Sarcoplasmic reticulum calcium pump

Site symmetry symbols, I, 128 Six-coordinate compounds stereochemistry, 1, 49-69 Six-membered rings metal complexes, 2, 79 Skeletal muscle sarcoplasmic reticulum calcium pump, 6, 565 Slags... 

Jencks has considered the mechanism of action of the sarcoplasmic reticulum calcium pump, which catalyzes the following reaction ... 

Table 8. Residual calcium levels and maximal concentration ratios accomplished by the sarcoplasmic reticulum calcium pump fueled with different substrates58,, 26 ...

Bers, D. M., and Bridge, J. H. (1989). Relaxation of Rabbit Ventricular Muscle by Na-Ca Exchange and Sarcoplasmic Reticulum Calcium Pump. Ryanodine and Voltage Sensitivity. Circ Res 65(2) 334-42. 

Kargacin, M.E., and Kargacin, G.J., 1996, The sarcoplasmic reticulum calcium pump is functionally altered in dystrophic muscle, Biochim Biophys Acta, 1290, pp 4—8. 

Dupont, Y. (1976). Fluorescence studies of the sarcoplasmic reticulum calcium pump. Biochem. Biophys. Res. Comm. 71,544-550. 

Tanford, C. (1984). Twenty questions concerning the reaction cycle of the sarcoplasmic reticulum calcium pump. Crit. Rev. Biochem. 17, 123-151. 

Tanford, C., Reynolds, J.A., Johnson, E.A. (1987). Sarcoplasmic reticulum calcium pump A model for Ca2+ binding and Ca2+-coupled phosphorylation. Proc. Natl. Acad. Sci. USA 84,7094-7098. 

The smooth endoplasmic reticulum calcium pumps (SERCA) found in brain were first identified in sarcoplasmic reticulum. The three isoforms of SERCA are products of separate genes SERCA-1 is expressed in fast-twitch skeletal muscle SERCA-2a in cardiac/slow-twitch muscle SERCA-2b, an alternatively spliced form, is expressed in smooth muscle and non-muscle tissues SERCA-3 is... 

ATP is used not only to power muscle contraction, but also to re-establish the resting state of the cell. At the end of the contraction cycle, calcium must be transported back into the sarcoplasmic reticulum, a process which is ATP driven by an active pump mechanism. Additionally, an active sodium-potassium ATPase pump is required to reset the membrane potential by extruding sodium from the sarcoplasm after each wave of depolarization. When cytoplasmic Ca2- falls, tropomyosin takes up its original position on the actin and prevents myosin binding and the muscle relaxes. Once back in the sarcoplasmic reticulum, calcium binds with a protein called calsequestrin, where it remains until the muscle is again stimulated by a neural impulse leading to calcium release into the cytosol and the cycle repeats. 

Calcium plays a vitally important part in possibly as many as three sequences of the contraction—relaxation cycle of vertebrate muscle (Taylor, Lymn and Moll, 1970). In the sarcoplasmic reticulum, calcium is stored in calsequestrin, a protein which holds, with high affinity, 43 atoms of calcium per molecule. The calcium is released in response to nervous impulses and triggers a sequence of reactions that lead to contraction of the muscle. After the contraction, the calcium is pumped back into this reticulum by the calcium/magnesium-dependent ATPase. 

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

The trigger for all musele eontraetion is an increase in Ca eoneentration in the vicinity of the muscle fibers of skeletal muscle or the myocytes of cardiac and smooth muscle. In all these cases, this increase in Ca is due to the flow of Ca through calcium channels (Figure 17.24). A muscle contraction ends when the Ca concentration is reduced by specific calcium pumps (such as the SR Ca -ATPase, Chapter 10). The sarcoplasmic reticulum, t-tubule, and sarcolemmal membranes all contain Ca channels. As we shall see, the Ca channels of the SR function together with the t-tubules in a remarkable coupled process. 

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). 

Fast-twitch muscle fibers develop tension two to three times faster than slow-twitch muscle fibers because of more rapid splitting of ATP by myosin ATPase. This enables the myosin crossbridges to cycle more rapidly Another factor influencing the speed of contraction involves the rate of removal of calcium from the cytoplasm. Muscle fibers remove Ca++ ions by pumping them back into the sarcoplasmic reticulum. Fast-twitch muscle fibers remove Ca++ ions more rapidly than slow-twitch muscle fibers, resulting in quicker twitches that are useful in fast precise movements. The contractions generated in slow-twitch muscle fibers may last up to 10 times longer than those of fast-twitch muscle fibers therefore, these twitches are useful in sustained, more powerful movements. 

Calcium ions are actively pumped back into the extracellular fluid as well as the sarcoplasmic reticulum. When the concentration of calcium falls below... 

Calcium is pumped back into the sarcoplasmic reticulum. This requires adenosine triphosphate (ATP). 

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