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PDBsum entry 1qgn
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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 crystal structure of cystathionine gamma-Synthase from nicotiana tabacum reveals its substrate and reaction specificity.
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Authors
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C.Steegborn,
A.Messerschmidt,
B.Laber,
W.Streber,
R.Huber,
T.Clausen.
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Ref.
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J Mol Biol, 1999,
290,
983-996.
[DOI no: ]
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PubMed id
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Abstract
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Cystathionine gamma-synthase catalyses the committed step of de novo methionine
biosynthesis in micro-organisms and plants, making the enzyme an attractive
target for the design of new antibiotics and herbicides. The crystal structure
of cystathionine gamma-synthase from Nicotiana tabacum has been solved by
Patterson search techniques using the structure of Escherichia coli
cystathionine gamma-synthase. The model was refined at 2.9 A resolution to a
crystallographic R -factor of 20.1 % (Rfree25.0 %). The physiological substrates
of the enzyme, L-homoserine phosphate and L-cysteine, were modelled into the
unliganded structure. These complexes support the proposed ping-pong mechanism
for catalysis and illustrate the dissimilar substrate specificities of bacterial
and plant cystathionine gamma-synthases on a molecular level. The main
difference arises from the binding modes of the distal substrate groups (O
-acetyl/succinyl versusO -phosphate). Central in fixing the distal phosphate of
the plant CGS substrate is an exposed lysine residue that is strictly conserved
in plant cystathionine gamma-synthases whereas bacterial enzymes carry a glycine
residue at this position. General insight regarding the reaction specificity of
transsulphuration enzymes is gained by the comparison to cystathionine
beta-lyase from E. coli, indicating the mechanistic importance of a second
substrate binding site for L-cysteine which leads to different chemical reaction
types.
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Figure 4.
Figure 4. Modelled complex
between tCGS and its substrate
HSP. The stereo plot shows the
active site of tCGS with the mod-
elled external aldimine between
HSP and PLP. The crystal structure
(blue) with the manually positioned
ligand molecule (yellow) is overlaid
with the minimised models (ligand:
grey; protein: green). The
Figure was produced with SETOR
(Evans, 1993).
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Figure 7.
Figure 7. Schematic drawing
illustrating the reaction mechanism
proposed for tCGS. After formation
of the Michaelis complex (I), trans-
aldimination leads to an external
aldimine (II); via a carbanionic
intermediate (III), a PLP substrate
ketimine (IV) is formed. After
release of the phosphate leaving
group from an a-b-unsaturated
intermediate (V), cysteine enters
the active site and reacts at C
g
of
the partitioning intermediate (qui-
ninoid form of PLP-bound vinyl-
glycine, VI). The resulting a-b-
unsaturated intermediate (VII) is
protonated to form the PLP pro-
duct ketimine (VIII). Finally, the
product PLP aldimine results from
protonation of a carbanionic inter-
mediate (IX) by the active site
lysine.
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The above figures are
reprinted
by permission from Elsevier:
J Mol Biol
(1999,
290,
983-996)
copyright 1999.
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