation reveals a close similarity to peptidoglycan deacetylases.
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Authors
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E.J.Taylor,
T.M.Gloster,
J.P.Turkenburg,
F.Vincent,
A.M.Brzozowski,
C.Dupont,
F.Shareck,
M.S.Centeno,
J.A.Prates,
V.Puchart,
L.M.Ferreira,
C.M.Fontes,
P.Biely,
G.J.Davies.
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Ref.
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J Biol Chem, 2006,
281,
10968-10975.
[DOI no: ]
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PubMed id
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Abstract
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The enzymatic degradation of plant cell wall xylan requires the concerted action
of a diverse enzymatic syndicate. Among these enzymes are xylan esterases, which
hydrolyze the O-acetyl substituents, primarily at the O-2 position of the xylan
backbone. All acetylxylan esterase structures described previously display a
alpha/beta hydrolase fold with a "Ser-His-Asp" catalytic triad. Here
we report the structures of two distinct acetylxylan esterases, those from
Streptomyces lividans and Clostridium thermocellum, in native and complex forms,
with x-ray data to between 1.6 and 1.0 A resolution. We show, using a novel
linked assay system with PNP-2-O-acetylxyloside and a beta-xylosidase, that the
enzymes are sugar-specific and metal ion-dependent and possess a single metal
center with a chemical preference for Co2+. Asp and His side chains complete the
catalytic machinery. Different metal ion preferences for the two enzymes may
reflect the surprising diversity with which the metal ion coordinates residues
and ligands in the active center environment of the S. lividans and C.
thermocellum enzymes. These "CE4" esterases involved in plant cell
wall degradation are shown to be closely related to the de-N-acetylases involved
in chitin and peptidoglycan degradation (Blair, D. E., Schuettelkopf, A. W.,
MacRae, J. I., and Aalten, D. M. (2005) Proc. Natl. Acad. Sci. U. S. A., 102,
15429-15434), which form the NodB deacetylase "superfamily."
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Figure 4.
FIGURE 4. Comparison of the S. lividans acetylxylan
esterase, SlCE4, and the peptidoglycan deacetylase from S.
pneumonia. A, ribbon representation of SlCE4 (green) overlapped
with the peptidoglycan de-N-acetylase from S. pneumonia
(purple). Active site residues and acetate molecules (gray) are
shown in ball-and-stick representation and Zn^2+ ions as
spheres. B, divergent stereo ball-and-stick representation of
SlCE4 active site residues (green) overlapped with those from
the peptidoglycan de-N-acetylase from S. pneumonia (purple and
labeled); the acetate molecules are shown in green/gray. Zn^2+
ions (cyan) and water molecules are shown as spheres. Figures
were made using MOLSCRIPT (44, 45) and rendered using RASTER3D
(46).
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Figure 7.
FIGURE 7. Active site interactions and catalysis in family
CE4 acetylxylan esterases. Schematic diagram of the metal ion
coordination of the S. lividans family CE4 esterase (acetate
shown in red)(A) and the C. thermocellum CE4 esterase (B) and a
putative reaction mechanism (C) based upon classical Zn^2+
hydrolase chemistry and the work of van Aalten and colleagues
(19) on the streptococcal peptidoglycan deacetylases. The
divalent metal plays the role of Lewis acid, with Asp and His
residues playing the roles of catalytic base (activating the
nucleophilic water) and acid (aiding sugar departure).
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2006,
281,
10968-10975)
copyright 2006.
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