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PDBsum entry 1fqb
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Sugar binding protein
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PDB id
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1fqb
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Contents |
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* Residue conservation analysis
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References listed in PDB file
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Key reference
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Title
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Crystal structures of the maltodextrin/maltose-Binding protein complexed with reduced oligosaccharides: flexibility of tertiary structure and ligand binding.
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Authors
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X.Duan,
J.A.Hall,
H.Nikaido,
F.A.Quiocho.
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Ref.
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J Mol Biol, 2001,
306,
1115-1126.
[DOI no: ]
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PubMed id
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Abstract
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The structure of the maltodextrin or maltose-binding protein, an initial
receptor for bacterial ABC-type active transport and chemotaxis, consists of two
globular domains that are separated by a groove wherein the ligand is bound and
enclosed by an inter-domain rotation. Here, we report the determination of the
crystal structures of the protein complexed with reduced maltooligosaccharides
(maltotriitol and maltotetraitol) in both the "closed" and
"open" forms. Although these modified sugars bind to the receptor,
they are not transported by the wild-type transporter. In the closed structures,
the reduced sugars are buried in the groove and bound by both domains, one
domain mainly by hydrogen-bonding interactions and the other domain primarily by
non-polar interactions with aromatic side-chains. In the open structures, which
abrogate both cellular activities of active transport and chemotaxis because of
the large separation between the two domains, the sugars are bound almost
exclusively to the domain rich in aromatic residues. The binding site for the
open chain glucitol residue extends to a subsite that is distinct from those for
the glucose residues that were uncovered in prior structural studies of the
binding of active linear maltooligosaccharides. Occupation of this subsite may
also account for the inability of the reduced oligosaccharides to be
transported. The structures reported here, combined with those previously
determined for several other complexes with active oligosaccharides in the
closed form and with cyclodextrin in the open form, revealed at least four
distinct modes of ligand binding but with only one being functionally active.
This versatility reflects the flexibility of the protein, from very large
motions of interdomain rotation to more localized side-chain conformational
changes, and adaptation by the oligosaccharides as well.
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Figure 2.
Figure 2. Binding of maltotriitol to the MBP closed form in
the C2 space group crystal. The sugar residues are labeled -G1
for the glucitol, followed by G1, G2, and G3 for the glucose
residues. (a) Difference (F[o] -F[c]) omit electron density map
(blue) at 2.3 Å resolution of the maltotriitol contoured
at 2s level. (b) Stereo view of the hydrogen bonds (  slant
3.4 Å) (broken lines) and stacking interactions between
the maltotriitol and MBP. With the exception of E111 and E153,
all the polar residues (blue color) that are involved in
hydrogen-bonding interactions originate from domain I. E111 and
E153 (red color) are located in domain II and the hinge
connecting the two domains, respectively. The aromatic residues
(colored green) originate from domain I (W62) and domain II
(W230, Y155, W340).
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Figure 3.
Figure 3. The binding site of MBP with bound maltotetraitol
in the C2 or closed structure. The identifications of the amino
acid residues and the sugar residues are identical with those
shown in Figure 2. The additional Tyr341 residue originates from
domain II. (a) The 2.3 Å difference electron density map
of maltotetraitol patterned after Figure 2(a). (b) Stereo view
of the hydrogen bonds ( slant
3.4 Å) (broken lines) and stacking interactions.
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The above figures are
reprinted
by permission from Elsevier:
J Mol Biol
(2001,
306,
1115-1126)
copyright 2001.
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