Comparative study of glutamine synthetase bound lanthanide(III) ions using NMR relaxation and lanthanide(III) luminescence techniques

Changes in the intrinsic fluorescence intensity of glutamine synthetase induced by lanthanide(III) ion binding demonstrate the existence of three types of sites for these ions. The sites are populated sequentially during titrations of the enzyme, and the first two have a stoichiometry of 1 per enzyme subunit. The number of water molecules coordinated to Eu(III) bound to the first site was determined by luminescence lifetime techniques to be 4.1 +/- 0.5. The hydration of Gd(III) bound to the same site was studied by magnetic field dependent water proton longitudinal relaxation rate measurements, and by water proton and deuteron relaxation measurements of one sample at single magnetic fields. The magnetic resonance techniques also yield a value of 4 for the hydration number.