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the shortening of a muscle that enables it to perform mechanical work. In animals and man, voluntary movements are made through muscle contraction.
Muscle tissue is comprised of 16.5 to 20.9 percent protein, including contractile proteins, which enable a muscle to contract. Of considerable interest are the mechanically active myofibrillar proteins that W. Kühne was the first to study, in 1864. Important data characterizing the physicochemical and biochemical properties of mechanically active muscle proteins were obtained by A. Ia. Danilevskii from 1881 to 1888. In 1939, V. A. Engel’gardt and M. N. Liubimova established that myosin, the main contractile protein of muscles, possesses adenosine triphosphatase (ATPase) activity. In 1942 and 1943, A. Szent-Györgyi and F. B. Straub showed that the protein of myofibrils consists of two components, myosin and actin. It is the interaction of these fibrillar proteins that accounts for contraction in a very great variety of contractile organelles and locomotive muscles.
The periodic change in the physical state of muscle proteins that is responsible for the alternating contraction and relaxation of muscles and for their ability to perform mechanical work is related to certain energy-producing biochemical processes. Between 1939 and 1942, Engel’gardt and Liubimova discovered that the elasticity and extensibility of artificially constructed myosin filaments change markedly when the filaments are allowed to react with adenosine triphosphate (ATP) in solution. ATP splits at the same time to form adenosine diphosphate (ADP) and inorganic phosphate. This discovery suggested a new direction for biochemical research, the mechano-chemistry of muscle contraction. Szent-Gyorgi and Straub subsequently showed that the actual contractile protein is not myosin but a complex of myosin and actin, actomyosin. Muscle fibers soaked in water or 50 percent glycerin contract upon reacting with ATP. Similar contraction occurs in filaments prepared from actomyosin gels that have undergone syneresis. These experiments confirm that the energy required for muscular contraction is liberated during the reaction of actomyosin with ATP, with the latter splitting into ADP and H3PO4. The quantity of energy released is large: 8 to 10 kcal or 33.5 to 41.9 kilojoules (kJ) per mole of ATP. However, the true mechanism of this reaction is still unknown. The terminal phosphate group of ATP in reacting with actomyosin is believed to be transferred to myosin without the intermediate formation of heat but with the formation of a high-energy phosphorylated form of actomyosin. It is this high-energy intermediate that is capable of contracting.
The molecular weight of myosin as determined by ultracentrifugation is almost 500,000. The myosin molecule can be split into smaller subunits without rupturing the covalent bonds. Figure 1 shows the resulting two heavy polypeptide chains with a molecular weight of over 210,000, and the two short, light polypeptides with a molecular weight of about 20,000 each. Other evidence supports the existence of three such short, light sub-units. When viewed through the electron microscope, the myosin molecule appears to consist of two parts, a thick head and a long tail. The overall length of the molecule is about 1600 angstroms (Å).
In striated muscle fiber, many myosin macromolecules are arranged side by side to form thick myosin filaments. The heads of the myosin molecules apparently participate directly in the formation of transverse bridges between the thick myosin and thin actin filaments. The molecular weight of the actin polypeptide chain is close to 46,000 (it was formerly believed to be about 70,000). Its primary structure, that is, the number, identity, and sequence of the amino acid radicals, has also been established. The fibrillar actin, or F-actin, molecule can be compared to a double-stranded bead necklace. Each strand forms a spiral, with each individual bead corresponding to one globular actin, or G-actin, monomer. The F-actin filaments in the sarcomeres of striated muscle fiber are spatially separate from the myosin filaments. The interaction of the two types of filament systems is accomplished by means of the energy liberated during the splitting of ATP in the presence of Ca2+ ions. Since ATP is constantly utilized during muscular work, prolonged, two-phase muscular activity requires the continuous resynthesis of ATP. This resynthesis from ADP and H3PO4 involves several energy-producing transformations. The most important of these are (1) transfer of the phosphate group from phosphocreatine to ADP (this reaction induces the rapid production of ATP at the expense of phosphocreatine during contraction of the muscle), (2) glycogenolysis or glycolysis (breakdown of glycogen or glucose with the formation of lactic acid) and (3) tissue respiration (formation of ATP in the mitochondria of muscle fibers using the energy released in the oxidation mostly of carbohydrates, fatty acids, and unsaturated phospholipids). Some ATP can also be formed from ADP as a result of the myokinase reaction: 2 ADP ≶ AMP + ATP. Phosphorylation of creatine at the expense of ATP resulting in the formation of phosphocreatine is accomplished during glycolysis and tissue respiration. Phosphocreatine and glycogen are resynthesized mainly during the resting phase after the muscle relaxes. Skeletal muscle under anaerobic or hypoxic conditions can perform some work. However, in such cases fatigue sets in much sooner than in the presence of oxygen and an accumulation of lactic acid in the muscle is the result.
