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
Famous quotes containing the word actions:
“Because impudence is a vice, it does not follow that modesty is a virtue; it is built upon shame, a passion in our nature, and may be either good or bad according to the actions performed from that motive.”
—Bernard Mandeville (1670–1733)
“The mob is man voluntarily descending to the nature of the beast. Its fit hour of activity is night. Its actions are insane like its whole constitution. It persecutes a principle; it would whip a right; it would tar and feather justice, by inflicting fire and outrage upon the houses and persons of those who have these. It resembles the prank of boys, who run with fire-engines to put out the ruddy aurora streaming to the stars.”
—Ralph Waldo Emerson (1803–1882)
“To be told that our child’s behavior is “normal” offers little solace when our feelings are badly hurt, or when we worry that his actions are harmful at the moment or may be injurious to his future. It does not help me as a parent nor lessen my worries when my child drives carelessly, even dangerously, if I am told that this is “normal” behavior for children of his age. I’d much prefer him to deviate from the norm and be a cautious driver!”
—Bruno Bettelheim (20th century)