Troponin structure

Herzberg, O., James, M.N.G. Structure of the calcium regulatory muscle protein troponin-C at 2.8 A resolution. Nature 313 653-659, 1985. 

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 addition to the major proteins of striated muscle (myosin, actin, tropomyosin, and the troponins), numerous other proteins play important roles in the maintenance of muscle structure and the regulation of muscle contraction. Myosin and actin together account for 65% of the total muscle protein, and tropomyosin and the troponins each contribute an additional 5% (Table 17.1). The other regulatory and structural proteins thus comprise approximately 25% of the myofibrillar protein. The regulatory proteins can be classified as either myosin-associated proteins or actin-associated proteins. 

FIGURE 17.30 (a) A ribbon diagram and (b) a molecular graphic showing two slightly different views of the structure of troponin C. Note the long a-helical domain connecting the N-terminal and C-terminal lobes of the molecule. 

The Ca2+-binding subunit TN-C is homologous to calmodulin with four EF-hands. In contrast to calmodulin, which is ubiquitously expressed in multicellular eukaryotic organisms and interacts with many targets, troponin specifically regulates muscle contraction. There are some structural differences between Troponin C in skeletal and cardiac muscles reflecting their physiological differences. 

The superstructure of smooth muscle actin filaments is differentiated from those of striated muscle by the absence of the troponins and the lateral organization by association of the filaments with dense bodies instead of with the Z-line. How these differences are encoded is again not at all clear. However, the myofibrillar structure and the alignment of the alternating actin and myosin filaments is apparently due primarily to dense bodies and the actin-actinin macrostructures. As the bent dumbbell shaped actins assemble into filaments they are all oriented in the same direction. The S-1 fragments of myosin will bind to actin filaments in vitro and in... 

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. 

Abnormalities of myocardial contractile and structural proteins P-Myosin heavy chains, troponin, tropomyosin, dystrophin... 

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

Upon entering the smooth muscle cell, Ca++ ions bind with calmodulin, an intracellular protein with a chemical structure similar to that of troponin. The resulting Ca++-calmodulin complex binds to and activates myosin kinase. This activated enzyme then phosphorylates myosin. Crossbridge cycling in smooth muscle may take place only when myosin has been phosphorylated. 

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.

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

Zot, A. S. and Potter, J. D. Structural aspects of troponin-tropomyosin regulation of skeletal muscle contraction. Annu. Rev. Biophys. Biophys. Chem. 16 535-560,1987. 

N. A. Malik, G. M. Anatharamaiah, A. Gawish, and H. C. Cheung, Structural and biological studies on synthetic peptide analogues of a low-affinity calcium-binding site of skeletal troponin C, Biochim. Biophys. Acta 911, 221-230 (1987). 

Metals in biological systems function in a number of different ways. Group 1 and 2 metals operate as structural elements or in the maintenance of charge and osmotic balance (Table 1.2). Transition metal ions that exist in single oxidation states, such as zinc(II), function as structural elements in superoxide dismutase and zinc fingers, or, as an example from main group +2 ions, as triggers for protein activity—that is, calcium ions in calmodulin or troponin C... 

Magnesium is also of interest as a replacement for Ca(ll) in calcium-requiring enzymes. In some of these, the replacement is simple (Lewinski and Lebioda, 1986), and in others it cannot occur. NMR studies show that magnesium can bind in the calcium sites of troponin C (Tsuda et al., 1990). The structure of turkey skeletal muscle troponin C has recently been reported (Herzberg and James, 1985). In one domain the replacement of Ca(II) by Mg(II) causes a conformational change, but in the other domain it does not. 

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