Muscle structure myosin

Alpha helices are sufficiently versatile to produce many very different classes of structures. In membrane-bound proteins, the regions inside the membranes are frequently a helices whose surfaces are covered by hydrophobic side chains suitable for the hydrophobic environment inside the membranes. Membrane-bound proteins are described in Chapter 12. Alpha helices are also frequently used to produce structural and motile proteins with various different properties and functions. These can be typical fibrous proteins such as keratin, which is present in skin, hair, and feathers, or parts of the cellular machinery such as fibrinogen or the muscle proteins myosin and dystrophin. These a-helical proteins will be discussed in Chapter 14. 

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. 

The muscle is a highly organized tissue, built up of individual cells known as fibres, which are held together by connective tissue. Each muscle fibre consists of a high number of single strands of myofibrils. The myofibrils are again comprised of myofilaments. The myofilaments are divided into thin and thick filaments, which mainly contain two filamentary proteins, actin and myosin, respectively. The myofibrils occupy approximately 80% of the muscle cell volume, and the majority of the water, which makes up about 75% of the muscle, is located in the spaces between thin and thick filaments. A schematic drawing of muscle structure is shown in Fig. 1. 

An individual polypeptide in the a-keratin coiled coil has a relatively simple tertiary structure, dominated by an a-helical secondary structure with its helical axis twisted in a left-handed superhelix. The intertwining of the two a-helical polypeptides is an example of quaternary structure. Coiled coils of this type are common structural elements in filamentous proteins and in the muscle protein myosin (see Fig. 5-29). The quaternary structure of a-keratin can be quite complex. Many coiled coils can be assembled into large supramolecular complexes, such as the arrangement of a-keratin to form the intermediate filament of hair (Fig. 4-1 lb). 

Microtubules in the long axons of nerve cells function as "rails" for the "fast transport" of proteins and other materials from the cell body down the axons. In fact, microtubules appear to be present throughout the cytoplasm of virtually all eukaryotic cells (Fig. 7-32) and also in spirochetes.311 Motion in microtubular systems depends upon motor proteins such as kinesin, which moves bound materials toward what is known as the "negative" end of the microtubule,312 dyneins which move toward the positive end.310 These motor proteins are driven by the Gibbs energy of hydrolysis of ATP or GTP and in this respect, as well as in some structural details (Chapter 19), resemble the muscle protein myosin. Dynein is present in the arms of the microtubules of cilia (Fig. 1-8) whose motion results from the sliding of the microtubules driven by the action of this protein (Chapter 19). 

The idea of conformational coupling of ATP synthesis and electron transport is especially attractive when we recall that ATP is used in muscle to carry out mechanical work. Here we have the hydrolysis of ATP coupled to motion in the protein components of the muscle. It seems reasonable that ATP should be formed as a result of motion induced in the protein components of the ATPase. Support for this analogy has come from close structural similarities of the F, ATPase P subunits and of the active site of ATP cleavage in the muscle protein myosin (Chapter 19). 

Fibrous proteins represent a substantial subset of the human proteome. They include the filamentous structures found in animal hair that act as a protective and thermoregulatory outer material. They are responsible for specifying much of an animal s skeleton, and connective tissues such as tendon, skin, bone, cornea and cartilage all play an important role in this regard. Fibrous proteins are frequently crucial in locomotion and are epitomised by the muscle proteins myosin and tropomyosin and by elastic structures like titin. Yet again the fibrous proteins include filamentous assemblies, such as actin filaments and microtubules, where these provide supporting structures and tracks for the action of a variety of molecular motors. 

The background ideas about muscle structure and the crossbridge cycle, together with some historical perspectives, are discussed in this volume in Squire et al. (2005) and Geeves and Holmes (2005) and also, for example, in Huxley (1969, 2004), Holmes (1997), Geeves and Holmes (1999), and the special Royal Society issue on Myosin, Muscle and Motility (Phil. Trans. Roy. Soc. B. volume 359, pp 1811-1964). 

