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PDBsum entry 1gx9
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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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The ligand-Binding site of bovine beta-Lactoglobulin: evidence for a function?
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Authors
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G.Kontopidis,
C.Holt,
L.Sawyer.
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Ref.
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J Mol Biol, 2002,
318,
1043-1055.
[DOI no: ]
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PubMed id
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Abstract
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Ever since the fortuitous observation that beta-lactoglobulin (beta-Lg), the
major whey protein in the milk of ruminants, bound retinol, the details of the
binding have been controversial. beta-Lg is a lipocalin, like plasma
retinol-binding protein, so that ligand association was expected to make use of
the central cavity in the protein. However, an early crystallographic analysis
and some of the more recent solution studies indicated binding elsewhere. We
have now determined the crystal structures of the complexes of the trigonal form
of beta-Lg at pH 7.5 with bound retinol (R=21.4% for 7329 reflections between 20
and 2.4 A resolution, R(free)=30.6%) and with bound retinoic acid (R=22.7% for
7813 reflections between 20 and 2.34 A resolution, R(free)=29.8%). Both ligands
are found to occupy the central calyx in a manner similar to retinol binding in
retinol-binding protein. We find no evidence of binding at the putative external
binding site in either of these structural analyses. Further, competition
between palmitic acid and retinol reveals only palmitate bound to the protein.
An explanation is provided for the lack of ligand binding to the orthorhombic
crystal form also obtained at pH 7.5. Finally, the possible function of beta-Lg
is discussed in the light of its species distribution and similarity to other
lipocalins.
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Figure 2.
Figure 2. A diagram of the binding site of bovine b-Lg
showing the contacts less than 3.9 Å made by retinol to
the protein. Notice that there are no obvious contacts of the
hydroxyl group to either Lys60 or Lys69, and that the only
hydrogen bond involving the ligand appears to be that to Glu62.
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Figure 4.
Figure 4. A cladogram derived from the amino acid sequences
of b-lactoglobulin, glycodelin and retinol-binding protein. The
alignment was carried out by Dr Andrew Coulson and TREEVIEW[58.]
was used to produce the diagram. All of the proteins used in the
alignment were full-length, mature (no signal sequence is
present) b-lactoglobulins with the exception of human
glycodelin, labelled Glycodelin, and the 71 amino acid fragment
from baboon endometrium, labelled Baboon fra. The sequences were
from the SwissProt databank [59.] unless otherwise noted: cow,
P02754; cow pseudo-gene; [48.] buffalo, P02755; goat, P02756;
goat pseudo-gene, Z47079; sheep, P02757; dolphin, B61590; pig,
P04119; dog, P33685; dog III, P33686; cat I, P33687; cat III,
P33688; donkey I, P13613; horse I, P02758; donkey II, P19647;
horse II, P07380; cat II, P21664; baboon, AF021261; glycodelin,
P09466; wallaby, Q29614; kangaroo, P11944; possum, Q29146. For
RBP: toad, P06172; chicken, P41263; rat, P27485; human, P02753;
horse P19, Q28388.
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The above figures are
reprinted
by permission from Elsevier:
J Mol Biol
(2002,
318,
1043-1055)
copyright 2002.
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