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PDBsum entry 1x2a
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References listed in PDB file
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Key reference
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Title
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Binding of c5-Dicarboxylic substrate to aspartate aminotransferase: implications for the conformational change at the transaldimination step.
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Authors
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M.M.Islam,
M.Goto,
I.Miyahara,
H.Ikushiro,
K.Hirotsu,
H.Hayashi.
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Ref.
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Biochemistry, 2005,
44,
8218-8229.
[DOI no: ]
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PubMed id
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Abstract
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The mechanism for the reaction of aspartate aminotransferase with the C4
substrate, l-aspartate, has been well established. The binding of the C4
substrate induces conformational change in the enzyme from the open to the
closed form, and the entire reaction proceeds in the closed form of the enzyme.
On the contrary, little is known about the reaction with the C5 substrate,
l-glutamate. In this study, we analyzed the pH-dependent binding of
2-methyl-l-glutamate to the enzyme and showed that the interaction between the
amino group of 2-methyl-l-glutamate and the pyridoxal 5'-phosphate aldimine is
weak compared to that between 2-methyl-l-aspartate and the aldimine. The
structures of the Michaelis complexes of the enzyme with l-aspartate and
l-glutamate were modeled on the basis of the maleate and glutarate complex
structures of the enzyme. The result showed that l-glutamate binds to the open
form of the enzyme in an extended conformation, and its alpha-amino group points
in the opposite direction of the aldimine, while that of l-aspartate is close to
the aldimine. These models explain the observations for 2-methyl-l-glutamate and
2-methyl-l-aspartate. The crystal structures of the complexes of aspartate
aminotransferase with phosphopyridoxyl derivatives of l-glutamate, d-glutamate,
and 2-methyl-l-glutamate were solved as the models for the external aldimine and
ketimine complexes of l-glutamate. All the structures were in the closed form,
and the two carboxylate groups and the arginine residues binding them are
superimposable on the external aldimine complex with 2-methyl-l-aspartate.
Taking these facts altogether, it was strongly suggested that the binding of
l-glutamate to aspartate aminotransferase to form the Michaelis complex does not
induce a conformational change in the enzyme, and that the conformational change
to the closed form occurs during the transaldimination step. The hydrophobic
residues of the entrance of the active site, including Tyr70, are considered to
be important for promoting the transaldimination process and hence the
recognition of the C5 substrate.
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