Contractile proteins: functions, examples. Structure of skeletal muscles See what “actin” is in other dictionaries

The main muscle proteins are actin and myosin

The mass of muscle fibrils consists of water (75%) and proteins (more than 20%). The main representatives of muscle proteins are actin and myosin, of which myosin accounts for 55%.

This protein (MW 460 kDa) has the form of an asymmetric hexamer. The myosin molecule has an elongated part, consisting of two helices wound one on top of the other. Each helix has a globular head at one end. The hexamer (6 subunits) includes one pair of heavy chains (MW 200 kDa) and two pairs of light chains (MW 15-27 kDa). Heavy chains consist of a linearly elongated, a-helical C-terminal domain (1300 amino acid residues) and a globular N-terminal domain (about 800 amino acid residues). Two a-helical domains belonging to two heavy chains form together a stable superhelical structure with two globular heads (Fig. 17.8).

The complete myosin molecule also contains 4 relatively small polypeptide chains (MW 16-24 kDa), which are associated with globular heads. Unlike actin, myosin skeletal muscles has enzymatic activity and catalyzes the hydrolysis of ATP by binding to F-actin. All light chains bind Ca 2+, are phosphorylated by a special kinase and, in general, take part in the regulation of the activity of myosin ATPase.

Fig.17.8. Schematic representation of the structure of thick filaments. The spatial configuration of myosin is shown.

The myosin molecule contains several functionally important regions. Not far from the middle of the linear supercoiled zone there is a place where the molecule is broken down by trypsin. This enzyme, as it were, cuts the molecule into 2 parts: one contains globular heads and some part of the supercoiled zone; the other consists of the remaining portion of the supercoiled zone at the C-terminus. The part containing the head is called “heavy meromyosin” (MW 350 kDa). The C-terminal fragment is called “light meromyosin” (MW 125 kDa).

The significance of the site of action of trypsin on the myosin molecule is that it surprisingly coincides with the place in the myosin molecule that works as a kind of hinge, converting the chemical energy of ATP into a purely mechanical phenomenon of contraction - relaxation. Another important site that performs a similar role is subject to the action of another proteolytic enzyme, papain. Papain cuts the myosin molecule very close to the globular heads. It turns out two fragments and it is the one where the head is located that exhibits ATPase activity.

Thick filaments are formed from myosin. The thick filament consists of approximately 400 myosin molecules, 200 on each side of the M line. These molecules are held together by the C protein (the “clip” protein), the M-line protein, and hydrophobic interactions with each other. At a point localized at the site of trypsin action, heavy meromyosin deviates from the main axis of the thick filament, forming sharp corner. Due to this, the head closely approaches the actin of thin filaments, localized in the space between the thick filaments. The most important molecular event preceding muscle contraction is the regulated binding of myosin heads to thin filament actin. Subsequently it happens rapid change myosin conformation around the already mentioned peculiar “hinge” points, and the bound actin moves in the direction of the M-line.

The share of actin in the total mass of muscle proteins is 25%. This is a globular protein monomer with a molecular weight of 43 kDa, called G-actin. In the presence of magnesium ions and the physiological concentration of ions in solution, G-actin polymerizes to form an insoluble filament, which is called F-actin (Fig. 17.9). Two F-actin polymers wrap around each other in a helix. This is how the basic structure of the thin thread is formed. The F-actin fiber has a thickness of 6–7 nm and a repeating structure with a periodicity of 35.5 nm. Neither G- nor F-actin have any catalytic activity.

Rice. 17.9. Structure of F-actin

Each G-actin subunit has an ATP/ADP binding site, which takes part in the polymerization of the thin filament. After polymerization is completed, the thin filament is coated and stabilized by a protein - b-actinin. In addition to the nucleotide binding site, each G-actin molecule has a high-affinity myosin head binding site. Its work in skeletal and cardiac muscles is regulated by additional thin filament proteins. Thus, additional proteins control the contractile cycle.

Actin and myosin

The interest of biochemistry in the processes occurring in contracting muscles is based not only on elucidating the mechanisms of muscle diseases, but what may be even more important is revealing the mechanism for converting electrical energy into mechanical energy, bypassing the complex mechanisms of traction and transmission.

