The calcium pump It is a structure of a protein nature that is responsible for the transport of calcium through cell membranes. This structure is dependent on ATP and is considered an ATPase-like protein, also called Catwo+-ATPase.
The Catwo+-ATPases are found in all cells of eukaryotic organisms and are essential for calcium homeostasis in the cell. This protein carries out primary active transport, since the movement of calcium molecules goes against their concentration gradient..
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The catwo+ plays important roles in the cell, so its regulation within them is essential for its proper functioning. Often acts as a second messenger.
In the extracellular spaces the concentration of Catwo+ it is approximately 10,000 times greater than within cells. An increased concentration of this ion in the cell cytoplasm triggers various responses, such as muscle contractions, release of neurotransmitters, and the breakdown of glycogen..
There are several ways of transferring these ions from cells: passive transport (nonspecific exit), ion channels (movement in favor of their electrochemical gradient), secondary active transport of the anti-support type (Na / Ca), and primary active transport with the pump. ATP dependent.
Unlike the other mechanisms of Ca displacementtwo+, the pump works in vector form. That is, the ion moves in only one direction so that it only works by expelling them.
The cell is extremely sensitive to changes in Ca concentrationtwo+. By presenting such a marked difference with its extracellular concentration, it is therefore so important to efficiently restore its normal cytosolic levels.
Three types of Ca have been describedtwo+-ATPases in animal cells, based on their locations in the cells; pumps located in the plasma membrane (PMCA), those located in the endoplasmic reticulum and nuclear membrane (SERCA), and those found in the Golgi apparatus membrane (SPCA).
SPCA pumps also carry Mn ionstwo+ which are cofactors of various enzymes of the Golgi apparatus matrix.
Yeast cells, other eukaryotic organisms, and plant cells present other types of Catwo+-Very particular ATPases.
In the plasma membrane we find the antiportic Na / Ca active transport, being responsible for the displacement of a significant amount of Catwo+ in cells at rest and activity. In most cells in a resting state, the PMCA pump is responsible for transporting calcium to the outside..
These proteins are made up of about 1,200 amino acids, and have 10 transmembrane segments. There are 4 main units in the cytosol. The first unit contains the terminal amino group. The second has basic characteristics, allowing it to bind to activating acid phospholipids..
In the third unit there is an aspartic acid with catalytic function, and "downstream" of this a fluorescein isotocyanate binding band, in the ATP binding domain.
In the fourth unit is the calmodulin-binding domain, the recognition sites of certain kinases (A and C) and the Ca-binding bands.two+ allosteric.
SERCA pumps are found in large quantities in the sarcoplasmic reticulum of muscle cells and their activity is related to contraction and relaxation in the muscle movement cycle. Its function is to transport the Catwo+ from the cell cytosol to the reticulum matrix.
These proteins consist of a single polypeptide chain with 10 transmembrane domains. Its structure is basically the same as that of PMCA proteins, but it differs in that they only have three units within the cytoplasm, with the active site being in the third unit..
The functioning of this protein requires a balance of charges during the transport of the ions. Two Catwo+ (by hydrolyzed ATP) are displaced from the cytosol to the reticulum matrix, against a very high concentration gradient.
This transport occurs in an antiportic manner, since at the same time two H+ are directed to the cytosol from the womb.
The transport mechanism is divided into two states E1 and E2. In E1 the binding sites that have a high affinity for Catwo+ they are directed towards the cytosol. In E2 the binding sites are directed towards the lumen of the reticulum presenting a low affinity for Catwo+. The two Ca ionstwo+ join after transfer.
During the binding and transfer of Catwo+, conformational changes occur, including the opening of the M domain of the protein, which is towards the cytosol. The ions then bind more easily to the two binding sites of said domain..
The union of the two Ca ionstwo+ promotes a series of structural changes in the protein. Among them, the rotation of certain domains (domain A) that reorganizes the units of the pump, enabling the opening towards the reticulum matrix to release the ions, which are uncoupled thanks to the decrease in affinity at the binding sites..
The H protons+ and water molecules stabilize the Ca binding sitetwo+, causing the A domain to rotate back to its original state, closing access to the endoplasmic reticulum.
This type of pump is found in all eukaryotic cells and is responsible for the expulsion of Catwo+ into the extracellular space in order to keep its concentration stable within cells.
In this protein a Ca ion is transportedtwo+ by hydrolyzed ATP. Transport is regulated by levels of the protein calmodulin in the cytoplasm.
By increasing the concentration of Catwo+ cytosolic, calmodulin levels increase, which bind to calcium ions. The Ca complextwo+-calmodulin, then assembles to the PMCA pump binding site. A conformational change occurs in the pump that allows the opening to be exposed to the extracellular space.
Calcium ions are released, restoring normal levels inside the cell. Consequently the complex Catwo+-calmodulin disassembles, returning the conformation of the pump to its original state.
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