A. V. Palladin, D. L. Ferdman, N. N. Iakovlev, and others studied the biochemistry of muscle training and development. S. E. Severin demonstrated the ability of the dipeptides carnosine and anserine to restore the efficiency of fatigued muscles and to promote the transmission of nerve impulses from nerve to muscle.
After contracting in response to stimulation from a nerve or an electric current, a muscle quickly relaxes even though the ATP content of the fibers barely changes. Myofibrils are capable of reacting with ATP and contracting in its presence only if Ca2+ ions are in the medium. Maximum contractility occurs at a Ca2+ concentration of about 10-6 to 10-5 moles per liter (M). When the Ca2+ concentration decreases to 10-7 M or less, the
muscle fibers lose their capacity for contraction and tensile strength in the presence of ATP. It is currently held that the concentration of Ca2+ ions is maintained in resting muscle below this threshold because the ions are bound to the tubules and vacuoles of the sarcoplasmic reticulum. The binding does not take pace through simple adsorption, but, rather, through an active physiological process effected by the energy released during the splitting of ATP in the presence of magnesium ions. This mechanism is called the calcium pump (by analogy with the sodium pump). Thus, the relaxation of live muscle in the presence of sufficient ATP is brought about by the calcium pump, which decreases the concentration of Ca2+ ions in the medium surrounding the myofibrils to below the limit at which the manifestation of ATPase activity and the contraction of the actomyosin structures of the fiber are still possible.
Contraction of the fiber in response to stimulation from a nerve or from an electric current is the result of a sudden change in permeability that permits escape of the Ca2+ ions from the cisterns and tubules of the sarcoplasmic reticulum and the T system into the interfibrillar space. The transverse tubules of the T system, which are situated at the level of the sarcomeric end plates and contain Ca2+ ions, communicate with the surface membrane of the fiber. The wave of depolarization quickly spreads through this system of tubules and reaches the deep portions of the fiber. After the nerve impulse is extinguished by the action of the calcium pump, the Ca2+ concentration in the interfibrillar space quickly decreases to the threshold value and the muscle returns to its original relaxed state until a new impulse elicits a repetition of the entire cycle.
The loss of the capacity of actomyosin to split ATP and contract upon a decrease in concentration of the Ca2+ ions below 10-7 M is related to the presence of a special protein, troponin, in the contractile system. If troponin is absent, actomyosin can still react in vitro with ATP in the virtual absence of Ca2+. Under physiological conditions, troponin is present as a constant component of the contractile system of muscle fiber in the form of the troponin-tropomyosin complex.
The two-phase model of muscle contraction applies to many muscle types. In some insects, for example, beetles, bees, flies, and mosquitoes, the frequency of contraction of the wing muscles is much higher than the frequency of incoming nerve impulses. These muscles respond to a myogenic rather than neurogenic rhythm. They may oscillate hundreds of times per second. The oscillation of these muscles is unrelated to a change in the Ca2+ concentration in the sarcoplasm of the muscle fibers. Automatic, two-phase activity of locomotor cell organelles may be observed on the cellular level: in spermatozoa, ciliated epithelium, and undulating membranes of trypanosomes, to name a few. The locomotor organelles oscillate at a rate characteristic of the particular kind of cell, provided that the Ca2+ concentration remains constant. The oscillation continues as long as a slight surplus of ATP in the solution exists.
Clearly, the mechanism of oscillation in the locomotor organelles and in myofibrils can be understood only in light of the relationship existing between the enzymatic activity (capacity to split ATP) and the conformational state of the macromolecules of the contractile substrate.
REFERENCESSee references under .
I. I. IVANOV