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From Proteopedia

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Introduction

All living organisms depend on P-type ATPase to pump cation across the membrane. They play a fundamental role in their metabolism and physiology. Ca2+ ATPase, a P-type ATPase, transports calcium ions across the membrane against a concentration gradient. These pumps clear cytoplasm of the second messenger, the calcium. It's very important to keep a low concentration of calcium in the cell for a good cell signaling. We can found the PMCA (Plasma Membrane Ca2+ ATPase) which remove the calcium from the cell and the SERCA (Sarcoplasmic Endoplasmic Reticulum Ca2+ ATPase) which pumps calcium into the endoplasmic reticulum. The hydrolysis of one adenosine triphosphate (ATP) is essential for the functioning of the pump and the transport of one calcium ion.

Structural Highlights

The calcium ATPase is a protein composed of 1001 aminoacids. The protein is composed of many , including 10 transmembrane alpha helices. It also contains a lot of beta strands.

Ligand and Interaction

The architecture of calcium ATPase (determined by X-Ray crystallography) allow to understand mechanisms by which the energy of ATP is coupled to the calcium transport across a membrane. Structurally, the pump contains 10 transmembrane domains (α helices), two large intracellular loops and amino/carboxy-terminal cytoplasmic tails. In the cytoplasm, the ATP binds to the cytosolic loop that connects transmembrane domains four and five (ATP binding site). It transfers its γ-phosphate to the aspartic acide 351 (phosphorylation site) and creates a acid-stable aspartyl phosphate intermediate. The binding of ATP is initiated by the cooperative fixing of two calcium ions to the transport site. The phosphorylation of Asp351 allows a large conformational changes in cytoplasmic domains which closes the ion gates from the cytoplasm and alters the affinity of the protein for the calcium. After releasing calcium (in the lumen of cytoplasm or out side the cell), two protons are bound to the transport sites (charges compensation) and the aspartyl phosphate is hydrolyzed to complete the cycle. To sum up, calcium pumps have two conformations, E1 and E2. E1 has the calcium binding site oriented toward the cytoplasm . E2 has the calcium binding site oriented toward the lumen of the endoplasmic reticulum or toward the extracellular background. These two conformations are characterized by different specificity for ion binding.

Biological Function and Localisation

The pumps exist in two major conformational states : E1 and E2. When the pump is in the E1 state, it has high calcium affinity and interacts with calcium at one side of the membrane. In the E2 state, the enzyme has a lower calcium affinity and that leads to the release of the ion at the opposite side.

Recently, some structural work on the SERCA pump has shown that the mechanism of the pump is a little bit more complex. When the calcium pump is unphosphorylated, two of the helices are disrupted, forming a cavity accessible from the cytosol. This cavity binds two calcium ions. When ATP binds to the pump, it allows the phosphorylation of the enzyme on its phosphorylation domain. The hydrolysis of ATP causes conformational changes that bring the nucleotide binding domain and the phosphorylation domain of the enzyme into close proximity. The activator domain rotates and the transmembrane helices 4 and 6 rearrange. Calcium is released into the lumen of the sarcoplasmic reticulum.

More recent structural work on the SERCA pump has increased the complexity of the conformational transitions that occur during the catalytic cycle. Upon binding of Ca2+, a series of structural changes occur that involve both the protruding cytoplasmic sector and the transmembrane domains, permitting the phosphorylation of the catalytic D-residue by the γ-phosphate of ATP. The dissociation of Ca2+ from the enzyme follows the transition of the high Ca2+ affinity E1∼P(Ca2+) enzyme to the lower affinity E2-P enzyme, the hydrolysis of which then regenerates the Ca2+-free E2 ATPase, completing the catalytic cycle.

Regulations

There are different kind of calcium ATPase regulations. For example, the phospholamban (PLN or PLB) is a membrane protein that regulates the calcium pump in cardiac muscle and skeletal muscle cells. This small phosphoprotein is a pentamer. The phospholamban can be phosphorylated at three distinct sites by various protein kinases (PKA, PKC, CamK...). The phosphorylation state is mediatedthrough beta-adrenergic stimulation. In unphosphorylate state, the phospholamban inhibits the activity of calcium pump in cardiac and skeletal muscle cells by decreasing the apparent affinity of the ATPase for calcium. The phosphorylation of the protein results in the dissociation of the protein from the ATPase. The phosphoprotein binds just downstream of the active ATPase site (asp351). The activity of the calcium pump is also regulated by by calmodulin, acidic phospholipids and phosphorylation by kinases A and C. Most of the activation mechanisms implicate the C-terminal region of the pump containing the high affinity calmodulin binding domain, which is involved in the autoinhibition of the pump.

Dysfunctions and Diseases

References

David H.MacLennan, William J.Rice and N. Michael Green, 1997 - The Mechanism of Ca2+ Transport by Sarco(Endo)plasmic Reticulum Ca2+-ATPases - The Journal of Biological Chemistry, p.272, 28815-28818

Marianela G.Dalghi, Marisa M.Fernández, Mariela Ferreira-Gomes, Irene C.Mangialavori, Emilio L.Malchiodi, Emanuel E.Strehler and Juan Pablo F.C.Rossi, 2013 - Plasma Membrane Calcium ATPase Activity Is Regulated by Actin Oligomers through Direct Interaction - The Journal of Biological Chemistry, p.288, 23380-23393

Marisa Brini and Ernesto Carafoli, 2010 - The Plasma Membrane Ca2+ ATPase and the Plasma Membrane Sodium Calcium Exchanger Cooperate in the Regulation of Cell Calcium - Cold Spring Harbor Perspectives in Biology

Marisa Brini and Ernesto Carafoli, 2009 - Calcium Pumps in Health and Disease- Physiological Reviews

Thomas D.Pollard and William C. Earnshaw, - Membrane, structure and function - Cell Biology (second edition), p.133-136