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From Proteopedia

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 ^[1] . 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 a dodecamer formed from two face-to-face hexameric rings of subunits, with 12 active sites formed between monomers ^[2] .

Overall Mechanism
The first step is the formation of the activated intermediate γ-glutamyl phosphate. The n2 ion coordinates the phosphate oxygens of ATP to allow phosphoryl transfer to the γ-carboxylate group of glutatmate, yielding the intermediate ^[3] . The second step is the attack on the intermediate by ammonia therefore releasing free phosphate to yield glutamine.
ATP binding site
Each active site of GS is described as a 'bifunnel in which ATP and glutamate bind at opposite ends. The ATP binding site is referred to as the top of the bifunnel because it opens to the external 6-fold surface of GS (figure below) ^[3]. At the the joint of the are two cation binding sites, n1 and n2, where either magnesium or manganese bind for catalysis. The n2 ion is involved in the phosphroyl transfer, while the n1 ion stabilizes an active GS and plays a role in binding glutamate ^[3] .
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 . Both the N-terminus and C-terminus of each subunit are helical. The N-terminal helix sits above the hexameric ring and is exposed to solvent ^[3] . The C-terminal hexlix (helical thong) is inserted into the hydrophobic hole in the subunit opposite hexameric ring ^[3] . The movement of Asp-50 aids in the formation of the ammonium binding site, and the movement of Arg-339 assist phosphoryl transfer and P_i binding. .

D. Eisneberg et al / Biochimica et Biophysica Acta 1477 (2000) 124

More Catalytic Residues^[1]

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

References

1. ↑ ^{1.0 1.1} Liaw, S-H, et.al.,Discovery of the ammonium substrate site on glutamine synthetase, a third cation binding site Protein Sci. 1995 4: 2358-2365
2. ↑ Gill, H & Eisenberg, D., Biochemistry 2001 40: 1903-1912
3. ↑ ^{3.0 3.1 3.2 3.3 3.4} D. Eisneberg et al / Biochimica et Biophysica Acta 1477 (2000) 124