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Glutamine synthetase (GS) () is an enzyme that plays an essential role in the metabolism of nitrogen by catalyzing the condensation of glutamate and ammonia to form glutamine: Glutamate + ATP + NH3 → Glutamine + ADP + phosphate Glutamine Synthetase uses ammonia produced by nitrate reduction, amino acid degradation, and photorespiration. The amide group of glutamate is a nitrogen source for the synthesis of glutamine pathway metabolites. Other reactions may take place via GS. Competition between ammonium ion and water, their binding affinities, and the concentration of ammonium ion, influences glutamine synthesis and glutamine hydrolysis. Glutamine is formed if an ammonium ion attacks the acyl-phosphate intermediate, while glutamate is remade if water attacks the intermediate. Ammonium ion binds more strongly than water to GS due to electrostatic forces between a cation and a negatively charged pocket.〔 Another possible reaction is upon NH2OH binding to GS, rather than NH4+, yields γ-glutamylhydroxamate.〔〔 == Structure == Glutamine Synthetase can be composed of 8, 10, or 12 identical subunits separated into two face-to-face rings.〔 Bacterial GS are dodecamers with 12 active sites between each monomer.〔 Each active site creates a ‘bifunnel’ which is the site of three distinct substrate binding sites: nucleotide, ammonium ion, and amino acid.〔〔〔 ATP binds to the top of the bifunnel that opens to the external surface of GS.〔 Glutamate binds at the bottom of the active site.〔 The middle of the bifunnel contains two sites in which divalent cations bind (Mn+2 or Mg+2). One cation binding site is involved in phosphoryl transfer of ATP to glutamate, while the second stabilizes active GS and helps with the binding of glutamate.〔 Hydrogen bonding and hydrophobic interactions hold the two rings of GS together. Each subunit possesses a C-terminus and an N-terminus in its sequence. The C-terminus (helical thong) stabilizes the GS structure by inserting into the hydrophobic region of the subunit across in the other ring. The N-terminus is exposed to the solvent. In addition, the central channel is formed via six four-stranded β-sheets composed of anti-parallel loops from the twelve subunits.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「glutamine synthetase」の詳細全文を読む スポンサード リンク
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