In order to understand the mechanism and biochemical processes occurring in contracting muscles, it is necessary to look into the structure of the muscle fiber. The structural unit of muscle fiber is Myofibrils - specially organized bundles of proteins located along the cell. Myofibrils, in turn, are built from protein threads (filaments) of two types - thick and thin. The main protein of thick filaments is myosin, and the main protein of thin filaments is actin. Myosin and actin filaments are the main component of all contractile systems in the body. Electron microscopic examination showed a strictly ordered arrangement of myosin and actin filaments in the myofibril. The functional unit of the myofibril is the sarcomere - the section of the myofibril between the two Z-plates. The sarcomere includes a bundle of myosin filaments, linked in the middle along the so-called M-plate, and actin filament fibers passing between them, which in turn are attached to the Z-plates.

Studying the structure of muscle fibers in a light microscope revealed their transverse striations. Electron microscopic studies have shown that cross-striations are due to the special organization of the contractile proteins of myofibrils - actin (molecular weight 42,000) and myosin (molecular weight about 500,000). Actin filaments are represented by a double filament twisted into a double helix with a pitch of about 36.5 nm. These filaments are 1 µm long and 6-8 nm in diameter, the number of which reaches about 2000, and are attached at one end to the Z-plate. Filament-like molecules of the protein tropomyosin are located in the longitudinal grooves of the actin helix. In increments of 40 nm, a molecule of another protein, troponin, is attached to the tropomyosin molecule. Troponin and tropomyosin play an important role in the mechanisms of interaction between actin and myosin. In the middle of the sarcomere, between the actin filaments, there are thick myosin filaments about 1.6 µm long. In a polarizing microscope, this area is visible as a strip of dark color (due to birefringence) - an anisotropic A-disk. A lighter stripe H is visible in its center. At rest, there are no actin filaments in it. On both sides of the A-disk, light isotropic stripes are visible - I-disks, formed by actin filaments. At rest, the actin and myosin filaments overlap each other slightly so that the total length of the sarcomere is about 2.5 μm. Electron microscopy revealed an M-line in the center of the H-band - a structure that holds myosin filaments. A cross section of a muscle fiber shows the hexagonal organization of the myofilament: each myosin filament is surrounded by six actin filaments.

Electron microscopy shows that on the sides of the myosin filament there are protrusions called cross bridges. They are oriented relative to the axis of the myosin filament at an angle of 120°. According to modern concepts, the transverse bridge consists of a head and a neck. The head acquires pronounced AT-phase activity upon binding to actin. The neck has elastic properties and is a hinged joint, so the head of the cross bridge can rotate around its axis. myosin actin biochemistry

The use of microelectrode technology in combination with interference microscopy has made it possible to establish that applying electrical stimulation to the Z-plate region leads to a contraction of the sarcomere, while the size of the A disk zone does not change, and the size of the H and I stripes decreases. These observations indicated that the length of myosin filaments does not change. Similar results were obtained when the muscle was stretched - the intrinsic length of actin and myosin filaments did not change. As a result of these experiments, it became clear that the area of ​​mutual overlap of actin and myosin filaments changed. These facts allowed N. Huxley and A. Huxley to independently propose the theory of thread sliding to explain the mechanism of muscle contraction. According to this theory, during contraction, the size of the sarcomere decreases due to the active movement of thin actin filaments relative to thick myosin filaments. Currently, many details of this mechanism have been clarified and the theory has received experimental confirmation.

Actin- a muscle tissue protein that, together with another protein - myosin - forms actomyosin - the main component of the contractile filaments of muscle fibers.

Actin is a globular structural protein. Molecular weight 42000 Da. There are two forms: globular and fibrillar, formed during the polymerization of globular actin in the presence of ATP and magnesium ions. Each actin molecule has regions that are complementary to certain regions on the heads of myosin molecules and are able to interact with them to form actomyosin, the main contractile protein of muscles. 1 cm of muscle contains about 0.04 g of actin. The actin-myosin system is common to the contractile structures of both vertebrates and invertebrates. In cyosol, actin is mainly associated with ATP, but can also bind with ADP. The ATP-actin complex polymerizes faster and dissociates more slowly than the actin-ADP complex. Actin is one of the most abundant proteins in many eukaryotic cells, with concentrations greater than 100 μM. It is also one of the best preserved proteins, differing by no more than 5% between organisms such as algae and humans.

