In table 32 stability constants for the binding of ca

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In Table 3.2, stability constants for the binding of Ca 2 +and Mg 2 + to var- ious ligands are collected. We may note that selectivity of Ca 2+ over Mg2+ is not very great for simple carboxylate ligands, but that it tends to increase for large multidentate ligands, such as EDTA and in particular EGTA. The Ca 2 + sites in many intracellular proteins with "EF-hand" binding sites (see Section V. C) bind Ca 2 + about 10 4 times more strongly than Mg 2 +. Another difference in ligand-binding properties of Mg 2+ and Ca2+ can be seen by comparing the rates of substitution of water molecules in the inner / , / , / , / , / .-' / (A) (B) (C) Figure 3.1 The three commonly observed modes of calcium carboxylate ligation. (A) The unidentate mode, in which the calcium ion interacts with only one of the two carboxylate oxygens. (B) The biden- tate mode, in which the calcium ion is chelated by both oxygen atoms. (C) The a-mode, in which the calcium ion is chelated by one carboxylate oxygen, and another ligand is attached to the a-carbon. Adapted from Reference II.
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II. BASIC FACTS ABOUT CALCIUM: ITS COMPOUNDS AND REACTIONS 111 Table 3.2 Ca 2 + and Mg 2+ (where available) stability constants (log K) for different organic and biochemical ligands. Most values are at ionic strength 0.1 and 25°C.5,6,12-15 Ligand Acetate Lactate Malonate Aspartate Citrate Nitrilotriacetate EGTAa EDTA b Glycine (Gly) y-Carboxyglutamic acid (Gla) Gly-Gly dipeptide Gla-Gla dipeptide Macrobicyclic amino cryptate [2.2.2] Fluo-3 Fura-2 BAPTA Quin-2 Phospholipase A2 Thrombin fragment I Trypsinogen Chymotrypsinogen Chymotrypsin Calmodulin, N-terminal Trypsin Calmodulin, C-terminal Protein kinase C a-Lactalbumin Rabbit skeletal muscle Troponin C, Ca 2 + /Mg 2 + sites Carp parvalbumin Bovine calbindin D 9k 0.5 l.l 1.5 1.6 3.5 6.4 10.9 10.6 1.4 1.3 1.2 3.2 4.5 6.2 6.9 7.0 7.1 3.6 3.7 3.8 3.9 4.1 4.5 4.6 5.3 ~7 ~7 7.3 ~8.5 8.8 0.5 0.9 2.1 2.4 3.4 5.5 5.3 8.8 3.4 2.0 2.0 1.8 2.7 3.0 3.3 3.6 4.2 ~4.3 " EGT A: ethylenebis(oxyethylenenitrilo)tetraacetate b EDT A: ethylenedinitrilotetraacetate hydration sphere by simple ligands, according to This rate (log k, with k in s- 1) has been determined to be 8.4 for Ca 2 + and 5.2 for Mg 2 + ,16
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112 3 I CALCIUM IN BIOLOGICAL SYSTEMS The formation of biominerals is a complex phenomenon. In order to obtain a feeling for the conditions under which inorganic solid phases in biological systems are stable, it is of some interest to look at solubility products. Solubility products, K~p, have a meaning only if the composition of the solid phase is specified. For a solid compound with the general composition (A) k(B) ,(C) m the solubility product is defined as (3.1) where [A], [B], etc., denote activities of the respective species, usually ionic, in equilibrium with the solid. Activities are concentrations multiplied by an ac- tivity coefficient, 'Y, nearly always less than unity. Activity coefficients for ions in real solutions can be estimated from Debye-Hiickel theory 17 if the ionic strength of the solution is known. In human blood plasma, the ionic strength, I, is about 0.16, and the activity coefficient for Ca 2 + at 37°C is 0.34. In many discussions it may be sufficient to equate concentrations with activities.
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