Fluorescence decay time measurements of Eu3+-ATP-enzyme complexes. Replacement of the metal hydration water by active site ligands.

Measurements of the fluorescent lifetimes of the rare earth metal Eu3+ in varying mole fractions of H2O/D2O were used to determine the hydration of the metal in the presence of ATP and/or hexokinase or chloroplast reversible ATPase. The number of water molecules coordinated to the metal in Eu3+-ATP was estimated to be 2.6; when this complex is bound to hexokinase, 1 water molecule is displaced. Upon binding to chloroplast reversible ATPase, the metal coordinates 1 water molecule while the Eu3+-ATP complex does not retain any associated solvent. These numbers are in contrast to the 9 solvent molecules coordinated to the naked metal ion. These results are discussed in reference to mechanistic and structural considerations of the two enzymes.