Microfilaments are threads of the actin protein of a non-metastatic nature in the cytoplasm of eukaryotic cells. Diameter 4...7nm. Under the plasma membrane, microfilaments form plexuses; in the cytoplasm, cells form bundles of parallel oriented filaments or a three-dimensional gel, forming the cytoskeleton. They include, in addition to actin, other contractile proteins myosin, tropomyosin, actinin, which differ from the corresponding muscle proteins, as well as specific proteins (vinculin, fragmin, filamin, etc.). Microfilaments are in dynamic equilibrium with actin monomers. Microfilaments are contractile elements of the cytoskeleton and are directly involved in changing the shape of the cell during flattening, attached to the substrate, amoeboid movement, endomitosis, cyclosis (for plant cells), formation of a cytotomy ring in animal cells, maintenance of microvilli in invertebrate intestinal cells. Some membrane receptor proteins are indirectly attached to microfilaments.

Myosin is a muscle tissue protein that, together with another protein, actin, forms actomyosin, the main component of the contractile filaments of muscle fibers. Myosin is a globular structural protein.

The myosin molecule consists of two parts: a long rod-shaped section (“tail”) and a globular section attached to one of its ends, represented by two identical “heads”. Myosin molecules are arranged in the myosin filament in such a way that the heads are regularly distributed along its entire length, except for a small middle section where they are not present (the “naked” zone). Where actin and myosin filaments overlap, myosin heads can attach to adjacent actin filaments, and muscle contraction can occur as a result of this interaction.

The energy to perform such work is released through the hydrolysis of ATP; All myosin heads exhibit ATPase activity; the attachment of myosin heads depends on the concentration of Ca2 + ions in the sarcoplasm. Myosin ATPase is activated by the interaction of actin with myosin. Mg2+ ions can inhibit this process.

References

• 1. G. Dugas, K. Penny “Bioorganic chemistry”, M., 1983
• 2. D. Metzler “Biochemistry”, M., 1980
• 3. A. Leninger “Fundamentals of Biochemistry”, M., 1985

BIOCHEMISTRY OF SPORTS

Structure and function of muscle fiber

There are 3 types of muscle tissue:

Striated skeletal;

Striated heart;

Smooth.

Functions of muscle tissue.

Striated skeletal tissue - makes up approximately 40% of the total body weight.

Its functions:

dynamic;

static;

receptor (for example, proprioceptors in tendons - intrafusal muscle fibers (fusiform));

depositing - water, minerals, oxygen, glycogen, phosphates;

thermoregulation;

emotional reactions.

Striated cardiac muscle tissue.

The main function is injection.

Smooth muscle - forms the wall of hollow organs and blood vessels.

Its functions: - maintains pressure in hollow organs; - maintains blood pressure;

Ensures the movement of contents through the gastrointestinal tract and ureters.

Chemical composition of muscle tissue

The chemical composition of muscle tissue is very complex and changes under the influence of various factors. Average chemical composition well-prepared muscle tissue is: water - 70-75% of the tissue mass; proteins - 18-22%; lipids - 0.5-3.5%; nitrogenous extractives - 1.0-1.7%; nitrogen-free extractives - 0.7-1.4%; minerals - 1.0-1.5%.

About 80% of the dry residue of muscle tissue consists of proteins, the properties of which largely determine the properties of this tissue.

MYOFIBRILS - contractile elements of muscle fiber. Fine structure of myofibrils

Myofibrils are thin fibers (their diameter is 1-2 microns, length 2-2.5 microns), containing 2 types of contractile proteins (protofibrils): thin filaments of actin and twice as thick filaments of myosin. They are arranged in such a way that there are 6 actin filaments around the myosin filaments, and 3 myosin filaments around each actin filament. Myofibrils are divided by Z-membranes into separate sections - sarcomeres, in the middle part of which there are predominantly myosin filaments, aactin filaments are attached to Z-membranes on the sides of the sarcomere. (The different abilities of actin and myosin to refract light give muscle a striated appearance in a light microscope when it is at rest.)

