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 Ca²⁺ 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. Essentially, calcium pumps use energy to transport calcium across cell membranes, which allows the body to perform tasks that would otherwise be difficult to perform. Failure for the body to transport sufficient amounts of calcium is one cause of muscle cramps.

The plasma membrane Ca²⁺ ATPase and the sodium-calcium exchanger are together the main regulators of cytoplasmic Ca²⁺ concentrations.

Biological role

Ca²⁺ has many important roles as an intracellular messenger. The release of a large amount of free Ca²⁺ can trigger a fertilized egg to develop, skeletal muscle cells to contract, secretion by secretory cells and interactions with Ca²⁺ -responsive proteins like calmodulin. To maintain low concentrations of free Ca²⁺ 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 Ca²⁺ to rush into the cytosol when a stimulus signal opens the Ca²⁺ channels in the membrane. The pumps are also necessary to actively pump the Ca²⁺ 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 Ca²⁺ binding sites.

Mechanism

Classical theory of active transport for P-type ATPases

Data from crystallography studies by Chikashi Toyoshima applied to the above cycle

References