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<font size=4 face ="Arial">Background</font>
<font size=4 face ="Arial">Background</font>
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<font size=3>Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of ammonia and
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<font size=2>Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of ammonia and
glutamate to yield glutamine, ADP, and inorganic phosphate in the presence of divalent cations.
glutamate to yield glutamine, ADP, and inorganic phosphate in the presence of divalent cations.
The reaction occurs in two steps with γ-glutamyl phosphate as an intermediate and is used by
The reaction occurs in two steps with γ-glutamyl phosphate as an intermediate and is used by

Revision as of 04:27, 17 December 2008

ATP Binding and Important Residues of Glutamine Synthetase




Background
Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of ammonia and glutamate to yield glutamine, ADP, and inorganic phosphate in the presence of divalent cations. The reaction occurs in two steps with γ-glutamyl phosphate as an intermediate and is used by bacteria to introduce reduced nitrogen into cellular metabolism. GS is an enzyme of 12 identical subunits, arranged in two rings of 6, with the active site between each pair of subunits in a ring. GS contains two divalent cation sites (n1,n2) and one monovalent cation site per subunit.

Overall Reaction of Glutamine Synthetase
Glutamate + NH4+ + ATP --> glutamine + ADP + Pi


Overall Mechanism
ATP first binds to GS, then glutamate binds and attacks ATP to form γ-glutamyl phosphate and ADP. An ammounium ion binds to GS and then loses aproton to form the more active species ammonia, which attacks the γ-glutamyl phosphate to yield glutamine.

ATP binding site
ATP binds at the top of the active site cavity and the glutamate binds at the bottom, adjacent to the n1 ion (Liaw 1994). The movement of Arg 359 toward the glutamate site, induced by ATP binding, increases the binding affinity of glutamate. The active site of GS is located at the subunit interface (which contains n1 & n2) and is constituted mainly by the C domain of one subunit (Liaw 1995). (insert wiki below that maps the ATP binding)

Involving Residues
Most residues involved in enzymatic catalysis are located at the C domain but Asp50 is contributed from the N domain of the other subunit. The binding of ADP induces Asp50’ in order to enhance the ammonium binding, and then to deprotonate the ammonium ion to form the active ammonia to attack the gamma-glutamyl phosphate.

More Catalytic Residues(Modified from Liaw 1995)
Residue Role in enzymatic mechanism
Arg-321 Coordinates the carboxylate of glutamate
Glu-327 Closes active site and shields intermediate from hydrolysis
His-269 Coordinates the n2 ion
Glu-220 Coordinates the n1 ion
Asp-50 Increases the affinity for ammonium binding






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Proteopedia Page Contributors and Editors (what is this?)

Grace Natalie, Eran Hodis

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