Glutamine Synthetase: Metals and Filament Coordination

Abstract

Glutamine has a central role in cellular metabolism because it is a key donor of carbon and nitrogen atoms to other key metabolic precursors. Nitrogen fixation and mobilization is controlled through the inter-conversion of glutamate and glutamine by glutamine synthetase (GS). GS converts glutamate and ammonium into glutamine in an ATP-dependent reaction. GS assembles into filaments whose formation is conserved among E. coli, yeast, and plants. The higher-order structural organization observed among GS’s and the importance of the glutamine pool within metabolism suggest these filaments may serve as a layer of regulation as seen with other filament-forming enzymes. Here we characterized in vitro assembly of E. coli and human glutamine synthetase. We show that the filament formation of E. coli GS is pH and divalent cation-dependent. Additionally, we show metals disrupt active site integrity through secondary structure changes. Finally, we formally show human GS can form filaments, define the residues involved in assembly, and provide structural evidence that these filaments bind and turnover substrates.