Pumping calcium

Sarcoplasmic calcium ATPase this enzyme utilizes the energy gained from hydrolysis of ATP to pump calcium from the cytosol into the stores of the sarcoplasmic reticulum. Its activity is negatively regulated by the closely associated protein phospholamban, and this inhibition is relieved upon phosphorylation of phospholamban by protein kinase A (PKA). 

Phosphatase Hydrolysis of phosphate Translocation of ATPases linked to cation Inorganic pyrophosphatase (pyrophosphate orthophosphate + P043-) Glucose-6-phosphatase Fructose-1,6-bisphosphatase Alkaline phosphatase Phosphoprotein phosphatase (Na+, K+)-ATPase (Mg2+, Ca2+)-ATPase Proton-translocating ATPase Zn2+ Zn2+ Sodium pump Calcium pump H+ pump... 

Abstract The plasma membrane calcium ATPase (PMCA) uses energy to pump calcium (Ca2+)... 

Chami, M., Gozuacik, D., Lagorce, D., Brini, M., Faison, P., Peaucellier, G., Pinton, P., Lecoeur, H., Gougeon, M. L., le Maire, M., Rizzuto, R., Brechot, C. and Paterlini-Brechot, P., 2001, SERCA1 truncated proteins unable to pump calcium reduce the endoplasmic reticulum calcium concentration and induce apoptosis. J Cell Biol 153, 1301-14. 

Schematic diagram of a cardiac muscle sarcomere, with sites of action of several drugs that alter contractility (numbered structures). Site 1 is Na+/K+ ATPase, the sodium pump. Site 2 is the sodium/calcium exchanger. Site 3 is the voltage-gated calcium channel. Site 4 is a calcium transporter that pumps calcium into the sarcoplasmic reticulum (SR). Site 5 is a calcium channel in the membrane of the SR that is triggered to release stored calcium by activator calcium. Site 6 is the actin-troponin-tropomyosin complex at which activator calcium brings about the contractile interaction of actin and myosin.

The SR membrane contains a very efficient calcium uptake transporter, which maintains free cytoplasmic calcium at very low levels during diastole by pumping calcium into the SR. The amount of calcium sequestered in the SR is thus determined, in part, by the amount accessible to this transporter. This in turn is dependent on the balance of calcium influx (primarily through the voltage-gated membrane calcium channels) and calcium efflux, the amount removed from the cell (primarily via the sodium-calcium exchanger, a transporter in the cell membrane). 

Darier disease (keratosis follicularis) is an autosomal dominant disorder of keratinization, which usually starts at puberty. Mutations in an endoplasmatic ATPase, which pumps calcium ions across membranes in keratinocytes, have been found to cause the disease.43... 

Jencks, W.P. (1989b). How does a calcium pump pump calcium J. Biol. Chem. 264,18855-18858. 

Maxwell, W. L., McCreath, B. J., Graham, D. I., and Gennarelli, T. A., Cytochemical evidence for redistribution of membrane pump calcium-ATPase and ecto-Ca-ATPase activity, and calcium influx in myelinated nerve fibres of the optic nerve after stretch injury, J. Neurocytol., 24, 925, 1995. 

P-Type ATPases Couple Phosphorylation and Conformational Changes to Pump Calcium Ions Across Membranes... 

Mcllrath, T. J. and Carlsten, J. L., "Production of large numbers of atoms in selected excited state by laser optical pumping calcium," Phys., 1973, B6, 697-708. 

The most physiologically important futile cycle results from cell membranes that are leaky. For example, in muscle cells ATP is consumed in pumping calcium ions into the sarcoplasmic reticulum. Leakage of calcium ions from this compartment forces the process to proceed faster than would be necessary if the membrane were totally impermeable to these ions. 

Structure determination of repetitive biomolecules has become the main thrust of NMR in solution. Based on assignment experiments discussed in Sect. 2, the several hundred to few thousand resonances of a protein can be assigned. This approach will be presented here on the example of a complex between calmodulin (CaM) and C20W, a cognate peptide out of the plasma membrane Ca -ATPase, which is responsible for pumping calcium ions out of the cell. The complex is formed from the completely N- and C-labelled CaM and unlabelled C20W [35]. 

The extracellular concentration of Ca + is approximately 1.5 him. The cytosolic concentration, on the other hand, is only 0.1 hm (note the units see Chemistry 111). This is a gradient of more than 10000 1 in concentration (Fig. 50.1) and again requires active transport, in this case by a Ca +-ATPase. This pumps calcium ions out of the cell across the plasma membrane. Alternatively Ca + can also be pumped out of the cytosol into an internal compartment, the endoplasmic reticulum. In the case of muscle, the highly specialised internal compartment is known as the sarcoplasmic reticulum and plays an important role in triggering contraction. 

This sudden increase in Ca concentration also constitutes an energy requirement for contraction. In fact, calcium pumps in the cell membrane use ATP to pump calcium ion out of the cell into nearby storage compartments to be held in readiness for release through transmembrane chaimels that are electrically triggered to open by a nerve impulse. Thus, the energy to drive contraction of the voluntary (or striated) muscle of the discus thrower comes directly from ATP and from an increase in calcium ion concentration, with the latter also being achieved through expenditure of ATP. 

With ATP as the source of energy, a particular protein in the cell membrane pumps two potassium ions, into the cell and three sodium ions, Na, out of the cell another protein pumps calcium ions, Ca % away from the contractile apparatus of muscle cells. Tlie result is a polarized membrane with more negative charge inside than outside of the cell. 

Figure 8.44. Drawing of a muscle fiber containing six myofibrils with each surrounded by the sarcoplasmic reticulum that releases calcium ion to trigger contraction and that pumps calcium ion back out to allow for the relaxation in preparation for the next contraction/relaxation cycle. Just inside the sar-colemma of the muscle fiber that surrounds the bundles of myofibrils are the mitochondria that supply the ATP required to convert the fiber from a contracted state to a relaxed but energized state in wait for the next release of calcium ion for trigger-...

During a turnover cycle of the pump, calcium ions and H i protons are transported across the membrane per mole... 

Jencks s (1989) comments in his minireview on How does a calcium pump pump calcium are worth quoting extensively as a testimony to the transient kinetic study of enzymes ... 

Transduction Chain Phosphorylation, Proton Pumps, Calcium Ions.132-14... 

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