Biochemical Foundations of Human Vital Activity - Volkov, N. I., & Nesen, E. N. 2000

Biochemistry of Sports
Biochemistry of Muscle and Muscle Contraction
Chemical Composition of Muscle Tissue

Human Muscle tissue contains 72–80% Water and 20–28% dry matter by muscle mass. Water is a component of most Cellular Structures and serves as a solvent for numerous substances. Proteins and other Organic compounds make up the major part of the dry matter (Table 21).

Class="center">TABLE 21. Chemical composition of mammalian skeletal Muscles (average values)

Principal Muscle Proteins

Three main groups of proteins are distinguished in muscle tissue: sarcoplasmic proteins, which account for about 35%; myofibrillar proteins, making up approximately 45%; and stromal proteins, whose content reaches 20%.

Sarcoplasmic proteins are soluble in water and dilute salt solutions. The bulk of them consists of enzyme proteins located mainly in the Cell/35.html">Mitochondria, which catalyze Oxidative Phosphorylation, as well as numerous Glycolysis Enzymes and those involved in nitrogen and Lipid METABOLISM found in the sarcoplasm. This group also includes Myoglobin, a protein that binds oxygen with a higher affinity than Hemoglobin and serves to store molecular oxygen in muscles. Recently, a group of sarcoplasmic proteins known as parvalbumins has been discovered; these are capable of binding Calcium Ions, though their physiological role remains elusive.

Myofibrillar proteins include the contractile proteins Myosin, Actin, and Actomyosin, as well as the regulatory proteins Tropomyosin, troponin, and α- and β-actinins. Myofibrillar proteins are responsible for the contractile function of muscles.

Myosin is one of the principal contractile proteins of muscle, accounting for about 55% of total muscle protein. It forms the thick filaments of myofibrils. The Molecular Weight of this protein is approximately 470,000. The myosin molecule comprises a long fibrillar region and globular structures (heads). The fibrillar part of the myosin molecule has a double-helical Structure (Fig. 117). The molecule contains six subunits: two heavy polypeptide chains (molecular weight 200,000) and four light chains (molecular weight 1,500–2,700) located in the globular region. The primary function of the fibrillar domain of the myosin molecule is its ability to form well-organized bundles of myosin filaments, or thick protofibrils (see Fig. 117). The myosin heads contain the ATPase Active Site and the actin-binding site, thereby facilitating ATP Hydrolysis and interaction with actin filaments.

Fig. 117 Diagram of The structure of a myosin molecule (a), a myosin bundle (b), and a thick myosin filament (c)

The myosin molecule contains a significant amount of glutamic acid and carries a strong negative charge, which enhances the binding of free Ca²⁺ and Mg²⁺ ions. In the presence of Ca²⁺ ions, the ATPase activity of myosin and The rate of ATP hydrolysis increase According to the equation

The chemical energy of ATP released during this enzymatic reaction is used to alter the conformation of the myosin protein and generate tension between the thick and thin myosin filaments in a contracting muscle. Mediated by Mg²⁺ ions, myosin is capable of binding ATP and ADP molecules, as well as interacting with actin molecules that form part of the myofibril thin filaments.

Fig. 118 Diagram of the structure of an actin, or thin, filament

Actin is the second contractile muscle protein, forming the backbone of thin filaments (Fig. 118). It exists in two known forms: globular G-actin and fibrillar F-actin (see Chapter 12). Globular actin is a spherical protein with a molecular weight of 42,000, accounting for about 25% of the total muscle protein mass. In the presence of Mg²⁺, actin undergoes non-covalent polymerization to form an insoluble helical filament known as F-actin. Both forms of actin lack enzymatic activity. Each G-actin molecule is capable of binding a single Ca²⁺ ion, which plays a crucial role in initiating contraction. In addition, the G-actin molecule tightly binds one molecule of ATP or ADP. ATP binding by G-actin is typically accompanied by its polymerization into F-actin, with the simultaneous hydrolysis of ATP to ADP and phosphate. ADP remains bound to the fibrillar actin. The actin polymerization process can be described by the following equation:

F-actin activates myosin ATPase, thereby providing the driving force for Muscle contraction.

Actin is capable of interacting with myosin to form an actomyosin complex. The molar ratio of actin to myosin in the actomyosin complex is approximately 1:1. An F-actin filament can bind A large number of myosin molecules. A key property of the actomyosin complex is its dissociation in the presence of ATP and Mg²⁺.

