Calcium pumps structure

Carafoli, E., and Brini, M., 2000, Calcium pumps structural basis for and mechanism of calcium transmembrane transport. Curr Opin Chem Biol, 4 152-61. 

Figure 13.3 Calcium-pump structure. The overall structure of the SERCA P-type ATPase. Notice the two calcium ions (green) that lie in the center of the transmembrane domain.

The sodium and calcium pumps can be isolated to near purity and still exhibit most of the biochemical properties of the native pump. Some kinetic properties of these pumps in native membranes are altered or disappear as membrane preparations are purified. For example, when measured in intact membranes, the time-dependencies of phosphorylation and dephosphorylation of the pump catalytic sites exhibit biphasic fast to slow rate transition this characteristic progressively disappears as the membranes are treated with mild detergents. One suggested explanation is that, as the pumps begin to cycle, the catalytic subunits associate into higher oligomers that may permit more efficient transfer of the energy from ATP into the ion transport process [29, 30], Some structural evidence indicates that Na,K pumps exist in cell membranes as multimers of (a 3)2 [31]. 

FIGURE 22-3 Structures of compounds that inhibit sarcoendoplas-mic reticulum Ca2+-ATPase (SERCA) calcium pumps. 

Stokes, D.L. Green, N.M. (2003) Structure and function of the calcium pump. Annu. Rev. Biophys. Biomol. Struct. 32, 445-468. Advanced review. 

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). 

Toyoshima, C., Nakasako, M., Nomura, H., and Ogawa, H., 2000, Crystal structure of the calcium pump of sarcoplasmic reticulum at 2.6 A resolution. Nature, 405 647-55. 

Hilfiker, H., Strehler-Page, M.A., Stauffer, T.P., Carafoli, E., Strehler, E.E., 1993, Structure of the gene encoding the human plasma membrane calcium pump isoform 1. J Biol Chem 268, 19717-19725. 

Toyoshima, C., Nomura, H., Tsuda, T., 2004, Lumenal gating mechanism revealed in calcium pump crystal structures with phosphate analogues. Nature 432, 361-368. 

The presence of PHB and polyP in a human calcium pump indicates that the role of these polymers in ion transport has been conserved, and further suggests that the polymers are likely constituents of other ion channels and pumps. Recently, the potassium channel of the Gram-positive soil bacterium, Streptomyces lividans (KcsA), attracted great interest as the first ion channel to have its structure analyzed... 

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. 

Sarcoplasmic reticulum Ca + ATPase, responsible for calcium ion transport. See Martonosi, A.N. and Pikula, S., The structure of the Ca +-ATPase of sarcoplasmic reticulum, Acta Biochim. Pol. 50, 337-365, 2003 Strehler, E.E. and Treiman, M., Calcium pumps of plasma membrane and cell interior, Curr. Mol. Med. 4, 323-335, 2004. 

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). 

Olesen C, Picard M, Winther AM, Gyrup C, Morth JP, Oxvig C, Mpller JV, Nissen P. The structural basis of calcium transport by the calcium pump. Nature. 2007 450 957-959. 

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.

Figure 13.4 Conformational changes associated with calcium pumping. This structure was determined in the absence of bound calcium and with a phosphorylaspartate analog present in the P domain. Notice how different this structure is from the ralcium-bound form shown in f igure 13.3 both the transmembrane part (yellow) and the A, P, and N domains have substantially rearranged. [Drawn from TWPG.pdb.]...

Toyoshima, C2, and Mizutani, T 2004. Crystal structure of the calcium pump with a bound ATP analogue. Nature 4. 0-529-535... 

An early report suggested that suramin can block the nucleotide-dependent calcium pump of rabbit skeletal sarcoplasmic reticulum by inhibition of the calcium uptake and the ATTase activity [51]. These results have been confirmed by Emmik et al. [52]. Baumert and Heider [53] found that pyridoxal-5-phosphate and a series of its analogues (for chemical structure. 

A comparatively small group of structurally unique guaianolide type STLs known from Thapisa species (Apiaceae/Umbelliferae) are known to inhibit with high specificity and very high affinity (low nanomolar concentrations) intracellular calcium-pumps termed sarco-endoplasmatic reticulum Ca2+ ATPases (SERCA). Thapsigargin (TG) was identified as the active principle in the roots of T. garganica [124-127] which cause severe inflammation of the skin after contact and which had been used in traditional medicine as a counter-irritant for centuries [128, 129]. 

---