Calcium pump protein

Figure 8 Localization of solute (propranolol) within the lipid bilayer. This solute-membrane interaction has been shown to influence the conformation and activity of a calcium-pump protein (X) embedded in the bilayer. (From Ref. 78.)... 

Figure 3.1 Proteins come in different shapes and sizes (a) the enzyme glutamine synthetase, (b) the protein fibrin, and (c) the calcium pump protein.

Calcium is absorbed from the intestine by facilitated diffusion and active transport. In the former, Ca " moves from the mucosal to the serosal compartments along a concentration gradient. The active transport system requires a cation pump. In both processes, a calcium-binding protein (CaBP) is thought to be required for the transport. Synthesis of CaBP is activated by 1,25-DHCC. In the active transport, release of Ca " from the mucosal cell into... 

The endoplasmic reticulum (ER) is responsible for the production of the protein and lipid components of most of the cell s organelles. The ER contains a large number of folds, but the membrane forms a single sheet enclosing a single closed sac. This internal space is called the ER lumen. The smooth endoplasmic reticulum (ER) in muscle cells contains the vesicles and tubules that serve as a store of calcium ions. These are released as one step in the muscle contraction process. Calcium pumps, Ca +-ATPases, serve to move the calcium from the cytoplasm to the ER or SR lumen. 

Upon binding calcium ions, the small acidic protein known as calmodulin can activate enzymes by binding to a wide variety of proteins containing cahnodulin-binding domains. Such proteins include cAMP phosphodiesterase, calmodulin-dependent nitric oxide synthase, calmodulin kinases, the plasma membrane calcium pump, calcineurin, and calmodulin-dependent inositol-(l,4,5)-trisphosphate 3-kinase. See also Activation Autoinhibition... 

The transporting protein in the sarcoplasmic membrane can be phosphorylated by ATP as well as by inorganic phosphate (cf.2,174 ). In the forward running mode of the pump, i. e. when the calcium pump accumulates calcium and concomitantly hydrolyzes ATP, the terminal phosphate residue of ATP is transferred to the transport protein. The reaction depends on the presence of calcium ions in the external medium. In the reverse mode of the pump inorganic phosphate is incorporated into the transport protein. This reaction is inhibited when calcium ions are present in the external medium,... 

The insoluble Ca(II) salts of weak acids, such as calcium phosphate, carbonate, and oxalate, serve as the hard structural material in bone, dentine, enamel, shells, etc. About 99% of the calcium found in the human body appears in mineral form in the bones and teeth. Calcium accounts for approximately 2% of body weight (18,19). The mineral in bones and teeth is mosdy hydroxyapatite [1306-06-5] having unit cell composition Ca10(PO4)6(OH)2. The mineralization process in bone follows prior protein matrix formation. A calcium pumping mechanism raises the concentrations of Ca(II) and phosphate within bone cells to the level of supersaturation. Granules of amorphous calcium phosphate precipitate and are released to the outside of the bone cell. There the amorphous calcium phosphate, which may make up as much as 30—40% of the mineral in adult bone, is recrystallized to crystallites of hydroxyapatite preferentially at bone collagen sites. These small crystallites do not exceed 10 nm in diameter (20). 

Activity is modulated by other proteins present in the membrane. These include a glycoprotein (MW 53 000) which stimulates ATPase activity 138 a 60 000 molecular weight protein, which is phosphorylated in a calmodulin-dependent fashion, affects accumulation of calcium 139 while the activity of the enzyme is affected by an endogenous kinase and phosphatase which phosphorylates and dephosphorylates the protein.140 Phospholamban is a proteolipid (MW 22 000) in cardiac SR which undergoes both cyclic AMP-dependent and calcium-calmodulin-dependent phosphorylation,141 but at different sites. All these proteins are probably involved in regulating the activity of the calcium pump. 

