Sarcoplasmic-endoplasmic reticulum ATPase

The mechanism of action for such peroxidic compounds involves a reductive activation by iron in haem, released as a result of hemoglobin digestion by Plasmodium. This irreversible redox reaction affords carbon-centered free radicals causing the alkylation of haem and of proteins. One such protein (the sarcoplasmic-endoplasmic reticulum ATPase PfATP6) appears to be critical for parasite survival, and there is no indication for resistance by the parasite. However, treatment is expensive and recrudescence of malaria occurs often. Moreover, it was found that at high doses such compounds are neurotoxic. 

Sarcoplasmic reticulum (SR) is a form of the smoothfaced endoplasmic reticulum (ER) in muscles. It functions as an intracellular Ca2+ store for muscle contraction. Ca2+ is energetically sequestered into the SR by Ca2+-pump/sarcoplasmic endoplasmic reticulum Ca2+-ATPase (SERCA) and released via Ca2+ release channels on stimuli (ryanodine receptor in striated muscles and inositol 1,4,5-trisphosphate receptor in most smooth muscles). Endoplasmic reticulum in non-muscle tissues also functions as an intracellular Ca2+ store. 

Dhitavat, J., Dode, L., Leslie, N., Sakuntabhai, A., Lorette, G., and Hovnanian, A., 2003b, Mutations in the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase isoform cause Darier s disease. J Invest Dermatol, 121 486—9. 

Li, Y., Ge, M., Ciani, L., Kuriakose, G., Westover, E.J., Dura, M., Covey, D.F., Freed, J.H., Maxfield, F.R., Lytton, J., and Tabas, I., 2004, Enrichment of endoplasmic reticulum with cholesterol inhibits sarcoplasmic-endoplasmic reticulum calcium ATPase-2b activity in parallel with increased order of membrane lipids implications for depletion of endoplasmic reticulum calcium stores and apoptosis in cholesterol-loaded macrophages../. Biol. Chem. 279, 37030—37039 Lin, P., Yao, Y., Hofmeister, R., Tsien, R.Y., and Farquhar, M.G., 1999, Overexpression of CALNUC (nucleobindin) increases agonist and thapsigargin releasable Ca2+ storage in the Golgi. J. Cell Biol. 145, 279-289... 

The artemisinins appear to kill the parasite by a free radical mechanism—not by the generation of ROS but, rather, by virtue of a free radical associated with the endoperoxide, possibly involving a carbon radical. Evidence points toward activation of the endoperoxide via an iron-dependent mechanism. The resulting free radical selectively targets sarcoplasmic/endoplasmic reticulum Ca " -ATPase of the Plasmodium falciparum (PfATP6), altering calcium stores (49). The artemisinins actually may form covalent adducts to specific membrane-associated proteins after concentrating in infected erythrocytes. 

This chapter will summarize recent developments on the structure of the Ca -ATPase of the sarcoplasmic reticulum with occasional references to the Ca -ATPases in the plasma membranes and endoplasmic reticulum of non-muscle cells. 

Our discussion here will concentrate on the various forms of the Ca " transport ATPases that occur in the sarcoplasmic reticulum of muscle cells of diverse fiber types and in the endoplasmic reticulum of nonmuscle cells (SERCA). The structure of these enzymes will be compared with the Ca transport ATPases of surface membranes (PMCA) [3,29-32,34] and with other ATP-dependent ion pumps that transport Na, K, andH [46,50-52]. 

Afonso A, Hunt P, Cheesman S, Alves AC, Cunha CV, do Rosario V, Cravo P. (2006) Malaria parasites can develop stable resistance to artemisinin but lack mutations in candidate genes atp6 (encoding the sarcoplasmic and endoplasmic reticulum Ca " ATPase), tctp, mdrl, and cglO. Antimicrob Agents Chemother 50 480 89. 

The cytosolic concentration of free Ca2+ is generally at or below 100 mi, far lower than that in the surrounding medium, whether pond water or blood plasma. The ubiquitous occurrence of inorganic phosphates (Pj and I l ,) at millimolar concentrations in the cytosol necessitates a low cytosolic Ca2+ concentration, because inorganic phosphate combines with calcium to form relatively insoluble calcium phosphates. Calcium ions are pumped out of the cytosol by a P-type ATPase, the plasma membrane Ca2+ pump. Another P-type Ca2+ pump in the endoplasmic reticulum moves Ca2+ into the ER lumen, a compartment separate from the cytosol. In myocytes, Ca2+ is normally sequestered in a specialized form of endoplasmic reticulum called the sarcoplasmic reticulum. The sarcoplasmic and endoplasmic reticulum calcium (SERCA) pumps are closely related in structure and mechanism, and both are inhibited by the tumor-promoting agent thapsigargin, which does not affect the plasma membrane Ca2+ pump. 

Lytton, J., Westlin, M., and Hanley, M. R., 1991, Ihapsigargin inhibits the sarcoplasmic or endoplasmic reticulum Ca-ATPase family of calcium pumps. J Biol Chem, 266 17067-71. 

