Distances between functional sites in cardiac sarcoplasmic reticulum (Ca2+ +Mg2+)-ATPase. Inter-lanthanide energy transfer.

The high-affinity Ca2+-binding sites of cardiac sarcoplasmic reticulum (Ca2+ +Mg2+)-ATPase have been probed using trivalent lanthanide ions. Non-radiative energy-transfer studies, using luminescent probe Eu3+ as a donor and Nd3+ or Pr3+ as acceptor, were carried out to estimate the distance between two high-affinity Ca2+-binding/transport sites. Eu3+ was excited directly with pulsed laser light and the energy-transfer efficiency to Nd3+ or Pr3+ was measured, under the conditions in which most donor-acceptor pairs occupied the high-affinity Ca2+ sites. The distance between two high-affinity Ca2+ sites is about 0.89 nm. In the presence of ATP the distance between the high-affinity sites is about 0.855 nm, whereas in the presence of adenosine 5'-[beta, gamma-methylene]triphosphate or adenosine 5'-[beta, gamma-imino]triphosphate the distance is about 0.895 nm. To estimate the distance between the high-affinity Ca2+ sites and ATP-binding/hydrolytic site, we have measured the energy-transfer efficiency between Eu3+ and Cr3+-ATP with Eu3+ at the high-affinity Ca2+ sites and Cr3+-ATP at the ATP-binding/hydrolytic site. Our results show that ATP-binding/hydrolytic site is separated by about 2.2 nm from each high-affinity Ca2+ site.