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PDBsum entry 1hk5
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Plasma protein
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PDB id
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1hk5
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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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Structural basis of albumin-Thyroxine interactions and familial dysalbuminemic hyperthyroxinemia.
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Authors
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I.Petitpas,
C.E.Petersen,
C.E.Ha,
A.A.Bhattacharya,
P.A.Zunszain,
J.Ghuman,
N.V.Bhagavan,
S.Curry.
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Ref.
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Proc Natl Acad Sci U S A, 2003,
100,
6440-6445.
[DOI no: ]
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PubMed id
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Abstract
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Human serum albumin (HSA) is the major protein component of blood plasma and
serves as a transporter for thyroxine and other hydrophobic compounds such as
fatty acids and bilirubin. We report here a structural characterization of
HSA-thyroxine interactions. Using crystallographic analyses we have identified
four binding sites for thyroxine on HSA distributed in subdomains IIA, IIIA, and
IIIB. Mutation of residue R218 within subdomain IIA greatly enhances the
affinity for thyroxine and causes the elevated serum thyroxine levels associated
with familial dysalbuminemic hyperthyroxinemia (FDH). Structural analysis of two
FDH mutants of HSA (R218H and R218P) shows that this effect arises because
substitution of R218, which contacts the hormone bound in subdomain IIA,
produces localized conformational changes to relax steric restrictions on
thyroxine binding at this site. We have also found that, although fatty acid
binding competes with thyroxine at all four sites, it induces conformational
changes that create a fifth hormone-binding site in the cleft between domains I
and III, at least 9 A from R218. These structural observations are consistent
with binding data showing that HSA retains a high-affinity site for thyroxine in
the presence of excess fatty acid that is insensitive to FDH mutations.
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Figure 1.
Fig. 1. Comparison of the structures of HSA-T[4] and (a)
HSA-myristate-T[4] (b). The protein secondary structure is shown
schematically with the subdomains color-coded as follows: IA,
red; IB, light red; IIA, green; IIB, light-green; IIIA, blue;
IIIB, light blue. This color scheme is maintained throughout.
Ligands are shown in a space-filling representation, colored by
atom type: carbon (fatty acid), gray; carbon (T[4]), brown;
nitrogen, blue; oxygen, red; iodine, magenta. T[4]-binding sites
are labeled Tr1-Tr5; fatty acid-binding sites are labeled
FA1-FA7. Except where stated otherwise, molecular graphics were
prepared by using BOBSCRIPT (40) and RASTER3D (41). (c)An F[obs]
- F[calc] simulated annealing omit map (29) contoured at 3  for T[4]
bound to site Tr1 in subdomain IIA of the R218P mutant. Selected
amino acid side chains are colored by atom type. Hydrogen bonds
are indicated by dashed orange lines. (d) Schematic structure of
T[4] hormone, indicating key nomenclature.
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Figure 2.
Fig. 2. Comparison of the bound configurations of T[4] in
HSA. (a) Site Tr1 in subdomain IIA. There is only very weak
electron density for the side chain of R218 because variation in
the position of this residue and atoms beyond C  were
omitted from the refined model; the position shown in the figure
is indicative only but is consistent with the location of the
main-chain atoms and steric constraints imposed by neighboring
residues and the T[4] ligand. (b) Site Tr2 in subdomain IIIA.
(c) Sites Tr3 and Tr4 in subdomain IIIB (from the R218H mutant
structure, because this model shows the least disorder of the
C-terminal helix). (d) Site Tr5 in the interdomain cleft of the
HSA-myristate complex. Figures were prepared by using MOLSCRIPT
(41) and PYMOL (42). Selected amino acid side chains are shown
colored by atom type. The van der Waals surface of the ligand is
represented by a semitransparent magenta surface. Hydrogen bonds
are indicated by dashed orange lines.
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