The large concentration gradient between extracellular spaces and cytosol is maintained by the active transport of Ca2+ across the plasma membrane, the endoplasmic reticulum (or the sarcoplasmic reticulum in muscle), and the mitochondrial inner membrane. Generally, plasma membrane and endoplasmic reticulum each contain a Ca2+-ATPase that actively pumps Ca2+ out of the cytosol at the expense of ATP hydrolysis (7, pp. 496-498). Mitochondria act as a buffer for cytosolic Ca2+ ... 

In order to provide a better understanding of the role of Ca + as an almost universal regulator of cellular function, we need to take a brief look at the many ways by which Ca ions can be transported in or out of eukaryotic cells. Although various transport pathways have been elucidated, the present picture is probably not complete, since the molecular structures and properties of the transport proteins are only partially known. The major pathways for transport across cellular membranes involve three membrane systems the plasma membrane, the inner mitochondrial membrane, and the membrane of the endoplasmic reticulum (ER) (or, in striated muscle cells, a specialized form of ER called the sarcoplasmic reticulum (SR) (Figure 3.9). Two of the membrane-bound transport systems are Ca " -ATPases, since they derive their main energy from the hydrolysis of ATP (1 and 2 in Figure 3.9). Their properties do, however, differ in many other respects, as we will see. 

Schematic representation of the major pathways for the transport of Ca across cellular membranes. PM, plasma membrane ER(SR), endoplasmic reticulum (sarcoplasmic reticulum) M, mitochondria A P, difference in membrane potential. The transport proteins shown are 1 and 2, PM and ER(SR) Ca -ATPases 3 and 4, PM and ER(SR) receptor-mediated Ca " channels 5 and 6, PM and M (inner-membrane) Na /Ca exchangers 7 and 8, PM and M voltage-sensitive Ca channels. In addition, some not-well-defined passive transport pathways are indicated by dashed arrows.

The Na K ATPase of the plasma membrane and the Ca " transporters of the sarcoplasmic and endoplasmic reticulums (the SERCA pumps) are examples of P-type ATPases they undergo reversible phosphorylation during their catalytic cycle and are inhibited by the phosphate analog vanadate. F-type ATPase proton pumps (ATP synthases) are central to energy-conserving mechanisms in mitochondria and chloroplasts. V-type ATPases produce gradients of protons across some intracellular membranes, including plant vacuolar membranes. 

The entry of into the cytoplasm is mediated by diverse channels Plasma membrane channels regulated by G proteins, membrane potential, K or Ca itself, and channels in specialized regions of endoplasmic reticulum that respond to IP or, in excitable cells, to membrane depolarization and the state of the release channel and its stores in the sarcoplasmic reticulum. Ccf is removed both by extrusion (Naf-Ccf exchanger and Ca ATPase) and by reuptake into the endoplasmic reticulum (SERCA pumps). propagates its signals through a... 

A large number of other active transporters also convert ATP chemical bond energy into an ion gradient (membrane potential). Vesicular ATPases pump protons into lyso-somes. Ca ATPases in the plasma membrane move Ca out of the cell against a concentration gradient. Similar Ca ATPases pump Ca into the lumen of the endoplasmic reticulum and the sarcoplasmic reticulum (in muscle). Thus, a considerable amount of energy is expended in maintaining a low cytoplasmic Ca level. 

Ca -ATPases located in the plasma membrane pump Ca - out of the cell. Ca -ATPases in the endoplasmic reticulum, and in the sarcoplasmic reticulum of heart and other muscles, sequester Ca within the membranes, where it is bound by a low-affinity binding protein. Ca is released from the sarcoplasmic reticulum in response to a nerve impulse, which signals contraction, and the increase of Ca stimulates both muscle contraction and the oxidation of fuels. Within the heart, another Ca transporter protein, the NaVCa exchange transporter, coordinates the efflux of Ca in exchange for Na, so that Ca is extraded with each contraction. 

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. 

There are several mammalian members of alkali cation ATPases the Na,K ATPases, the H,K ATPases, the SERCA (sarcoplasmic and endoplasmic reticulum), Ca -transport ATPases, and the PM (plasma membrane) Ca ATPase. The fungal H ATPases also belong to this family. Characteristically, the mammalian enzymes all perform countertransport—exchange of a cellular cation with an extracellular cation. The fungal enzymes transport only H outward. The parietal cell possesses a variety of these small alkali cation ATPases, each subserving a different function, but the pump of clinical interest is the pump responsible for acid secretion, the H,K ATPase. 

Bayle D, Weeks D, Sachs G. The membrane topology of the rat sarcoplasmic and endoplasmic reticulum caldum ATPases by in vitro translation scanning. J Biol Chem 1995 270 25678-25684. 

Afonso, A. Hunt, P. Cheesman, S. Alves, A. C. Cunha, C. V. Rosario, Vd. Cravo, P. Malaria Parasites Can Develop Stable Resistance to Artemisinin but Lack Mutations in Candidate Genes atp6 (Encoding the Sarcoplasmic and Endoplasmic Reticulum Ca2+ ATPase), tctp, mdrl, and cglO. Antimicrob. Agents Chemother. 2006,50, 480M89. 

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