EF hand sites

Fig. 13. The binding sites of calcium in (a) parvalbumin (41a), (b) annexin (41) and (c) calmodulin (42). The drawings show two bidentate carboxylates coordinated to Ca2 in the EF-hand site of parvalbumin, and one bidentate carboxylate coordinated to Ca2 in annexin and calmodulin. All the donor atoms coordinated to the calciums are oxygen donor atoms from carboxylates of asp = aspartate, or glu = glutamate, or else peptide carbonyl oxygens from gly = glycine or met = methionine. Redrawn after Refs. (41-42).

Parvalbumin is a muscle protein with a single polypeptide chain of 109 amino acids. Its function is uncertain, but calcium binding to this protein probably plays a role in muscle relaxation. The helix-loop-helix motif appears three times in this structure, in two of the cases there is a calcium-binding site. Figure 2.13 shows this motif which is called an EF hand because the fifth and sixth helices from the amino terminus in the structure of parvalbumin, which were labeled E and F, are the parts of the structure that were originally used to illustrate calcium binding by this motif. Despite this trivial origin, the name has remained in the literature. 

Why does EDTA cause only 90% inhibition, leaving 10% of the activity intact Buffer solutions usually contain 0.1 1 pM of contaminating Ca2+ when special precaution is not taken, and this concentration is much greater than the molar concentration of luciferase used in the experiments. Thus, one of the possibilities would be that Ca2+ interacts with the molecule of luciferase and can increase the activity of luciferase about 10 times, in spite of the fact that the molecule of luciferase lacks the Ca2+ binding site of EF-hand type (Thompson et al., 1989). Another possibility would be that EDTA interacts directly with the molecules of luciferase, to cause the inhibition. The question remains unresolved. 

Calcineurin homologous protein (CHP) is ubiquitously expressed and has four EF-hand domains and one putative site of myristoylation. 

TPR) domains. PP2B (calcineurin) is a heterodimer of a catalytic A-subunit together with a regulatory, Ca2+-binding B-subunit. The A-subunit additionally carries a calmodulin binding site and an autoinhibitory domain. PP7 also contains EF-hand motifs. Both, PP2B and PP7 are stimulated by Ca2+-ions. 

Many calcium-binding proteins can be described in terms of loops containing potential metal-binding sites linked by helices of various shapes and lengths. There are several families of closely related proteins, in particular the groups of EF-hand species and... 

Obelin is a Ca2+-activated bioluminescent photoprotein that has been isolated from the marine polyp Obelia longissima. Binding of calcium ions determines a luminescent emission. The protein consists of 195 amino acid residues [264] and is composed of apoobelin, coelenterazine, and oxygen. As aequorin, it contains three EF-hand Ca2+-binding sites and the luminescent reaction may be the result of coelenterazine oxidation by way of an intramolecular reaction that produces coelenteramide, C02, and blue light. As for aequorin, the luminescent reaction of obelin is sensitive to calcium and the protein was used in the past as an intracellular Ca2+ indicator. More recently, the cloning of cDNA for apoobelin led to the use of recombinant obelin as a label in different analytical systems. 

Figure 11.9 (Left) The EF hand helix-loop-helix motif (centre) rat testes calmodulin. The globular domains each have two Ca2+-binding sites, indicated by white spheres, connected by a seven-turn a-helix (right) two views of the (Ca2+)4 fruit fly calmodulin in complex with its 26-residue target peptide from rabbit skeletal muscle myosin light chain kinase, ((left, centre) From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc. and (right) Carafoli, 2002. Copyright (2002) National Academy of Sciences, USA.)...

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