Calcium ATPase
# Calcium ATPase

$Ca^{2+}$ ATPase is a form of P-ATPase that transfers calcium after a muscle has contracted. The two kinds of calcium ATPase are:

* Plasma membrane $Ca^{2+}$ ATPase (PMCA)
* Sarcoplasmic reticulum $Ca^{2+}$ ATPase (SERCA)

## Plasma membrane $Ca^{2+}$ ATPase (PMCA)

Plasma membrane $Ca^{2+}$ ATPase (PMCA) is a transport protein in the plasma membrane of cells that serves to remove calcium $(Ca^{2+})$ from the cell. It is vital for regulating the amount of $Ca^{2+}$ within cells. In fact, the PMCA is involved in removing $Ca^{2+}$ from all eukaryotic cells. There is a very large transmembrane electrochemical gradient of $Ca^{2+}$ driving the entry of the ion into cells, yet it is very important for cells to maintain low concentrations of $Ca^{2+}$ for proper cell signalling; thus it is necessary for the cell to employ ion pumps to remove the $Ca^{2+}.$ The PMCA and the sodium calcium exchanger (NCX) are together the main regulators of intracellular $Ca^{2+}$ concentrations. Since it transports $Ca^{2+}$ into the extracellular space, the PMCA is also an important regulator of the calcium concentration in the extracellular space.

The PMCA belongs to a family of P-type primary ion transport ATPases that form an aspartyl phosphate intermediate.

The PMCA is expressed in a variety of tissues, including the brain.

## Sarcoendoplasmic Reticulum $Ca^{2+}$ ATPase (SERCA)

In myocytes (muscle cells) $Ca^{2+}$ is normally sequestered (isolated) in a specialized form of endoplasmic reticulum (ER) called sarcoplasmic reticulum (SR). It is a $Ca^{2+}$ ATPase that transfers $Ca^{2+}$ 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 0.1µM. 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 $Ca^{2+}$ pumps.