Myosin filament structure has been described by Squire et al. (2005). In vertebrate striated muscles the myosin filaments can be described approximately as three-stranded 9/1 helices. The helix pitch is 1287 A, but, because there are three strands and nine subunits in each strand, the structure repeats after C = 1287/3 = 429 A. Figure 12 shows the expected form of the low-angle diffraction pattern from such filaments. The modeling of this structure by X-ray diffraction was described by Squire et al. in terms of the three crowns of heads within each 429 A repeat. The crown repeat of 143 A gives rise to an m = +1 meridional reflection, which has been labeled as the M3 reflection in many muscle studies (as in Fig. 12). The myosin head array also gives rise to layer lines at orders of the repeat of 429 A. The first myosin layer line (ML1) is at 1/429 A-1, the second (ML2) at 2/429 = 1/ 214.5 A-1, and so on. The M3 reflection occurs on the third layer line at 3/429 = 1/143 A-1. 

How does this cycle apply to muscle contraction Myosin molecules self-assemble into thick bipolar structures with the myosin heads protruding at both ends of a bare region in the center (Figure 34.19). Approximately 500 head domains line the surface of each thick filament. These domains are paired in myosin dimers, but the two heads within each dimer act independently. Actin filaments associate with each head-rich region, with the barbed ends of actin toward the Z-line. In the presence of normal levels of ATP, most of the myosin heads are detached from actin. Each head can independently hydrolyze ATP, bind to actin, release Pj, and undergo its power stroke. Because few other heads are... 

To understand the physiological nature of muscle contractions, it is helpful to examine muscles microscopically. Muscle fibers have an outside membrane called the plasmalemma, an interior structure called a sar-colemma, transverse tubules across the fibers, and an inner network of muscle tissue called sarcoplasma. When a nerve impulse reaches the muscle, an action potential is set up and the current quickly travels in both directions from the motor end plate through the entire length of the muscle fiber. The whole inside of the muscle tissue becomes involved as the current spreads and, aided by calcium, the contractile protein called actin causes the muscle component (myosin) to contract. An enzyme, ATP-ase, helps provide the energy needed for the muscular filaments to slide past each other. Relaxation occurs promptly when Ca flows into the muscle tissue and the cycle is completed. The muscle fiber is now ready to be stimulated again by a nerve impulse. 

There is an important difference between the macroscopic actin/myosin structure in muscle cells discussed above and the separate actin/myosin threads involved in the locomotion of individual cells. This form of myosin (myosin I) is also quite different, with one head instead of two as in muscle cells (myosin II), further it lacks a tail. Another interesting property of myosin I is its membrane... 

The muscle proteins myosin and tropomyosin also both consist of pairs of identical chains oriented in the same direction. The two 284-residue tropomyosin chains each contain 40 heptads and are linked by a single disulfide bridge. X-ray crystallographic studies and electron microscopy show tiiat the molecule is a rod of 2.0 nm diameter and 41 nm length, the dimensions expected for the coiled coil. However, as with other regular" protein structures, there are some irregularities. Myosin chains (Chapter 19) contain 156 heptads. 

C -Keratin, which is the primary component of wool and hair, consists of two right-handed o helices intertwined to form a type of left-handed superhelix called an a coiled coil, ot-Keratin is a member of a superfamily of proteins referred to as coiled-coil proteins (Figure 2,43). In these proteins, two or more a helices can entwine to form a verv stable structure, which can have a length of 1000 A (100 nm, or 0.1 jiim) or more. There are approximately 60 members of this family in humans, including intermediate filaments, proteins that contribute to the cell cytoskeleton (internal scaffolding in a cell), and the muscle proteins myosin and tropomyosin (Section 34.2). Members of this family are characterized by a central region of 300 amino acids that contains imperfect repeats ol a sequence of seven amino acids called a heptad repeal. 

The susceptibility to conformational changes is also governed by the formation of FA in the fish muscle. Structural changes during frozen storage in cod myosin, for instance, were noted to be induced by the addition of FA (Careche and Li-Chan,... 

It thus seems true to say that the extractability of L-myosin and actin depends solely on the mutual combination of these proteins, and the hindrance to diffusion by the surrounding muscle structures (cf. Dubuisson, 1947). 

The a-helix which is found in proteins which need to be extensible as well as strong, such as the keratins present in hair and other epithelial structures, myosin in muscle and fibrin in clotted blood. 

Rayment, 1., et al. Structure of the actin-myosin complex and its implications for muscle contraction. Science 261 58-65, 1996. 

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