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PDBsum entry 2gwd
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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 for the redox control of plant glutamate cysteine ligase.
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Authors
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M.Hothorn,
A.Wachter,
R.Gromes,
T.Stuwe,
T.Rausch,
K.Scheffzek.
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Ref.
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J Biol Chem, 2006,
281,
27557-27565.
[DOI no: ]
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PubMed id
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Abstract
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Glutathione (GSH) plays a crucial role in plant metabolism and stress response.
The rate-limiting step in the biosynthesis of GSH is catalyzed by glutamate
cysteine ligase (GCL) the activity of which is tightly regulated. The regulation
of plant GCLs is poorly understood. The crystal structure of substrate-bound GCL
from Brassica juncea at 2.1-A resolution reveals a plant-unique regulatory
mechanism based on two intramolecular redox-sensitive disulfide bonds. Reduction
of one disulfide bond allows a beta-hairpin motif to shield the active site of
B. juncea GCL, thereby preventing the access of substrates. Reduction of the
second disulfide bond reversibly controls dimer to monomer transition of B.
juncea GCL that is associated with a significant inactivation of the enzyme.
These regulatory events provide a molecular link between high GSH levels in the
plant cell and associated down-regulation of its biosynthesis. Furthermore,
known mutations in the Arabidopsis GCL gene affect residues in the close
proximity of the active site and thus explain the decreased GSH levels in mutant
plants. In particular, the mutation in rax1-1 plants causes impaired binding of
cysteine.
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Figure 1.
FIGURE 1. Plant GCL shows unique structural features. Front
and side views of BjGCL shown in ribbon representation. The
central  -sheet is depicted in
dark blue, the N- and C-terminal helical regions in light blue,
and the plant unique arms in dark and light green, respectively.
The L-glutamate bound in the active site is represented in bond
representation along with the Mg^2+ ion (in cyan). The two
disulfide bridges CC1 and CC2 are highlighted in yellow; the
-hairpin module is shown
in red.
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Figure 2.
FIGURE 2. Substrate binding in plant GCL. A, close-up view
of the glutamate binding site with the inhibitor BSO (in yellow;
sulfur depicted in magenta) in bond representation and including
the final 2F[obs] - F[calc] electron density map contoured at
1.5  . Residues reaching
from the central -sheet (in blue) to
coordinate the Mg^2+ ion (in cyan) are depicted in blue.
Residues contributed by the helical arms are shown in light
green. B, schematic representation of the inhibitor BSO binding
to BjGCL. The LigPlot diagram (50) summarizes key interactions
between the BSO ligand and active site residues. Yellow lines,
BSO ligand; green lines, BjGCL residues; semicircles with
radiating lines; atoms or residues involved in hydrophobic
contacts between protein and ligand. C, stereo close-up view of
the plant GCL cysteine binding pocket formed by mostly
hydrophobic residues (in blue) around the aliphatic side chain
of BSO (in light gray). The corresponding secondary structure
elements and residues in E. coli GCL (PDB-ID: 1VA6) are shown in
orange. D, known mutations in the Arabidopsis GCL gene are in
proximity of the substrate binding sites in plant GCL. BjGCL in
ribbon representation is shown with BSO and ADP (modeled) in
bonds representation (in yellow). Small spheres indicate the
positions of residues affected in AtGCL mutant plants (in
magenta). Enlarged versions provide models on how the affected
residues in rax1-1 and rml1 mutants may interact with GCL
substrates. The rax1-1 arginine residue (Arg^220) is shown in a
modeled rotamer configuration bringing its guanidinium group in
close proximity to the terminal methyl of BSO that corresponds
to the sulfhydryl group of cysteine (in green). Interactions are
highlighted by dotted lines (in magenta).
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2006,
281,
27557-27565)
copyright 2006.
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