Along with actin, thin filaments contain other minor proteins, namely tropomyosin, troponins, and actinins.

Tropomyosin (Tm) is a structural protein of the actin filament, existing as an elongated, strand-like molecule. Its two polypeptide chains wind around the actin filaments (see Fig. 118). At the ends of each tropomyosin molecule are Proteins of the troponin system, the presence of which is characteristic exclusively of striated muscle.

Troponin (Tn) is a regulatory protein of the actin filament. It consists of three subunits: TnT, TnI, and TnC. Troponin T (TnT) ensures

the binding of these proteins to tropomyosin. Troponin I (TnI) blocks (inhibits) the interaction between Actin and myosin. Troponin C (TnC) is a Ca²⁺-binding protein whose STRUCTURE AND Functions are similar to calmodulin, a protein widely distributed in nature. Like calmodulin, troponin C binds four Ca²⁺ ions per protein molecule and has a molecular weight of 17,000. In the presence of Ca²⁺, the conformation of troponin C changes, leading to a shift in THE POSITION OF Tn relative to actin, which exposes the actin-myosin interaction site.

Thus, the thin filament of a striated muscle myofibril is composed of F-actin, tropomyosin, and three troponin components: TnC, TnI, and TnT. In addition to these proteins, actinin is involved in muscle contraction. It is found in the Z-line region, where the ends of the F-actin molecules of the myofibril thin filaments are anchored.

Stromal proteins in Skeletal Muscle are represented primarily by Collagen and Elastin, which are Components of the sarcolemma and the Z-lines of myofibrils. These proteins possess high elasticity and resilience, which are essential for muscle contraction and relaxation. Their structure is discussed in Chapter 12.

Non-protein components of muscle

In addition to proteins, the dry residue of muscle tissue contains other substances (see Table 15), among which nitrogenous and non-nitrogenous extractive substances, as well as minerals, are distinguished.

The nitrogenous compounds of skeletal muscle include ATP and its breakdown products—ADP and AMP—as well as creatine phosphate, creatine, creatinine, carnosine, anserine, free amino acids, and others.

ATP, present in amounts of 0.25–0.40%, and creatine phosphate, which ranges from 0.4 to 1.0%, serve as Energy Sources for muscle contraction. Their breakdown products—ADP, AMP, and creatine—regulate metabolic processes within the muscles. Carnosine is a dipeptide involved in the Transfer of phosphate groups; it stimulates ion pump activity and increases the amplitude of muscle contraction, which decreases during fatigue, thereby helping to restore performance. Carnitine participates in The transport of Fatty acids—important energy sources—across mitochondrial membranes, thus facilitating their oxidation and energy production.

Various cell membranes of muscle tissue contain A number of nitrogenous phospholipids, including phosphatidylcholine (lecithin), phosphatidylethanolamine (cephalin), phosphatidylserine, and others. Phospholipids take part in metabolic processes by supplying Choline and fatty acids, which act as substrates for tissue Respiration. Other nitrogenous substances—such as urea, uric acid, and purine bases (adenine, guanine)—are intermediate or End products of Nitrogen metabolism and are found in muscles in small quantities.

Non-nitrogenous compounds of muscle tissue include glycogen, which is located in the sarcoplasm in a free or protein-bound state and serves as the primary energy substrate during intensive exercise. Depending on diet and conditioning, its level ranges from 0.3 to 3.0% of total muscle mass. Physical training primarily increases The amount of free glycogen. Muscles also contain several intermediate products of Carbohydrate Metabolism, such as hexose phosphates, pyruvic acid, and lactic acid.

Among Lipids, muscle tissue contains triglycerides in the form of fat droplets, as well as cholesterol.

Minerals account for 1–1.5% of total muscle mass and feature a diverse composition. The principal cations are Na⁺, К⁺, Мg²⁺, and Са²⁺. Potassium ions (К⁺) are concentrated mainly inside muscle fibers, whereas sodium ions (Na⁺) reside predominantly in the extracellular fluid. They play a key role in muscle excitation and the initiation of contraction. The prevailing anions include Сl^-, Н₂РO^-₄, HPO^2-₄, SO₄²⁺ (SO₄^2-), and НСО₃^-.

Muscle tissue contains a range of Trace Elements, including cobalt, iron, nickel, boron, and zinc. These elements either function as Structural components of complex protein molecules or act as enzyme activators. All minerals play a vital role in regulating various biochemical processes in muscles.