Relaxation of the muscle is brought about by removal of the ionic calcium from the sarcoplasm. This calcium is transported across the membrane of the sarcoplasmic reticulum, in an energy requiring process. In addition to the calcium pumping ATPase, the sarcoplasmic reticulum also contains a calcium binding protein called calsequestrin (Section 4.3.3). Some of the calcium segregated by the sarcoplasmic reticulum is apparently bound to this protein within the lumen of the sarcoplasmic reticulum. As sequestration of calcium ions into sarcoplasmic reticulum proceeds, more calcium ions dissociate from their binding sites on troponin C, re-... 

This obvious dependence on extracellular calcium is somewhat unexpected because (1) the sustained enhancement of calcium influx rate is adequately balanced by an increase in calcium efflux rate so that (2) the calcium concentration in the bulk cytosol is maintained near the basal value. This apparent paradox may be resolved by a model [54] which postulates that during the sustained phase of cellular response the high rate of calcium cycling across the plasma membrane raises the calcium concentration in a region just below the plasma membrane, often called the submembrane domain (see Rasmussen and Barrett, Chapter 4). Because the elevated calcium level in this domain is not conducted into the bulk cytosol, it cannot activate calcium-dependent response elements in the cytosol. Rather it regulates the activity of calcium-sensitive, plasma membrane-associated enzymes such as the calcium pump and PKC, the previously described phospholipid-dependent, calcium-activated protein kinase. 

The recent discoveries of PHB and polyP in a human calcium pump and bacterial potassium channel suggest that the naked PHB/polyP complexes found in bacteria are progenitors of protein ion transporters. The process by which protein channels and pumps may have evolved from PHB/polyP complexes is unknown however, one may surmise that over time proteins surrounded the complexes to support and regulate their activity. At first, the association may have been nonco-valent, but subsequently PHB may have become tethered to the protein by a covalent bond. By this view, many of the channels and pumps of prokaryotes and eukaryotes may be supramolecular structures in which protein, polyP, and PHB join together for efficient regulation of transmembrane ion transport. 

Calcium ions are also transported into the cell by a pump, which is a Ca +-dependent ATPase. This pump is necessary because the calcium ion concentration is four orders of magnitude higher outside than inside living cells. Calmodulin regulates the level of calcium ions and hence the calcimn pump. When the calcium concentration decreases, calcium is dissociated from calmodulin and the calcium pump is inactivated. The structure of such a pump from the sarcoplasmic reticulum is reported at 8 A resolution. This pump couples ATP hydrolysis with cation transport. The protein contains 10 transmembrane helices. A distinct cavity was located that led to the putative calcium-binding site, suggesting a path for a calcium passage. 

The information derived from the analysis of this example was used as distance restraints for calculation of the 3D structure of the complex of calmodulin, a calcium binding protein, and a peptide ligand. The amino acid sequence of the peptide ligand, C20W, corresponds to the N-terminal part of the calmodulin-binding domain of the plasma membrane calcium pump (125). 

Phospholamban is a homopentameric membrane protein involved in muscle contraction through regulation of the calcium pump in cardiac muscle cells. The stmcture of the unphospho-rylated protein solved in DPC micelles reveals a symmetric pentamer of phospholamban monomers (Fig. 2g) stabilized by leucine/isoleucine zipper motifs along the transmembrane domains (51). Notably, another stmcture was produced for phospholamban (Fig. 2h) that used a variant of the traditional simulated annealing and molecular dynamics protocol that reduced the chances of entrapment in local minima (52). 

Egan ME, Glockner-Pagel J, Ambrose C, Cahill PA, Pappoe L, Balamuth N, Cho E, Canny S, Wagner CA, Geibel J, Caplan MJ. Calcium-pump inhibitors induce functional surface expression of Delta F508-CFTR protein in cystic fibrosis epithelial cells. Nat. Med. 2002 8 485-192. 

Figure 13.4. Structure of SR CA " ATPase. This enzyme, the calcium pump of the sarcoplasmic reticulum, comprises a membrane-spanning domain of 10 a helices and a cytoplasmic headpiece consisting of three domains (N, P, and A). Two calcium ions (green) bind within the membrane-spanning region. The aspartate residue characteristic of this protein family is indicated.

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