Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Muscle Tissue
Chemical Composition of Striated Muscle
Muscle Proteins

A.Ya. Danilevsky was the first to divide Muscle-extracted Proteins into 3 classes: Water-soluble, those extractable with an 8–12% ammonium chloride solution, and proteins extractable with dilute acid and alkali solutions. Currently, Muscle tissue proteins are divided into three main groups: sarcoplasmic, myofibrillar, and stromal proteins. The first group accounts for about 35%, the second for 45%, and the third for 20% of the total muscle protein. These protein groups differ sharply from one another in their solubility in water and ionic media of varying Ionic strength.

Proteins that make up the sarcoplasm belong to proteins soluble in media with low ionic strength. The previously accepted Classification of sarcoplasmic proteins into myogen, globulin X, myoalbumin, and pigment proteins has largely lost its meaning, since the existence of globulin X and myogen as individual proteins is currently denied. It has been established that globulin X is a mixture of various protein substances with globulin properties. The term "myogen" is also a collective concept. In particular, the myogen protein group includes A number of proteins endowed with enzymatic activity, such as glycolytic Enzymes. Sarcoplasmic proteins also include the respiratory pigment Myoglobin and various enzyme proteins located mainly in Cell/35.html">Mitochondria and catalyzing the processes of tissue Respiration, Oxidative Phosphorylation, as well as many aspects of nitrogen and Lipid METABOLISM. Recently, a new group of sarcoplasmic proteins—parvalbumins—has been discovered, which are capable of binding Ca²⁺ ions. Their physiological role remains unclear.

The myofibrillar protein group includes Myosin, Actin, and Actomyosin—proteins soluble in ionic media of high ionic strength—as well as the so-called regulatory proteins: Tropomyosin, troponin, and a- and ß-actinin, which form a single complex with actomyosin in the muscle. These listed myofibrillar proteins are closely associated with the contractile function of Muscles.

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Fig. 20.3. Structure OF THE myosin molecule. Explanation in the text.

Myosin constitutes 50–55% of the dry weight of myofibrils. THE CONCEPT OF myosin as the principal myofibrillar protein emerged from the works of A.Ya. Danilevsky, O. Fürth, E. Weber, and a number of other researchers. However, universal attention to myosin was drawn only after the publication of the works of V.A. Engelhardt and M.N. Lyubimova (1939–1942). These works first demonstrated that myosin possesses ATPase activity, i.e., The ability to catalyze the Cleavage of ATP into ADP and H₃PO₄. The chemical energy of ATP released during this enzymatic reaction is converted into the mechanical energy of the contracting muscle. The Molecular Weight of Skeletal Muscle myosin is about 500,000 (470,000 for rabbit myosin). The myosin molecule (Fig. 20.3) has a highly elongated shape with a length of 150 nm. It can be cleaved without breaking covalent bonds into subunits: two heavy polypeptide chains with a molecular weight of 205,000–210,000 and several short light chains with a molecular weight of about 20,000. The heavy chains form a long coiled a-helix (the "tail" of the molecule), and the end of each heavy chain, together with the light chains, creates a globule (the "HEAD" of the molecule) capable of binding to actin. These "heads" protrude from the main shaft of the molecule. The light chains located in the head of the myosin molecule and participating in the manifestation of myosin ATPase activity are heterogeneous in composition. The number of light chains in the myosin molecule varies among different animal species and in different muscle types.

Brief Treatment with Trypsin cleaves the myosin molecule into two fragments. Light meromyosin (LMM)—a 90 nm long fragment—is formed from the tail region (the C-terminal region of the molecule), while heavy meromyosin (HMM) is formed from the remaining part, which includes the "heads". Similar to myosin, LMM forms filaments, but it lacks ATPase activity and does not bind actin. HMM catalyzes the Hydrolysis of ATP and binds actin. HMM can be further cleaved by prolonged treatment with trypsin or Papain, resulting in one 40 nm long S₂ fragment with a molecular weight of 62,000 and two S₁ fragments with a molecular weight of 110,000, which represent the myosin "heads".

Thick filaments (thick myofilaments) in the sarcomere should be understood as a structure formed by the assembly of A large number of myosin molecules spatially oriented in a specific manner (Fig. 20.4).

Actin, which accounts for 20% of the dry weight of myofibrils, was discovered by F. Straub in 1942. Two forms of actin are known: globular actin (G-actin) and fibrillar actin (F-actin). The G-actin molecule, with a molecular weight of 42,000, consists of a single polypeptide chain (globule) formed by 374 amino acid residues. When the ionic strength increases to the physiological level, G-actin polymerizes into F-actin (the fibrillar form). In electron micrographs, F-actin fibers appear as two strands of beads twisted around each other (Fig. 20.5).

Fig. 20.4. Structure of the thick myosin filament.

Actomyosin is formed by the combination of myosin and F-actin. Actomyosin—both natural and artificial, i.e., obtained in vitro by combining highly purified preparations of myosin and F-actin—possesses ATPase activity, which differs from that of myosin. The ATPase activity of myosin increases significantly in the presence of stoichiometric amounts of F-actin. The actomyosin enzyme is activated by Mg²⁺ ions and inhibited by ethylenediaminetetraacetate (EDTA) and high concentrations of ATP, whereas myosin ATPase is inhibited by Mg²⁺ ions, activated by EDTA, and not inhibited by high concentrations of ATP. The optimal pH values for both enzymes also differ.

As noted, In addition to the main proteins discussed, myofibrils also contain tropomyosin, troponin, and several other regulatory proteins.

Tropomyosin was discovered by K. Bailey in 1946. The tropomyosin molecule consists of two a-helices and has the appearance of a 40 nm long rod; its molecular weight is 65,000. Tropomyosin accounts for about 4–7% of all myofibrillar proteins.

Troponin is a globular protein discovered by S. Ebashi in 1963; its molecular weight is 80,000. In adult animal and human skeletal muscles, troponin (Tn) accounts for only about 2% of all myofibrillar proteins. It consists of three subunits (Tn-I, Tn-C, Tn-T). Tn-I (inhibitory) can inhibit ATPase activity, Tn-C (calcium-binding) has a high affinity for Calcium Ions, and Tn-T (tropomyosin-binding) ensures connection with tropomyosin. By binding to tropomyosin, troponin forms a complex called native tropomyosin. This complex attaches to Actin filaments and confers Ca²⁺ sensitivity on vertebrate skeletal muscle actomyosin (Fig. 20.6).

It has been established that troponin (its Tn-T and Tn-I subunits) can be phosphorylated by cAMP-dependent protein Kinases. The question of whether in vitro troponin phosphorylation is relevant to the Regulation of Muscle contraction remains open.

Stromal proteins in striated muscle are represented mainly by Collagen and Elastin. It is known that the skeletal muscle stroma, which remains after exhaustive extraction of muscle mince with high-ionic-strength salt solutions, consists largely of Connective Tissue elements from vessel and nerve walls, as well as the sarcolemma and certain other structures.

Fig. 20.5. Schematic representation of F-actin.

Fig. 20.6. Structure of the thin filament.

1 - actin; 2 - tropomyosin; 3 - troponin C; 4 - troponin I; 5 - troponin T.