Calcium pump
Calcium pumps are a family of ion transporters found in the cell membrane of all animal cells. They are responsible for the active transport of calcium out of the cell for the maintenance of the steep Ca electrochemical gradient across the cell membrane. Calcium pumps play a crucial role in proper cell signalling by keeping the intracellular calcium concentration roughly 10,000 times lower than the extracellular concentration. Failure to do so is one cause of muscle cramps. The plasma membrane Ca ATPase and the sodium-calcium exchanger are together the main regulators of intracellular Ca concentrations. Biological role . Ca has many important roles as an intracellular messenger. The release of a large amount of free Ca can trigger a fertilized egg to develop, skeletal muscle cells to contract, secretion by secretory cells and interactions with Ca -responsive proteins like calmodulin. To maintain low concentrations of free Ca in the cytosol, cells use membrane pumps like calcium ATPase found in the membranes of sarcoplasmic reticulum of skeletal muscle. These pumps are needed to provide the steep electrochemical gradient that allows Ca to rush into the cytosol when a stimulus signal opens the Ca channels in the membrane. The pumps  are also necessary to actively pump the Ca back out of the cytoplasm and return the cell to its pre-signal state. Crystallography of calcium pumps. The structure of calcium pumps found in the sarcoplasmic reticulum of skeletal muscle was elucidated in 2000 by Toyoshima, et al. using microscopy of tubular crystals and 3D microcrystals. The pump has a molecular mass of 110,000 amu, shows three well separated cytoplasmic domains, with a transmembrane domain consisting of ten alpha helices and two transmembrane Ca binding sites. Mechanism. Classical theory of active transport for P-type ATPases Data from crystallography studies by  Chikashi Toyoshima applied to the above cycle