Calcium ion binding sites

Fig. 13. Location of calcium ion binding site in the Gramicidin A channel —. For comparison, the sodium ion binding site is included...

Calcium ion binding site is 1 to 2 A closer to the aqueous solution and pushed away from the... 

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. 

Coordination chemistry of RE(III) with amino acids has been attracting much interest since the early 1970s after the discovery that certain RE(III) ions could be used as probes of calcium ion binding sites in proteins and enzymes [118, 119], Since then, a large amount of work on the solution and structural chemistry of rare earth-amino acid complexes has been published. The solution studies involve all of the rare earth elements and 13 (Gly, Ala, Val, Leu, Phe, Met, Pro, Ser, Tyr, His, Lys, Trp, and Arg) of the 20 standard amino acids, and more than 100 of the RE(III)-amino acid complexes have been structurally characterized. This section will cover the synthetic, structural, and solution chemistry of these complexes. 

Damall, D.W and Birnbaum, E.R. (1970) Rare earth metal ions as probes of calcium ion binding sites in proteins neodunium(III) acceleration of the activation of trypsinogen. Journal of Biological Chemistry, 245 (23), 6484-6486. 

Two calcium ion binding sites have been identified in the protein, one on the four-fold symmetry axis near the outermost surface of the tetramer, and one approximately 8 A from the active site. Calcium has been shown to increase the catalytic rate of the enzyme, although it is not absolutely necessary for activity [21,22]. Four carbohydrate attachment sites have also... 

Calmodulin is a small protein (Mr -17,000) which contains four calcium ion binding sites (Figure... 

Johnson, J.D., Collins, J.H., and Potter, J.D. (1978) Dansylaziridine labeled troponin C. A fluorescent probe of calcium ion binding to the calcium ion-specific regulatory sites./. Biol. Chem. 253, 6451. 

Urry, D. W. Neutral sites for calcium ion binding to elastin and collagen A charge neutralization theory for calcification and its relationship to arteriosclerosis. Proc. Nat Acad. Sci. (Wash.) 68, 810 (1971)... 

Contrary to the previous case, parvalbumin binds only two Ca ions. Compared to the technical difficulties encountered at the K-absorption edge of calcium due to increased absorption of 3 A wavelengths, the quantitative replacement of Ca by terbium ions (Lj-edge at 1.648 A) offers an ideal way to obtain structural information about the ion binding sites of parvalbumin by resonance scattering experiments. In fact, Miake-Lye, Doniach and Hodgson were able to determine for the first time the distance between the center of mass of the parvalbumin and its two terbium ion binding sites in solution. 

Annexins isolated from chondroblast matrix vesicles may be reconstituted with phospholipids to form calcium ion channels in the complete absence of Ca2+ ions. Indeed, annexin V has domains that directly bind calcium ions glutamate and aspartate residues provide the ion binding site (EF-hand domains). Figure 9.6b illustrates putative annexin V channels that mediate an influx of Ca2+ ions into artificial bilayers and liposomes (detectable with a calcium-sensitive fluorescent dye). These in vitro annexin Ca2+ channels, and also the Ca2+ influx into matrix vesicles in cell culture and in vivo, are blocked by Zn2+ ions, or a derivative of 1,4-benzothiazepine (inhibitor K201). 

The block was intensified by hyperpolarization. The voltage dependence of block indicates that calcium ions bind to sodium channels at a site located 37 + 2% of the electrical distance from the outside. Sodium ions also blocked sodium channels in a voltage-dependent manner, with dissociation constants of 185 and 204 mM at -50 and 0 mV, respectively. A model consisting of a one-ion pore with four barriers and three wells can account for the observations that deviate from the independence principle,... 

Fig. 8. Stereo view of the phospholipid binding site in the third domain of rat annexin V, including calcium ions in sites AB and AB, (a) with bound glycerophospho-serine (GPS), and (b) with bound glycerophosphoethanolamine (GPE). GPS and GPE as well as the calcium ions are drawn dark. The oxygens in the protein fragment are drawn gray. (Redrawn from Swaiijo et al. 1995).

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