Calcium binding protein troponin

Ferm VH, Layton WM Jr (1979) Reduction in cadmium teratogenesis by prior cadmium exposure. Environ Res 18 347-350 Forsen S, Thulin E, Lilja H (1979) Cd NMR in the study of calcium binding proteins troponin C. FEBS Lett 104 123-126 Foulkes EC (1978) Renal tubular transport of cadmium-metallothionein. Toxicol Appl Pharmacol 45 505-512... 

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. 

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.

The calcium mediated contraction of smooth muscle, which unlike striated muscle does not contain troponin, is quite different and requires a particular calcium-binding protein called calmodulin. Calmodulin (CM) is a widely distributed regulatory protein able to bind, with high affinity, four Ca2+ per protein molecule. The calcium—calmodulin (CaCM) complex associates with, and activates, regulatory proteins, usually enzymes, in many different cell types in smooth muscle the target regulatory proteins are caldesmon (CDM) and the enzyme myosin light chain kinase (MLCK). As described below, CaCM impacts on both actin and myosin filaments. 

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... 

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... 

These are soluble, sarcoplasmic calcium-binding proteins found in vertebrates. Invertebrates contain different sarcoplasmic calcium-binding proteins.247 Parvalbumins are low molecular weight (10 000-12 000), acidic proteins which contain two binding sites for Ca2+. The structure of parvalbumin has been determined, and details of the calcium sites are given in Table 6. XANES studies indicate that the calcium sites are similar to those in calmodulin, but different from troponin C.244,248 The two parvalbumin sites have different coordination numbers, six and eight.249... 

Drabikowski, W., Barylko, B., Dabroska, R., Nowak, E., and Szpacenko, A. Studies on the properties of TN-C component of troponin and on its effect on the interaction between the constituents of thin filament. In Calcium binding proteins (eds. W. Drabikowski,... 

Fuchs, F. Chemical properties of the calcium receptor site of troponin as determined from binding studies. In Calcium binding proteins (eds. W. Drabikowski, H. Strzelecka-Golaszewska and E. Carafoli), pp. 1-27. Amsterdam Elsevier Scientific Publ. and Warszawa PWN-Polish Scientific Publ. 1974. 

Collins, J. H., Potter, J. D., Horn, M. J., Wilshire, G., and Jackman, N. Structural studies on rabbit skeletal muscle troponin C Evidence for gene replication and homology with calcium binding proteins from carp and hake muscle. In calcium binding proteins (eds. 

Relaxation of the muscle is brought about by removal of the ionic calcium from the sarcoplasm. This calcium is transported across the membrane of the sarcoplasmic reticulum, in an energy requiring process. In addition to the calcium pumping ATPase, the sarcoplasmic reticulum also contains a calcium binding protein called calsequestrin (Section 4.3.3). Some of the calcium segregated by the sarcoplasmic reticulum is apparently bound to this protein within the lumen of the sarcoplasmic reticulum. As sequestration of calcium ions into sarcoplasmic reticulum proceeds, more calcium ions dissociate from their binding sites on troponin C, re-... 

The contractile proteins of the myofibril include three troponin regulatory proteins. The troponin complex includes three protein subunits, troponin C (the calcium-binding component), troponin I (the inhibitory component), and troponin T (the tropomyosin-binding component). The subunits exist in a number of isoforms. The distribution of these isoforms varies between cardiac muscle and slow- and fast-twitch skeletal muscle. Only two major isoforms of troponin C are found in human heart and skeletal muscle. These are characteristic of slow- and fast-twitch skeletal muscle. The heart isoform is identical with the slow-twitch skeletal muscle isoform. Isoforms of cardiac-specific troponin T (cTnT) and cTnl also have been identified and are the products of unique genes. All cardiac troponins are localized primarily in the myofibrils (94%-97%), with a smaller cytoplasm fraction (3%-6%). 

Figure 6.3. Stractuie and function of myofilaments, a Arrangement of proteins within the filaments, b, c Mechanism of myofilament motion. Calcium binds to troponin, which in tnm causes tropomyosin to move and expose the myosin binding site on actin. Binding to actin canses the myosin heads to kink, which translates into a sliding motion, c The kinked conformation of myosin cleaves ATP. In the process, myosin releases itself from actin and letnms to the extended conformation it then binds to another actin monomer, and the cycle is repeated.

Redesign of Ca fMg Specificity. Proteins in the calmodulin superfamily (including troponin C, parvalbumin (PV) and oncomodulin (OM)) share similar overall structure and yet have different selectivity for Ca and Mg see Calcium-binding Proteins, Cation-activated Enzymes). For example, PV and OM have four helix-tum-helix domains, two of which contain mixed Ca /Mg sites and two Ca -specific sites. The mixed Ca /Mg sites have been converted to Ca -specific sites by replacing amino acids with the corresponding residues in the Ca -specific site. " ... 

Calmodulin, a calcium-binding protein homologous to troponin C (Kakiuchi et al., 1970 Cheung, 1970 Kretsinger and Barry, 1975), has been shown to neutralize the inhibitory action of troponin I (without troponin T) on the actomyosin ATPase only in the presence of Ca " (Amphlett et al., 1976 Dedman et al., 1977 Yamamoto, 1983). The presence of troponin T weakens the neutralizing action of calmodulin (Yamamoto, 1983) Ca or Sr sensitivity of actomyosin ATPase is depressed and shifted to higher concentration. Thus calmodulin cannot replace troponin C under physiological conditions in the presence of troponin I—T. 

Intracellular ionized calcium acts as a second messenger, coupling the action of a hormone or electrical impulse (the first messenger) on the outside of the cell to intracellular events, such as hormone or protein secretion, protein kinase activity, or muscle contraction. The effect of Ca on intracellular processes is often mediated by a small calcium-binding protein, such as troponin C in muscle (Chapter 21) or calmodulin in many other cells (Chapters 15 and 30). Synthesis of these calciumbinding proteins is not directly affected by vitamin D or any of its metabolites. Many stimuli that affect permeability to calcium also activate membrane-bound adenylate cyclase and increase the intracellular concentration of cAMP (Chapter 30). 

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