Calcium-binding proteins structure

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. 

Two types of stabilized coelenterazine are known to exist in biolu-minescent organisms, i.e. the protein-bound form (usually bound to a calcium-binding protein) and the enol ester form (usually enol-sulfate see Structure I, Fig. 5.5). 

Calcium-binding proteins, 6, 564, 572, 596 intestinal, 6, 576 structure, 6, 573 Calcium carbonate calcium deposition as, 6, 597 Calcium complexes acetylacetone, 2, 372 amides, 2,164 amino acids, 3, 33 arsine oxides, 3, 9 biology, 6, 549 bipyridyl, 3, 13 crown ethers, 3, 39 dimethylphthalate, 3, 16 enzyme stabilization, 6, 549 hydrates, 3, 7 ionophores, 3, 66 malonic acid, 2, 444 peptides, 3, 33 phosphines, 3, 9 phthalocyanines, 2,863 porphyrins, 2, 820 proteins, 2, 770 pyridine oxide, 3,9 Schiff bases, 3, 29 urea, 3, 9... 

In striated muscle, there are two other proteins that are minor in terms of their mass but important in terms of their function. Tropomyosin is a fibrous molecule that consists of two chains, alpha and beta, that attach to F-actin in the groove between its filaments (Figure 49-3). Tropomyosin is present in all muscular and muscle-fike structures. The troponin complex is unique to striated muscle and consists of three polypeptides. Troponin T (TpT) binds to tropomyosin as well as to the other two troponin components. Troponin I (Tpl) inhibits the F-actin-myosin interaction and also binds to the other components of troponin. Troponin C (TpC) is a calcium-binding polypeptide that is structurally and functionally analogous to calmodulin, an important calcium-binding protein widely distributed in nature. Four molecules of calcium ion are bound per molecule of troponin C or calmodulin, and both molecules have a molecular mass of 17 kDa. 

Table XI (346-390) lists a number of calcium-binding proteins and indicates very succinctly their role in biological systems. This table both illustrates the range of functions of calcium-binding proteins and serves to introduce those which appear subsequently in this chapter. The structures and functions of particularly important calcium-binding proteins such as calmodulin, parvalbumin, and troponin C are described in standard texts on biochemistry. The minimal Table XI entry for the particularly important calmodulins is amplified in the next paragraph. Table XI provides a sprinkling of references to enable readers to gain entry into the literature, for these and for most of the less-familiar species.

Malisauskas, M., Zamotin, V.,Jass, J., Noppe, W., Dobson, C. M., and Morozova-Roche, L. A. (2003). Amyloid protofilaments from the calcium-binding protein equine lysozyme Formation of ring and linear structures depends on pH and metal ion concentration. / Mol. Biol. 330, 879-890. 

D. M. E. Szebenyi, S. K. Obendorf, and K. Moffat, Structure of vitamin D-dependent calcium-binding protein from bovine intestine, Nature 294, 327-332 (1981). 

Nalefski, E.A., and FaUce, J.J., 1996, The C2 domain calcium-binding motif structural and functional diversity. Protein Sci. 5 2375-2390. 

Kretsinger, R. H. (1972). Gene triplication deduced from the tertiary structure of a muscle calcium binding protein. Nature (London) New Biol. 240,85-88. 

Kretsinger, R. H., and Nockolds, G. E. (1973)., Carp muscle calcium-binding protein. II. Structure determinations and general description./. Biol. Chem. 248,3313-3326. 

Strynadka, N. C. J., and James, M. N. G. (1989). Crystal structures of the helix-loop-helix calcium-binding proteins. Annu. Rev. Biochem. 58, 951-958. 

Spectroscopic, that is, fluorescence and circular dichroism (CD), fourier transform infared (FTIR) measurements have been widely applied to analyzing changes in enzyme structures in an attempt to explain the stabilization or denaturation phenomena associated with enzyme environments, for example, temperature, solvents, and so on. All these measurements can also be performed in ILs where fluorescence and CD spectroscopy demonstrated the conformational changes in the native structure of calcium binding proteins (CALB) which resulted in the higher s)mthetic activity and stability in ILs as compared to those obtained in classical organic solvents [18].The stabilization of enzymes by ILs may be related to the associated structural changes of proteins [19]. 

We now consider two calcium binding proteins, parvalbumin and troponin. The first, carp parvalbumin, is a small protein of known crystal structure.41 It binds two Ca2+ ions per molecule. One of the Ca2+ ions is essential for a folded structure to be maintained the second may be lost with a relatively small conformational change.42... 

Price, P. A., Otsuka, A. S., Poser, J. W. Comparison of 7-carboxyglutamic acid-containing proteins from bovine and swordfish bone Primary structure and Ca++ binding, In calcium binding protein and calcium function, p.333. Wassermann, R. A., Corradino, E., Carafoli,... 

A variety of other calcium transport systems are associated with Ca21-activated ATPases. The extraembryonic structure, the chorioallantoic membrane, of the chick embryo is responsible for the translocation of over 120 mg of eggshell calcium into (he embryo during development. The enzyme responsible for this is a (Ca2+, Mg2+)-ATPase with Km values for Ca2+ of 30 p,mol dm-3 and 0.3 mmol dm-3, and a molecular weight of 170 000. The enzyme can be crossiinked and co-isolated with a calcium-binding protein.158 Transport of Ca2+ is also associated with (Ca2+, Mg2+)-ATPases in neutrophil plasma membranes,159 transverse tubule membranes from rabbit skeletal muscle,160 rabbit myocardial membrane,161 endoplasmic reticulum,162 sar-colemma,163 brain microsomes,164 the Golgi apparatus165 and rat liver plasma membranes.166... 

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