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PDBsum entry 1nux
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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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Metaphosphate in the active site of fructose-1,6-Bisphosphatase.
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Authors
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J.Y.Choe,
C.V.Iancu,
H.J.Fromm,
R.B.Honzatko.
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Ref.
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J Biol Chem, 2003,
278,
16015-16020.
[DOI no: ]
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PubMed id
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Abstract
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The hydrolysis of a phosphate ester can proceed through an intermediate of
metaphosphate (dissociative mechanism) or through a trigonal bipryamidal
transition state (associative mechanism). Model systems in solution support the
dissociative pathway, whereas most enzymologists favor an associative mechanism
for enzyme-catalyzed reactions. Crystals of fructose-1,6-bisphosphatase grow
from an equilibrium mixture of substrates and products at near atomic resolution
(1.3 A). At neutral pH, products of the reaction (orthophosphate and fructose
6-phosphate) bind to the active site in a manner consistent with an associative
reaction pathway; however, in the presence of inhibitory concentrations of K+
(200 mm), or at pH 9.6, metaphosphate and water (or OH-) are in equilibrium with
orthophosphate. Furthermore, one of the magnesium cations in the pH 9.6 complex
resides in an alternative position, and suggests the possibility of metal cation
migration as the 1-phosphoryl group of the substrate undergoes hydrolysis. To
the best of our knowledge, the crystal structures reported here represent the
first direct observation of metaphosphate in a condensed phase and may provide
the structural basis for fundamental changes in the catalytic mechanism of
fructose-1,6-bisphosphatase in response to pH and different metal cation
activators.
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Figure 1.
Fig. 1. Stereoviews of FBPase active sites. A, complex of
Mg2+, F6P, and orthophosphate at pH 7 (control complex). B,
complex of Mg2+, K+/Zn2+, F6P, and metaphosphate at pH 7 (high
K+ complex). C, complex of Mg2+, F6P, and metaphosphate at pH
9.6 (high pH complex). Electron density (blue) covers
metaphosphate/hydroxide anions or orthophosphate at a contour
level of 3  , using a
cutoff radius of 1 Å. Anomalous difference density (red)
covers site M1 at a contour level of 3 , using a
cutoff radius of 1 Å. MOLSCRIPT (39) and RASTER3D (35)
were used for the illustration.
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Figure 2.
Fig. 2. Distance relations between selected atoms in the
active site. A, control complex. B, high K+ complex. C, high pH
complex. The dotted outline in panel C represents a channel of
electron density that extends between metal sites 3 and 4, which
in the refined model is represented by a discrete set of water
molecules and magnesium cations.
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2003,
278,
16015-16020)
copyright 2003.
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