Phosphoryl Transfer and Calcium Ion Occlusion in the Calcium Pump

A tight coupling between adenosine triphosphate (ATP) hydrolysis and vectorial ion transport has to be maintained by ATP-consuming ion pumps. We report two crystal structures of Ca²⁺-bound sarco(endo)plasmic reticulum Ca²⁺-adenosine triphosphatase (SERCA) at 2.6 and 2.9 angstrom resolution in complex with (i) a nonhydrolyzable ATP analog [adenosine (β-γ methylene)-triphosphate] and (ii) adenosine diphosphate plus aluminum fluoride. SERCA reacts with ATP by an associative mechanism mediated by two Mg²⁺ ions to form an aspartyl-phosphorylated intermediate state (Ca₂-E1~P). The conformational changes that accompany the reaction with ATP pull the transmembrane helices 1 and 2 and close a cytosolic entrance for Ca²⁺, thereby preventing backflow before Ca²⁺ is released on the other side of the membrane.