Relationship between Ca2+-affinity and shielding of bulk water in the Ca2+-pump from molecular dynamics simulations.

The sarcoplasmic reticulum Ca(2+)-ATPase transports two Ca(2+) per ATP hydrolyzed from the cytoplasm to the lumen against a large concentration gradient. During transport, the pump alters the affinity and accessibility for Ca(2+) by rearrangements of transmembrane helices. In this study, all-atom molecular dynamics simulations were performed for wild-type Ca(2+)-ATPase in the Ca(2+)-bound form and the Gln mutants of Glu771 and Glu908. Both of them contribute only one carboxyl oxygen to site I Ca(2+), but only Glu771Gln completely looses the Ca(2+)-binding ability. The simulations show that: (i) For Glu771Gln, but not Glu908Gln, coordination of Ca(2+) was critically disrupted. (ii) Coordination broke at site II first, although Glu771 and Glu908 only contribute to site I. (iii) A water molecule bound to site I Ca(2+) and hydrogen bonded to Glu771 in wild-type, drastically changed the coordination of Ca(2+) in the mutant. (iv) Water molecules flooded the binding sites from the lumenal side. (v) The side chain conformation of Ile775, located at the head of a hydrophobic cluster near the lumenal surface, appears critical for keeping out bulk water. Thus the simulations highlight the importance of the water molecule bound to site I Ca(2+) and point to a strong relationship between Ca(2+)-coordination and shielding of bulk water, providing insights into the mechanism of gating of ion pathways in cation pumps.