Calcium-bound

Figure 3. Reporting of intracellular calcium sequestration by chlorotetracycline (CTC). CTC preferentially partitions into cell membranes and its fluorescence in this environment is sensitive to calcium bound to the membrane therefore its signal (excitation AOO nm, emission 530 nm) will come largely from organelles that bind or sequester calcium, such as smooth endoplasmic reticulum or mitochondria. Release of calcium from such organelles is accompanied by dissociation of the calcium-CTC complex, a decrease in CTC fluorescence and efflux of unbound probe from the organelle and from the cell.

In order to understand these effects in a detailed way for the calcium-binding proteins, it is necessary to compare the structure of the calcium-free and calcium-bound forms of each protein. The structures of parvalbumin and the intestinal calcium-binding protein (Wasserman protein) are known (see Table 6), but at present there are no crystal structures available for calcium-free proteins. Some structural information has been deduced from changes in the NMR spectrum as calcium is removed from the calcium-bound proteins.209,210... 

By analogy with other calcium-binding proteins, it is expected that the Ca2+-bound intestinal protein is then able to interact with some receptor protein. Suggestions have been made for interaction with alkaline phosphatase or carbonic anhydrase, but another possibility is that the calcium-bound protein is able to activate a gating mechanism for the transport of calcium. 

Each of the three EF-hand structures solved from the spectrin family proteins exhibit unique structural and functional differences, even though all are fundamentally similar. The a-spectrin (nonerythroid) EF hands bind calcium and presumably perform some kind of regulatory role regarding the actin-binding function of spectrin (Trave et al., 1995). Due to the low calcium affinity, it is expected that calcium regulatory events involving spectrin would occur in areas of the cell that would experience a transient but significant fluctuation of calcium concentration (Buevich et al., 2004). It is possible that the calcium-bound form of... 

Drohat AC, Baldisseri DM, Rustandi RR, Weber DJ. 1998. Solution structure of calcium-bound rat S100B(betabeta) as determined by nuclear magnetic resonance spectroscopy. Biochemistry 37(9) 2729-2740. 

In the case of red blood cells, it is assumed that the progressive loss of lipid asymmetry, possibly associated with the entry of calcium, is a signal that the cell is aging. This signal, in turn, is recognized by macrophages and leads to cell destruction. Dmgs which, for example, compete for calcium bound to phosphatidylserine could interfere with these processes and many other Ca2+-dependent processes such as protein kinase C activation. The influence of asymmetry in membranes of different phospholipid composition on the fusion of liposomes has been studied and reported [22]. 

Fura-2 is currently the most popular Ca2+ indicator for microscopy of individual cells. It retains the 1 1 stoichiometry, calcium selectivity, and pH insensitivity of the latter (6). The most important feature of fura-2 is the shift in excitation maximum upon binding Ca2+. The excitation maxima of the calcium-free fura-2 is at 360 nm whereas the calcium-bound form excites at 340 nm. In the original method, fura-2 was used for producing and measuring... 

Aminoglycoside binding to membrane acidic phospholipids Changes in content and metabolism of membrane phospholipids Displacement of calcium bound to phospholipids Formation of myeloid bodies... 

The catalytic activity of porcine pancreatic a-amylase, another member of the glucosyl hydrolase family, is metal cofactor-dependent [53]. A calcium ion-binding site is located at the interface of an antiparallel f-sheet, inserted in one of the loop regions of the /ia-barrel, and the core structure. In the calcium-bound state, the insertion stabilizes the substrate-binding site, and indirectly constrains part of the active site in a catalytically competent conformation. 

Metarhodopsin II is inactivated by phosphorylation of three serine residues at the carboxyl terminal of the protein, catalyzed by rhodopsin kinase. In transgenic mice with carboxyl terminal-truncated rhodopsin, lacking the phosphorylation sites, there is a prolonged response to a single photon. Rhodopsin kinase is activated by its substrate, metarhodopsin II, and is inhibited by calcium bound to the protein recoverin, which thus prolongs the photoresponse. 

Figure 10 (a) The structures of sTnC with two calciiun ions bound. (b) The NMR structures of four-calcium-bound cTnC with the C-terminal of the protein superimposed ... 

Figure 13 The structures of the calcium-free (a, ISYM) and the calcium-bound (b, IQLK) of SIOOB dimer (c). The structure of SlOO dimer complexed to the target peptide (cyan in bottom figure, IMWN) at the cleft of helices III, IV, T, and the helices II-III hinge...

Figure 14 The structure of calpain domain VI dimer (lALV) with one chain in light gray and the other in color. Each EF-hand motif is in a separate color. From EF-1 to EF-5 they are in blue, purple, cyan, orange, and dark green. Helices 4 and 7 are shared by two EF-hand motifs. The calcium bound to the EF-loop is represented by green balls and that bound to other positions is represented by orange ball...

The calcium ion in a-LA plays a structural role in stabilizing the protein. The thermal stability of the calcium-bound form of a-LA increases more than 40 °C compared to that of the apo-form. At low pH (e.g. pH 2), a-LA releases the calcium ion and becomes partially unfolded (molten globule state). This partially unfolded protein loses its tertiary structure but retains its secondary structure. Other metals, such as manganese or magnesium, are able to compete with calcium at the same site with a similar stabilizing effect. However, the binding of zinc, which is proposed to bind at different locations, decreases a-LA stability. ... 

Figure 15.22. Conformational Changes in Calmodulin on Calcium Binding. In the absence of calcium (top), the EF hands have hydrophobic cores. On binding of a calcium ion (green sphere) to each EF hand, structural changes expose hydrophobic patches on the calmodulin surface. These patches serve as docking regions for target proteins. Acidic residues are shown in red, basic residues in blue, and hydrophobic residues in black. The central helix in calmodulin remains somewhat flexible, even in the calcium-bound state.

To bind dietary calcium in the gut sodium cellulose phosphate (Calcisorb) is an oral ion exchange substance with a particular affinity for calcium. Bound calcium is eliminated in the faeces. It is used particularly for patients who overabsorb dietary calcium and who develop hypercalciuria and renal stones. 

About 40% of serum calcium is protein bound, with most of it 80%) being bound to albumin. Generally, one or two calcium ions are associated with serum albumin. Albumin serves as a calcium buffer. Jt can bind more calcium ions when excessive concentrations of calcium appear in the bloodstream. About 13% of the calcium in scrum is weakly complexed with phosphate, citrate, and sulfate. About half (47%) of serum calcium occurs as the free calcium ion. The level of free scrum Ca is maintained within narrow limits, 1.0 to 1.25 mM (40 to 50 pg/mJ). The normal concentration of total serum calcium (bound plus free) is 85 to 105 pg/ml. Conditions in which the level of free serum calcium fails below and rises above the normal range are called hypocalcemia and hypercalcemia, respectively. The term ionized calcium is often used to refer to the concentrations of free calcium. This term is not scientifically accurate, because all of the calcium in the body is ionized. Calcium does not engage in the formation of covalent bonds (Cotton and Wilkinson, 1966). 

Chelator-soluble pectins (calcium-bound pectins)... 

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