Actions
The pump is powered by the hydrolysis of adenosine triphosphate (ATP), with a stoichiometry of one Ca2+ ion removed for each molecule of ATP hydrolysed. It binds tightly to Ca2+ ions (has a high affinity, with a Km of 100 to 200 nM) but does not remove Ca2+ at a very fast rate. This is in contrast to the NCX, which has a low affinity and a high capacity. Thus, the PMCA is effective at binding Ca2+ even when its concentrations within the cell are very low, so it is suited for maintaining Ca2+ at its normally very low levels. Calcium is an important second messenger, so its levels must be kept low in cells to prevent noise and keep signalling accurate. The NCX is better suited for removing large amounts of Ca2+ quickly, as is needed in neurons after an action potential. Thus the activities of the two types of pump complement each other.
The PMCA functions in a similar manner to other p-type ion pumps. ATP transfers a phosphate to the PMCA, which forms a phosphorylated intermediate.
Ca2+/calmodulin binds and further activates the PMCA, increasing the affinity of the protein's Ca2+-binding site 20 to 30 times. Calmodulin also increases the rate at which the pump extrudes Ca2+ from the cell, possibly up to tenfold.
In brain tissue, it has been postulated that certain types of PMCA are important for regulating synaptic activity, since the PMCA is involved in regulating the amount of calcium within the cell at the synapse, and Ca2+ is involved in release of synaptic vesicles.
Read more about this topic: Plasma Membrane Ca2+ ATPase
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