Binding of Eu3+ to cardiac sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase-laser excited Eu3+ spectroscopic studies

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The binding of Eu3+ with Ca2+-stimulated, Mg2+-dependent adenosine triphosphatase ([Ca2+ + Mg2+]-ATPase) of cardiac sarcoplasmic reticulum (SR) has been investigated using direct laser excited Eu3+ luminescence. Eu3+ is found to inhibit both Ca2+-dependent ATPase activity and Ca2+-uptake in a parallel manner. This is attributed to the binding of Eu3+ to the high affinity Ca2+-binding sites. The Ki for Ca2+-dependent ATPase is approximately 50 nM. The 7F0----5D0 excitation spectrum of Eu3+ in cardiac SR shows a peak at 579.3 nm, as compared to 578.8 nm in potassium-morpholino propane sulfonic acid (K-MOPS) pH 6.8. Upon binding with cardiac SR, Eu3+ shows an increase in fluorescence intensity as well as in lifetime values. The fluorescence decay of bound Eu3+ exhibits a double-exponential curve. The apparent number of water molecules in the first coordination sphere of Eu3+ in SR is 2.8 for the short component and 1.0 for the long component. In the presence of ATP, a further increase in fluorescence lifetimes is observed, and the number of water molecules in the first coordination sphere of Eu3+ is reduced further to 1.3 and 0.5. The double exponential nature of the decay curve and the different number of water molecules coordinated to Eu3+ for both decay components suggest that Eu3+ binds to two sites and that these are heterogeneous. The reduction in the number of H2O ligands in the presence of ATP shows a change in the molecular environment of the Eu3+-binding sites upon phosphoenzyme formation, with a movement of Eu3+ to an occluded site on the enzyme.