Calcium ATPase

Ca²⁺ ATPase is a form of P-ATPase that transfers calcium after a muscle has contracted. The three kinds of calcium ATPase are:

Plasma membrane Ca²⁺ ATPase
Sarcoplasmic reticulum Ca²⁺ ATPase
Secretory Pathway Ca²⁺ ATPase

Calcium ATPases belong to a family of P-type primary ion transport ATPases that form an aspartyl phosphate intermediate.

Plasma membrane Ca²⁺ ATPase (PMCA)

[Image:Calcium atpase.png|thumb|250px|left|Rendered image of the Ca²⁺ pump]
Plasma membrane Ca²⁺ ATPase is a transport protein in the plasma membrane of cells that serves to remove calcium from the cell. It is vital for regulating the amount of Ca²⁺ within cells.
In fact, the PMCA is involved in removing Ca²⁺ from all eukaryotic cells.
There is a very large transmembrane electrochemical gradient of Ca²⁺ driving the entry of the ion into cells, yet it is very important for cells to maintain low concentrations of Ca²⁺ for proper cell signalling; thus it is necessary for the cell to employ ion pumps to remove the Ca²⁺.
The PMCA and the sodium calcium exchanger are together the main regulators of intracellular Ca²⁺ concentrations.
Since it transports Ca²⁺ into the extracellular space, the PMCA is also an important regulator of the calcium concentration in the extracellular space.
The PMCA is expressed in a variety of tissues, including the brain.

Sarcoendoplasmic Reticulum Ca²⁺ ATPase (SERCA)

In myocytes Ca²⁺ is normally sequestered in a specialized form of endoplasmic reticulum called sarcoplasmic reticulum. It is a Ca²⁺ ATPase that transfers Ca²⁺ from the cytosol of the cell to the lumen of the SR at the expense of ATP hydrolysis during muscle relaxation. In the skeletal muscles the calcium pump in the sarcoplasmic reticulum membrane works in harmony with similar calcium pumps in the plasma membrane. This ensures that the cytosolic concentration of free calcium in resting muscle is below 0.1 μM. The sarcoplasmic and endoplasmic reticulum calcium pumps are closely related in structure and mechanism, and both are inhibited by the tumor-promoting agent thapsigargin, which does not affect the plasma membrane Ca²⁺ pumps.