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

Abstract
Changes in the intrinsic fluorescence intensity of glutamine synthetase induced by lanthanide(III) ion binding demonstrate the existence of 3 types of sites for these ions. The sites are populated sequentially during titrations of the enzyme, and the first 2 have a stoichiometry of 1/enzyme subunit. The number of water molecules coordinated to Eu(III) bound to the 1st 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 1 sample at single magnetic fields. The magnetic resonance techniques also yield a value of 4 for the hydration number.