Phosphoryl Transfer and Calcium Ion Occlusion in the Calcium Pump

Abstract

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 Ca 2+ -bound sarco(endo)plasmic reticulum Ca 2+ –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 Mg 2+ ions to form an aspartyl-phosphorylated intermediate state (Ca 2 -E1∼P). The conformational changes that accompany the reaction with ATP pull the transmembrane helices 1 and 2 and close a cytosolic entrance for Ca 2+ , thereby preventing backflow before Ca 2+ is released on the other side of the membrane.