Actin filaments make up about 20% of the dry weight of myofibrils. Actin consists of two forms of protein: 1) globular form - in the form of spherical molecules and 2) rod-shaped tronomyosin molecules, twisted in the form of double-stranded helices into a long chain. Along this double actin filament, each turn contains 14 molecules of globular actin (7 molecules on both sides), like a string of beads, as well as Ca2+ binding sites. These centers contain a special protein (troponin) that is involved in the formation of actin-myosin bonds.

Myosin is composed of parallel protein filaments (this part is the so-called light meromyosin). At both ends there are necks extending to the sides with thickenings - heads (this part is heavy meromyosin), thanks to which cross bridges are formed between myosin and actin.

Physicochemical characteristics and structural organization of contractile proteins (myosin and actin). Tropomyosin and troponin.

Myofibrillar proteins include the contractile proteins myosin, actin, and actomyosin, as well as the regulatory proteins tropomyosin, troponin, and alpha and beta actin. Myofibrillar proteins provide muscle contractile function.

Myosin is one of the main muscle contractile proteins, making up about 55% of the total muscle proteins. It consists of thick threads (filaments) of myofibrils. The molecular weight of this protein is about 470,000. The myosin molecule has a long fibrillar part and globular structures (heads). The fibrillar part of the myosin molecule has a double-helix structure. The molecule consists of six subunits: two heavy polypeptide chains (molecular weight 200,000) and four light chains (molecular weight 1500-2700), located in the globular part. The main function of the fibrillar part of the myosin molecule is the ability to form well-ordered bundles of myosin filaments or thick protofibrils. The active center of ATPase and the actin-binding center are located on the heads of the myosin molecule, so they ensure ATP hydrolysis and interaction with actin filaments.

Actin is the second contractile muscle protein that forms the basis of thin filaments. Two of its forms are known: globular G-actin and fibrillar F-actin. Globular actin is a spherical protein with a molecular weight of 42,000. It accounts for about 25% of the total mass of muscle protein. In the presence of magnesium cations, actin undergoes noncovalent polymerization to form an insoluble filament in the form of a helix, called F-actin. Both forms of actin do not have enzymatic activity. Each G-actin molecule is capable of binding one calcium ion, which plays an important role in initiating contraction. In addition, the G-actin molecule tightly binds one molecule of ATP or ADP. The binding of ATP by G-actin is usually accompanied by its polymerization with the formation of F-actin and the simultaneous cleavage of ATP to ADP and phosphate. ADP remains bound to fibrillar actin.

Tropomyosin is a structural protein of the actin filament, which is an elongated molecule in the form of a strand. Its two polypeptide chains seem to wrap around actin filaments. At the ends of each tropomyosin molecule there are proteins of the troponin system, the presence of which is characteristic of striated muscles.

Troponin is an actin filament regulatory protein. It consists of three subunits: TnT, Tnl and TnS. Troponin T (TnT) mediates the binding of these proteins to tropomyosin. Troponin I (Tnl) blocks (inhibits) the interaction of actin with myosin. Troponin C (TnC) is a calcium-binding protein with a structure and function similar to the widely occurring naturally occurring protein calmodulin. Troponin C, like calmodulin, binds four calcium ions per protein molecule and has a molecular weight of 17,000. In the presence of calcium, the conformation of troponin C changes, which leads to a change in the position of Tn in relation to actin, resulting in the opening of the center of interaction between actin and myosin.

Thus, the thin filament of the striated muscle myofibril consists of F-actin, tropomyosin and three troponin components. In addition to these proteins, the protein actin is involved in muscle contraction. It is found in the Z-line zone, to which the ends of the F-actin molecules of the thin filaments of myofibrils are attached.

Of course, the main function of a smooth muscle cell is contraction. Contractile proteins are primarily responsible for the implementation of this function - actin And myosin . The interaction between actin and myosin is regulated by a number of processes, which are discussed in the chapter “Regulation of Contraction”.

Actin

The actin protein is an important component of the cell cytoskeleton and is found in almost all animal and plant cells. Actin got its name due to its ability to activate ATP hydrolysis. Actin myofilaments - have a length of more than 1 micron, a thickness of 3-8 nm, and are attached to dense bodies. About 12 actin filaments surround the myosin filaments in a rosette pattern. Actin microfilaments are composed of globular subunits G-actin - actin monomers (diameter 5.6 nm and molecular weight 42,000 daltons), which polymerize into fibrillar F-actin . Actin is formed by helically intertwined chains of F-actin.

