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PDBsum entry 2i4b
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Transport protein
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PDB id
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2i4b
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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 structure of a cyanobacterial bicarbonate transport protein, Cmpa.
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Authors
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N.M.Koropatkin,
D.W.Koppenaal,
H.B.Pakrasi,
T.J.Smith.
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Ref.
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J Biol Chem, 2007,
282,
2606-2614.
[DOI no: ]
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PubMed id
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Abstract
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Cyanobacteria, blue-green algae, are the most abundant autotrophs in aquatic
environments and form the base of the food chain by fixing carbon and nitrogen
into cellular biomass. To compensate for the low selectivity of Rubisco for CO2
over O2, cyanobacteria have developed highly efficient CO2-concentrating
machinery of which the ABC transport system CmpABCD from Synechocystis PCC 6803
is one component. Here, we have described the structure of the
bicarbonate-binding protein CmpA in the absence and presence of bicarbonate and
carbonic acid. CmpA is highly homologous to the nitrate transport protein NrtA.
CmpA binds carbonic acid at the entrance to the ligand-binding pocket, whereas
bicarbonate binds in nearly an identical location compared with nitrate binding
to NrtA. Unexpectedly, bicarbonate binding is accompanied by a metal ion,
identified as Ca2+ via inductively coupled plasma optical emission spectrometry.
The binding of bicarbonate and metal appears to be highly cooperative and
suggests that CmpA may co-transport bicarbonate and calcium or that calcium acts
a cofactor in bicarbonate transport.
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Figure 1.
FIGURE 1. Schematic representation of the assembled CmpABCD
bicarbonate transporter. CmpA is tethered to the periplasmic
membrane by a flexible linker and captures bicarbonate in the
periplasm for delivery to the transmembrane complex created by
CmpB. In many ABC transporters, the transmembrane pore is
created by a dimer of two transmembrane-spanning polypeptides.
CmpC and CmpD are ATPases that couple ATP hydrolysis to
bicarbonate transport through the pore. CmpC is unique in that
it contains a C-terminal solute-binding domain homologous to
CmpA. SBP, solute-binding protein.
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Figure 2.
FIGURE 2. The structure of CmpA. a, stereo ribbon diagram
of the CmpA crystal structure at pH 5.0 with carbonic acid. The
protein is gradiently colored blue to red as the chain extends
from the N to the C terminus. The view is of the front of the
C-clamp, which opens to the ligand-binding cleft. Carbonic acid
is depicted as spheres. b, stereo ribbon diagram of the CmpA
crystal structure at pH 8.0 with Ca^2+ and bicarbonate, with the
identical view and coloring as in a. Bicarbonate and Ca^2+ are
depicted as spheres. Note the difference in the position of
carbonic acid and bicarbonate in the ligand-binding cleft.
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2007,
282,
2606-2614)
copyright 2007.
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