How Ca2+-ATPase pumps ions across the sarcoplasmic reticulum membrane

Ca2+-ATPase of muscle sarcoplasmic reticulum is an ATP-powered Ca2+-pump that establishes a > 10,000 fold concentration gradient across the membrane. Its crystal structures have been determined for 9 different states that cover nearly the entire reaction cycle. Presented here is a brief structural account of the ion pumping process, with emphasis on why the structure has to be so.