Calcium-binding proteins specific protein

The diverse range of activities found for Ca2+ in biology often involves the binding of Ca2+ to a specific calcium-binding protein. These proteins may be the first receptor sites for intracellular... 

Duplat, D., Puissegur, M., Bedouet, L., Rousseau, M., Boulzaguet, H., Mdet, C., SeDos, D., Van Wormhoudt, A., and Lopez, E. (2006) Identification of calconectin, a calcium-binding protein specifically expressed by the mantle of Pinctada margaritifera. FEES Lett., 580, 2435-2441. 

Calcium-binding protein is not found in the intestinal mucosa of vitamin D-deficient animals. It is synthesized only in response to the presence of a material with vitamin D activity. Thus, using antisemm specific to intestinal calcium-binding protein, a radioimmunodiffusion assay (98) conducted on ... 

Nonrepetitive but well-defined structures of this type form many important features of enzyme active sites. In some cases, a particular arrangement of coil structure providing a specific type of functional site recurs in several functionally related proteins. The peptide loop that binds iron-sulfur clusters in both ferredoxin and high potential iron protein is one example. Another is the central loop portion of the E—F hand structure that binds a calcium ion in several calcium-binding proteins, including calmodulin, carp parvalbumin, troponin C, and the intestinal calcium-binding protein. This loop, shown in Figure 6.26, connects two short a-helices. The calcium ion nestles into the pocket formed by this structure. 

Calcium ions diffuse into the nerve terminal and bind with specific proteins on the vesicular and neuronal membranes. 

Effect of vitamin D on the intestine 1,25-diOH D3 stimulates intestinal absorption of calcium and phosphate. 1,25-diOH D3 enters the intestinal cell and binds to a cytosolic receptor. The 1,25-diOH D3-receptor complex then moves to the nucleus where it selectively interacts with the cellular DNA. As a result, calcium uptake is enhanced by an increased synthesis of a specific calcium-binding protein. Thus, the mechanism of action of 1,25-diOH D3 is typical of steroid hormones (see p. 238). 

Troponin C from rabbit skeletal and bovine cardiac muscle has a molecular weight of about 18 000. Skeletal TN-C has four sites for Ca2+. Two of these (III and IV) are high affinity sites (K < 107 dm3 mol-1) which also bind Mg2+ competitively, with K = 103 dm3 mol-1. The remaining two (I and II) appear to be specific for Ca2+, although of lower affinity (K 105 dm3 mol-1).234 These two sites are the only sites in calcium-binding proteins that do not bind Mg2+ with constants in the range 102-103 dm3 mol-1. Cardiac TN-C contains two Ca2+-Mg2+ sites, one Ca2+-specific site and one low affinity site for Ca2+, in which the two aspartate residues in the skeletal TN-C protein are replaced by leucine and alanine residues.235... 

The next stage involves the synthesis of specific calcium-binding proteins, typified by the intestinal CaBP253 discussed in Section 62.1.3.4.5, which probably stimulates the transport of calcium. The role of the protein in vitamin D-dependent absorption of calcium is supported by the good correlation between the concentration of CaBP and the rate of calcium absorption. Under conditions of low calcium or phosphorus diets, chicks and other animals produce more intestinal CaBP to increase the efficiency of uptake of calcium. In general, adaptation to a low calcium diet involves increased synthesis of l,25-(OH)2D3 and the intestinal CaBP. Lowered requirements for calcium in old age are manifested by lower levels of both factors.449,450... 

Eue I, Sorg C. 2001. Arachidonic acid specifically regulates binding of S100A8/9, a heterodimer complex of the S100 class of calcium binding proteins, to human microvascular endothelial cells. Atherosclerosis 154(2) 505—508. 

However, calcitriol is well established as the most potent agent with respect to stimulation of intestinal calcium and phosphate transport and bone resorption. Calcitriol appears to act on the intestine both by induction of new protein synthesis (eg, calcium-binding protein) and by modulation of calcium flux across the brush border and basolateral membranes by a means that does not require new protein synthesis. The molecular action of calcitriol on bone has received less attention. However, like PTH, calcitriol can induce RANK ligand in osteoblasts and proteins such as osteocalcin, which may regulate the mineralization process. The metabolites 25(OH)D and 24,25(OH)2D are far less potent stimulators of intestinal calcium and phosphate transport or bone resorption. However, 25(OH)D appears to be more potent than l,25(OH)2D in stimulating renal reabsorption of calcium and phosphate and may be the major metabolite regulating calcium flux and contractility in muscle. Specific receptors for l,25(OH)2D exist in target tissues. However, the role and even the existence of receptors for 25(OH)D and 24,25(OH)2D remain controversial. 

Ito D, Imai Y, Ohsawa K, Nakajima K, Fukuuchi Y, Kohsaka S (1998) Microglia-specific localization of a novel calcium binding protein, Ibal. Brain Res Mol Brain Res 57 1-9... 

The entry of calcium into neurons via presynaptic calcium channels is a key step in evoked neurotransmitter release. Compromised calcium channel function can lead to severe neurological consequences, and yet the pharmacological inhibition of specific calcium channel subtypes can be beneficial in the treatment of conditions such as neuropathic pain. Because of the importance of these channels, neurons have evolved complex means for regulating calcium channel activity, including activation of second messenger pathways by G protein coupled receptors and feedback inhibition by calcium binding proteins. By these means, neurons are able to maintain the fine balance of cytoplasmic calcium levels that is required for optimal neurotransmitter release. 

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