Troponin, in skeletal muscle

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

In skeletal muscle, calcium binds to troponin and causes the repositioning of tropomyosin. As a result, the myosin-binding sites on the actin become uncovered and crossbridge cycling takes place. Although an increase in cytosolic calcium is also needed in smooth muscle, its role in the mechanism of contraction is very different. Three major steps are involved in smooth muscle contraction ... 

Troponin is present only in skeletal muscle it is a complex of three monomeric proteins troponin C, troponin I and... 

An enhancement of ATPase action comes through the phosphorylation of myosin light chains (MW 18,000). The phosphorylation is achieved because the high cellular [Ca2+] activates myosin kinase, an enzyme that contains calmodulin, a Ca2+-binding subunit. Phosphorylation of myosin is absolutely required for smooth muscle contraction, though not for the contraction of skeletal or cardiac muscle, because smooth muscle has no troponin. Thus, whereas contraction and relaxation in skeletal and cardiac muscle are achieved principally via the action of Ca2+ on troponin, in smooth muscle they must depend solely on the Ca2+-dependent phosphorylation of myosin. In skeletal and cardiac muscle, once the stimulus to the sarcolemma is removed, [Ca2+] in sarcoplasm drops rapidly back to 10 7 or 10 8 M via various Ca2+ pump mechanisms present in the sarcoplasmic reticulum, and tropomyosin can once again interfere with the myosin-actin interaction. 

In skeletal muscle in the relaxed state, the sarcoplasm has a high Mg ATP2 -concent rat ion, but the concentration of calcium is below the threshold required for initiation of contraction. The myosin head, under resting conditions is unable to react with actin of the thin filaments because in the absence of calcium the tropomyosin molecule masks the myosin binding site on G-actin monomer or holds it in a conformation that is unreactive, through the action of TN-1 subunit of troponin. One tropomyosin molecule inhibits the myosin binding activity of seven G-actin monomers. 

Cardiac troponin subunits I and T are encoded by different genes than the respective skeletal muscle isoforms and have different amino acid sequences, giving them unique cardiac specificity. cTnl has never been shown to be expressed in normal, regenerating, or diseased human or animal skeletal muscle. By contrast, small amounts of cTnT are expressed as one of four identified isoforms in skeletal muscle during human fetal development, in regenerating rat skeletal muscle, and in diseased human skeletal muscle. cTnT isoform expression has been demonstrated in skeletal muscle specimens... 

McLaurin MD, Apple PS, Voss EM, Herzog CA, Sharkey SW. Cardiac troponin I, cardiac troponin T, and creatine kinase MB in dialysis patients without ischemic heart disease evidence of cardiac troponin T expression in skeletal muscle. Clin Chem 1997 43 976-82. 

The answer is b. (Murray, pp 48-62. Scriver, pp 3-45. Sack, pp 1-3. Wilson, pp 101-120.) Two kinds of interacting protein filaments are found in skeletal muscle. Thick filaments 15 nm in diameter contain primarily myosin. Thin filaments 7 nm in diameter are composed of actin, troponin, and tropomyosin. The thick and thin filaments slide past one another during muscle contraction. Myosin is an ATPase that binds to thin filaments during contraction, ot-actinin can be found in the Z line. 

The distribution of CP along actin filaments has been reported to be similar to that of troponin T in skeletal muscle (Takahashi et al., 1988). This localization is intriguing because the TN-TM system confers coopera-tivity in cross-bridge attachment in striated tissue. However, the function of CP in SM remains unknown and there is no direct evidence that this role affects cross-bridge cycling. CP is also found in cytoskeletal... 

Parvalbumins, water-soluble, monomeric proteins (Mr 12 kDa) with high-affinity sites for Ca +, predominantly found in the skeletal muscle of vertebrates. The parvalbumins also bind Mg + competitively. They occur not only in skeletal muscle of fish and amphibia, but also in mammalian muscle. Parvalbumins are related in structure and function to calmodulin and troponin C. They contain six o -helical regions (A to F), and the binding sites for two Ca + are formed by helix-loop-helix motifs (EF hands) between helices C/D and E/F. Human alpha and beta parvalbumins belong to the EF-hand type proteins [C. H. Heizmann, Experientia 1984, 40,910 U. G. Fohr et al., Eur.]. Biochem. 1993, 215, 719]. 

Calmodulin, an intracellular calcium-combining protein, is involved in many bodily processes such as secretion, activation of myosin kinase, and cyclic nucleotide metabolism. A similar protein, troponin-r, regulates conformational changes in skeletal muscle. The control of skeletal muscle contraction depends entirely on intracellular calcium. Hence those drugs such as nifedepine (Section 14.2) which block calcium channels, have no effect. On the other hand, smooth and cardiac muscles are much influenced by external calcium levels. 

Proteins can be broadly classified into fibrous and globular. Many fibrous proteins serve a stmctural role (11). CC-Keratin has been described. Fibroin, the primary protein in silk, has -sheets packed one on top of another. CoUagen, found in connective tissue, has a triple-hehcal stmcture. Other fibrous proteins have a motile function. Skeletal muscle fibers are made up of thick filaments consisting of the protein myosin, and thin filaments consisting of actin, troponin, and tropomyosin. Muscle contraction is achieved when these filaments sHde past each other. Microtubules and flagellin are proteins responsible for the motion of ciUa and bacterial dageUa. 

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. 

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. ... 

Ca " plays a key role in the initiation of muscle contraction by binding to troponin C. In skeletal mus-... 

Contractile proteins which form the myofibrils are of two types myosin ( thick filaments each approximately 12 nm in diameter and 1.5 (im long) and actin ( thin filaments 6nm diameter and 1 (Am in length). These two proteins are found not only in muscle cells but widely throughout tissues being part of the cytoskeleton of all cell types. Filamentous actin (F-actin) is a polymer composed of two entwined chains each composed of globular actin (G-actin) monomers. Skeletal muscle F-actin has associated with it two accessory proteins, tropomyosin and troponin complex which are not found in smooth muscle, and which act to regulate the contraction cycle (Figure 7.1). 

Tropomyosin is a fibrous molecule which twists around the F-actin strands. The troponin (Tn) complex is composed of three proteins Tnl (I = inhibitory) which prevents myosin binding to actin in the resting muscle, TnT which binds tropomyosin and TnC (C for calcium-binding). Cardiac muscle troponins are different from those of skeletal muscle and are designated cTnl, cTnT and cTnC. 

Smooth muscle differs from skeletal muscle in various ways. Smooth muscles—which are found, for example, in blood vessel walls and in the walls of the intestines—do not contain any muscle fibers. In smooth-muscle cells, which are usually spindle-shaped, the contractile proteins are arranged in a less regular pattern than in striated muscle. Contraction in this type of muscle is usually not stimulated by nerve impulses, but occurs in a largely spontaneous way. Ca (in the form of Ca -calmodulin see p.386) also activates contraction in smooth muscle in this case, however, it does not affect troponin, but activates a protein kinase that phosphorylates the light chains in myosin and thereby increases myosin s ATPase activity. Hormones such as epinephrine and angiotensin II (see p. 330) are able to influence vascular tonicity in this way, for example. 

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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