Interaction among substrates, inhibitors and Mn2+ bound to glutamine synthetase as studied by NMR relaxation rate measurements.
The interaction of Mn2+ with the substrate glutamate and several transition state analog inhibitors of glutamine synthetase has been studied. With Mn2+ bound to the tight binding site, the frequency and temperature dependence of the paramagnetic contribution to solvent water proton relaxation rates demonstrate changes in the structure of the metal ion environment induced by substrate or inhibitor binding. The water proton relaxation rate data also show differences in the metal ion environment in the presence of glutamate compared to methionine sulfoximine, a structural analog of an intermediate in the reaction mechanism. Additionally, the distance between the metal ion and the phosphorus atom of an inhibitor, 2-amino-4-phosphonobutyric acid, was estimated (approximately 5 A) using NMR measurements. These data are in accord with our recent hypothesis that the role of the metal ion is to stabilize the tetrahedral adduct formed on the reaction pathway.