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PDBsum entry 2c4x
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References listed in PDB file
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Key reference
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Title
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Xyloglucan is recognized by carbohydrate-Binding modules that interact with beta-Glucan chains.
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Authors
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S.Najmudin,
C.I.Guerreiro,
A.L.Carvalho,
J.A.Prates,
M.A.Correia,
V.D.Alves,
L.M.Ferreira,
M.J.Romão,
H.J.Gilbert,
D.N.Bolam,
C.M.Fontes.
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Ref.
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J Biol Chem, 2006,
281,
8815-8828.
[DOI no: ]
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PubMed id
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Abstract
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Enzyme systems that attack the plant cell wall contain noncatalytic
carbohydrate-binding modules (CBMs) that mediate attachment to this composite
structure and play a pivotal role in maximizing the hydrolytic process. Although
xyloglucan, which includes a backbone of beta-1,4-glucan decorated primarily
with xylose residues, is a key component of the plant cell wall, CBMs that bind
to this polymer have not been identified. Here we showed that the C-terminal
domain of the modular Clostridium thermocellum enzyme CtCel9D-Cel44A (formerly
known as CelJ) comprises a novel CBM (designated CBM44) that binds with equal
affinity to cellulose and xyloglucan. We also showed that accommodation of
xyloglucan side chains is a general feature of CBMs that bind to single
cellulose chains. The crystal structures of CBM44 and the other CBM (CBM30) in
CtCel9D-Cel44A display a beta-sandwich fold. The concave face of both CBMs
contains a hydrophobic platform comprising three tryptophan residues that can
accommodate up to five glucose residues. The orientation of these aromatic
residues is such that the bound ligand would adopt the twisted conformation
displayed by cello-oligosaccharides in solution. Mutagenesis studies confirmed
that the hydrophobic platform located on the concave face of both CBMs mediates
ligand recognition. In contrast to other CBMs that bind to single polysaccharide
chains, the polar residues in the binding cleft of CBM44 play only a minor role
in ligand recognition. The mechanism by which these proteins are able to
recognize linear and decorated beta-1,4-glucans is discussed based on the
structures of CBM44 and the other CBMs that bind single cellulose chains.
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Figure 5.
FIGURE 5. The three-dimensional structure and the
hydrophobic platform of PKD-CBM44. The overall structure of the
PKD-CBM44 highlighting the secondary structural elements, with
the  -strands of each -sheet
colored the same as is shown in a. The Trps in the binding cleft
are depicted as sticks, and the calcium atoms depicted as red
spheres. The hydrogen bond between the OH of Ser^92 and carbonyl
O of Thr^94 in the linker region is shown in cyan. Note -strand 2
of PKD has a kink at residues 29/30. b, comparison of the
calcium-binding sites Ca1 in the PKD and Ca2 in the CBM44
domains and their corresponding coordinating amino acid
residues. The electron density was contoured at 2  . c
depicts a top down view of the CBM44 binding cleft showing the
amino acids (as sticks) that are in the vicinity of the three
tryptophan residues (as balls and sticks) that comprise the
hydrophobic platform. All the ribbon figures in Figs. 5, 6, and
8 were prepared using MOLSCRIPT (58) and RASTER3D (59) and the
electron density figures with TURBO-FRODO (60).
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Figure 8.
FIGURE 8. Superpositioning of CBM30 and CBM29 on CBM44.
CBM44 is shown in blue, CBM30 in green and CBM29 (Protein Data
Bank code 1gwm) in sky blue trace, with the aromatics (Trp^189,
Trp^194, and Trp^198 for CBM44; Trp^27, Trp^68, and Trp^78 for
CBM30; and Trp^24, Trp^26, and Tyr^46 for CBM29) as ball and
sticks.
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2006,
281,
8815-8828)
copyright 2006.
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Secondary reference #1
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Title
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Overexpression, Purification and crystallization of the two c-Terminal domains of the bifunctional cellulase ctcel9d-Cel44a from clostridium thermocellum.
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Authors
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S.Najmudin,
C.I.Guerreiro,
L.M.Ferreira,
M.J.Romão,
C.M.Fontes,
J.A.Prates.
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Ref.
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Acta Crystallograph Sect F Struct Biol Cryst Commun, 2005,
61,
1043-1045.
[DOI no: ]
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PubMed id
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Note: In the PDB file this reference is
annotated as "TO BE PUBLISHED". The citation details given above have
been manually determined.
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Figure 2.
Figure 2 Crystals of PKD-CBM44 protein obtained by hanging-drop
vapour diffusion in the presence of 0.2 M CaCl[2], 0.1 M
sodium acetate pH 4.5 and 30% ethanol with 6% glycerol as
additive. The largest crystals are approximately 0.1 × 0.1
× 0.2 mm in size.
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The above figure is
reproduced from the cited reference
which is an Open Access publication published by the IUCr
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