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PDBsum entry 2o7u
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Metal transport
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PDB id
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2o7u
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References listed in PDB file
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Key reference
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Title
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Structures of two mutants that probe the role in iron release of the dilysine pair in the n-Lobe of human transferrin.
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Authors
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H.M.Baker,
D.Nurizzo,
A.B.Mason,
E.N.Baker.
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Ref.
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Acta Crystallogr D Biol Crystallogr, 2007,
63,
408-414.
[DOI no: ]
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PubMed id
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Abstract
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Iron uptake by humans depends on the ability of the serum protein transferrin
(Tf) to bind iron as Fe(3+) with high affinity but reversibly. Iron release into
cells occurs through receptor-mediated endocytosis, aided by the lower endosomal
pH of about 5.5. The protonation of a hydrogen-bonded pair of lysines, Lys206
and Lys296, adjacent to the N-lobe iron site of Tf has been proposed to create a
repulsive interaction that stimulates domain opening and iron release. The
crystal structures of two mutants, K206E (in which Lys206 is mutated to Glu) and
K206E/K296E (in which both lysines are mutated to Glu), have been determined.
The K206E structure (2.6 A resolution; R = 0.213, R(free) = 0.269) shows that a
salt bridge is formed between Glu206 and Lys296, thus explaining the drastically
slower iron release by this mutant. The K206E/K296E double-mutant structure (2.8
A resolution; R = 0.232, R(free) = 0.259) shows that the Glu296 side chain moves
away from Glu206, easing any repulsive interaction and instead interacting with
the iron ligand His249. The evident conformational flexibility is consistent
with an alternative model for the operation of the dilysine pair in iron release
in which it facilitates concerted proton transfer to the tyrosine ligand Tyr188
as one step in the weakening of iron binding.
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Figure 1.
Figure 1 Dilysine interaction in the N-lobe of transferrin. (a)
Ribbon diagram showing the approach of Lys296 from domain N1
(lower) to Lys206 from domain N2 (upper) adjacent to the bound
Fe atom (red sphere). (b) Interactions made by the  -amino
group of Lys296: hydrogen bonds (broken lines) to Lys206 N^  ,
Tyr188 O^  and
the aromatic ring of Tyr95. This and the other figures were
prepared using PyMOL (DeLano, 2002[DeLano, W. L. (2002). The
PyMOL Molecular Graphics System. http://www.pymol.org/ .]).
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Figure 3.
Figure 3 Mutation site in the K206E/K296E double mutant. (a)
Bias-removed OMIT density (F[o] - F[c], contoured at 3  )
for the mutated residues. (b) Side chains of Glu206 and Glu296
in the mutant structure, shown in gold, superimposed on Lys206
and Lys296 in the wild-type structure, shown in silver. Other
residues common to both proteins are shown in green with atoms
coloured by type. Hydrogen bonds are shown as broken lines and
covalent metal-ligand bonds as solid black lines.
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The above figures are
reprinted
by permission from the IUCr:
Acta Crystallogr D Biol Crystallogr
(2007,
63,
408-414)
copyright 2007.
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