The process of polymerization of globular subunits of G-actin is possible due to the ability of actin to form intermolecular contacts after hydrolysis of ATP to ADP and inorganic phosphate. Actin monomers assemble into a polymer in a specific order, with actin polymerization being initiated by activation of contraction. The process of actin polymerization and depolymerization is regulated by special proteins. For example, there is a special protein, profilin, which, by forming a complex with globular actin, counteracts actin polymerization. There are special proteins (for example, cytochalasin D) that bind to actin and “cap” it, i.e. form a kind of cap at one end of the polymerizing actin, thereby regulating the polymerization process. There are proteins (latrunculin A) that prevent the polymerization of globular actin and proteins that “cut” actin filaments into short fragments. Conversely, there are proteins that “cross-link” already formed actin filaments, forming ordered rigid bundles of actin filaments or large-mesh flexible networks (Fig. 3). .

In vertebrate tissues, 6 actin isoforms were found, which are derivatives of various genes and differ in amino acid sequence. The α-isoform is present in vascular smooth muscle cells, and the γ-isoform of actin is present in the smooth muscles of the gastrointestinal tract.

Fig.3. Fibrillar F-actin (a). Scheme of the process of polymerization and depolymerization of actin filaments (b). P – inorganic phosphate.

Myosin

Currently, more than ten different isoforms of myosin have been discovered. Skeletal muscle myosin has been studied in most detail. Smooth muscle has its own isoforms of myosin.

Myosin filaments - have a length of about 0.5 µm and a thickness of 12-15 nm; they consist of several molecules of myosin monomers. Smooth muscle myosin belongs to class II myosins, the so-called classical myosin, which consists of two heavy chains (with a molecular weight of 200 - 250 kDa, a length of 150 nm and a thickness of 1.52 nm). The myosin molecule is composed of meromyosin subunits: 1) light meromyosin, which forms the rod or tail of the myosin filament; and 2) heavy meromyosin, consisting of the S-1 fragment, which forms the head, and the S-2 fragment (hinge region), which is adjacent to the rod of the myosin filament and connects the S-1 fragment to the light meromyosin subunit (Fig. 4). The tendency of monomer tails to interact with each other in an orderly manner underlies the formation of filaments. On the myosin head there are two light chains - regulatory and main, with a molecular weight of 18 - 28 kDa, which are involved in the interaction of myosin with actin. It is hypothesized that in the absence of Ca 2+ ions, the light chains are wrapped around the hinge region of the myosin heavy chain, which significantly limits its mobility. In this state, the myosin head is unable to move relative to the actin filament. In the presence of Ca 2+ ions, mobility in the head region increases sharply and after ATP hydrolysis, the myosin head can move along the actin filaments.

Fig.4. The structure of the myosin macromolecule (explanation in the text).

Myosin filaments in a smooth muscle cell are not always detectable under a microscope, so they are believed to be formed and reversibly disintegrated with each contraction of the smooth muscle. Smooth muscle myosin differs significantly from skeletal myosin in that in the presence of physiological concentrations of ATP it is in the so-called folded (10S) conformation. In this conformation, a portion of the myosin monomer approximately 1/3 of the way from the end of the tail interacts with the neck region). In this case, intramolecular interactions in smooth muscle myosin prevail over intermolecular ones, tail association does not occur, and the equilibrium is shifted towards monomeric myosin. Myosin molecules enter the polymerization reaction in an unfolded (6S) conformation (Fig. 5). Smooth muscle myosin polymerizes into filaments when its light chain is phosphorylated by a special enzyme, myosin light chain kinase, or when it interacts with the KRP (kinase-related protein) protein.

Well-studied skeletal muscle myosin filaments assemble into bipolar dumbbell-shaped filaments, in which the myosin heads are located radially around the filament axis on both sides, while the central part of the molecule does not contain heads. Unlike skeletal ones, smooth muscle myosin filaments have lateral polarity, i.e. the heads of myosin molecules are located in the same plane on both sides of the filament along its entire length and have the opposite orientation (Fig. 5).

