Mechanism
GS catalyzes the ATP-dependent condensation of glutamate with ammonia to yield glutamine. The hydrolysis of ATP drives the first step of a two-part, concerted mechanism. ATP phosphorylates glutamate to form ADP and an acyl-phosphate intermediate, γ-glutamyl phosphate, which reacts with ammonia, forming glutamine and inorganic phosphate. ADP and Pi do not dissociate until ammonia binds and glutamine is released.
ATP binds first to the top of the active site near a cation binding site, while glutamate binds near the second cation binding site at the bottom of the active site. The presence of ADP causes a conformational shift in GS that stabilizes γ-glutamyl phosphate atom. Ammonium binds strongly to GS only if the acyl-phosphate intermediate is present. Ammonium, rather than ammonia, binds to GS because the binding site is polar and exposed to solvent. In the second step, deprotonation of ammonium allows ammonia to attack the intermediate from its nearby site to form glutamine. Phosphate leaves through the top of the active site, while glutamine leave through the bottom (between two rings).Goodsell, DS (June 2002). "Glutamine Synthetase". RCSB Protein Data Bank. http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb30_1.html. Retrieved 8 May 2010.
Read more about this topic: Glutamine Synthetase
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