The rate of dissociation of dimers from the filament is directly proportional to its length, so the growth of bipolar myosin filaments in skeletal muscles is self-limiting. This effect is not observed in smooth muscle myosin filaments (which have lateral polarity) and therefore they can change their length over a wide range due to the equivalent addition of new myosin molecules, allowing the movement of actin filaments over long distances. Most likely, a similar organization of smooth muscle myosin filaments underlies the ability of smooth muscles to develop significant shortening.

Fig.5. Model of smooth muscle myosin filament. A – folded conformation, B – unfolded conformation, C – polymerized smooth muscle myosin, D – polymerized skeletal muscle myosin.

Smooth muscle cells lack the protein troponin; instead, a structurally similar protein is present in the sarcoplasm calmodulin . Ca 2+ realizes most of its physiological functions by interacting with specific Ca 2+ binding proteins, which perform both Ca 2+ sensing and regulatory functions. This protein in smooth muscle cells is calmodulin. In essence, calmodulin is involved in all Ca 2+ -dependent processes in the cell. The total intracellular concentration of calmodulin in the cell is significantly lower than the total concentration of its intracellular targets, which allows it to be a kind of limiting regulatory factor. The Ca 2+ /calmodulin complex is required for activation of myosin light chain kinase and initiation of contraction. On the other hand, Ca 2+ /calmodulin-dependent protein phosphatase initiates dephosphorylation of myosin light chains, which leads to relaxation. Ca 2+ /calmodulin-dependent protein kinase II, present in smooth muscle cells, is a mediator of many Ca 2+ -dependent intracellular signaling pathways.

Function tropomyosin in the absence of troponin in a smooth muscle cell is not entirely clear, however, there is now experimental evidence of the participation of tropomyosin in the regulation of the cycle of cross-bridge formation and in the process of inhibition of the ATPase activity of actomyosin by caldesmon.

Thus, if we compare the contractile apparatus of a smooth muscle cell with skeletal muscle, it can be noted that the distinctive structural features are: 1) the absence of a sarcomere; 2) inadequate ratio of actin and myosin filaments in the resting state: there are significantly more actin filaments; 3) actin filaments are longer than in skeletal muscle; 4) an analogue of the Z-line is dense bodies and dense plaques; 5) an analogue of troponin C is a protein calmodulin; 6) an analogue of the T-tubule – caveolae; 7) the sarcoplasmic reticulum in the smooth muscle cell is less developed than in the skeletal cell.

Microfilaments(actin filaments) consist of actin, a protein most abundant in eukaryotic cells. Actin can exist as a monomer ( G-actin, “globular actin”) or polymer (F-actin, “fibrillar actin”). G-actin is an asymmetric globular protein (42 kDa), consisting of two domains. As ionic strength increases, G-actin reversibly aggregates to form a linear, coiled-coil polymer, F-actin. The G-actin molecule carries a tightly bound ATP molecule (ATP), which, when converted to F-actin, is slowly hydrolyzed to ADP (ADP), that is, F-actin exhibits the properties of an ATPase.

When G-actin polymerizes into F-actin, the orientation of all monomers is the same, so F-actin has polarity. F-actin fibers have two oppositely charged ends - (+) and (-), which polymerize at different rates. These ends are not stabilized by special proteins (as, for example, in muscle cells), and at a critical concentration of G-actin, the (+) end will lengthen and the (-) end will shorten. Under experimental conditions, this process can be inhibited by fungal toxins. For example, phalloidin(venom of the toadstool) binds to the (-)-end and inhibits depolymerization, while cytochalasin(a toxin from mold fungi with cytostatic properties) attaches to the (+) end, blocking polymerization.

Actin-associated proteins. There are more than 50 different types of proteins in the cytoplasm of cells that specifically interact with G-actin and F-actin. These proteins perform different functions: they regulate the volume of the G-actin pool ( profilin), influence the rate of G-actin polymerization ( villin), stabilize the ends of F-actin filaments ( fragin, β-actinin), stitch the filaments together or with other components (such as villin, α-actinin, spectrin, MARCKS) or destroy the F-actin double helix ( gelsolin). The activity of these proteins is regulated by Ca 2+ ions and protein kinases.

Articles in the section “Cytoskeleton: composition”:

• A. Aktin

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