MUSCLES

40-45 % of body mass only system converting chemical energy into mechanical 2 types -skeletal (striated) -smooth

Structural unit - muscle fiber (myocyte) Contains many nuclei located along the cell

Muscle structure

Types of Muscle

Skeletal Muscle Organization

A single muscle fiber

Chemical composition of skeletal muscles

Proteins of muscles 3 types: proteins of sarcoplasma proteins of miofibrils proteins of stroma

ACTIN & THIN FILAMENTS G-actin is the monomer which will form the thin filament. It is a protein with a molecular weight of 43,000. It has a prominent site for cross-linkage with myosin. G-actin ↓ F-actin (6-7 nm long polymerized G-actin, double stranded in structure) ↓ Thin filaments PROTEINS OF MUSCLE:

Proteins of Sarcoplasma Miogen fraction (enzymes of glycolysis etc.) Albumins Globulins Myoglobin (chromoprotein, provides the red color to muscles, responsible for oxygen storage)

Proteins of Stroma collagen keratin elastin are constituents of connective tissue of vessel walls, nerves, sarcolema.

Proteins of Miofibrils Myosin (56-60 %) Actin (20-25 %) Tropomyosin (10-15 %) Troponin complex (4-6 %)

Regulatory Proteins of the Muscles TROPOMYOSIN Rod-like protein Mol. Weight: 70,000 2 chains: alpha & beta chains Under resting conditions, it covers the site for myosin attachment on F-actin molecule. Forms part of Thin filaments TROPONIN Globular protein complex made of 3 polypeptides Forms part of thin filaments Binds to Ca2+ Inhibitory in function Attached to Tropomyosin

Structure of filaments and myofibrils Sarcoplasma of striated muscle fibers contains myofibrils oriented along which are built of 2 types protein filaments: thick and thin

Muscle contraction is carried out due to the sliding of thick and thin filaments Chemical energy – ATP hydrolysis Contraction is regulated by Ca 2+ concentration

Structure of Thick Filament Thick filaments consist of myosin molecules Myosin molecule built of 2 heavy ( Da) and 4 light ( Da) chains Heavy chains are coiled around each other and form the “tail” of the molecule 2 light chains form the globular head of the molecule The head has ATP-ase properties

About 400 molecules of myosin are combined in the thick filament

About half of molecules is directed to one end of filament, another half – to another end

Structure of Thin Filament Three proteins: actin, tropomyosin, troponin Two forms of actin: globular G-actin and fibril F-actin Molecules of globular actin are joined to form F-actin Two chains of F-actin are coiled in spiral In the groove of spiral of F-actin tropomiosin is located One molecule of tropomiosine contacts with 7 pairs of G-actin 1 molecule of troponin drops on 1 molecule of tropomiosin There are three subunits of troponin

Miofibrils contain about 2500 filaments There are 6 thin filaments for 1 thick filament

Structural unit of miofibril sarcomer Both ends of thick miosin filaments are free One end of thin filaments is attached to Z-plate

BIOMECHANISM OF MUSCLE CONTRACTION Potential spreads along miofiber Signal is transferred to cisterna of endoplasmic reticulum Permeability of membranes for Ca 2+ ions is changed and they get out into sarcoplasma During the rest concentration of Ca 2+ in sarcoplasma is less than mol/L After Ca 2+ exit from cisternas the concentration reaches mol/L

Molecule of tropomiosin moves along groove of thin filament As result centers for connection with heads of myosin are opened on the molecules of G-actin Ca 2+ binds to Ca-binding subunit of troponin Conformation of protein is changed

Myosin heads combined with ATP bind to the closest molecules of G-actin ATPase center is activated and ATP is hydrolized Head bent Sliding of thin filament along myosin New ATP molecule binds to head of myosin Bridge is torn In the condition of Ca presence the head binds to the next actin molecule Frequency - 50 times/s Heads works not synchronously Nervous impulses stop to come Ca-ATPase transfers Ca 2+ from sarcoplasma into cisternas Complex Ca 2+ -troponin is dissotiated Tropomiosin moves Molecules of actin are blocked Bridges are torn Muscle relaxation

ATP is required both for contraction and relaxation of muscles In ATP deficiency the bridges between actin and myosin are not torn Filaments are fixed in connected state – muscle contraction (cadaver rigidity)

SOURSES OF ENERGY FOR MUSCLE WORK ATP (5 umol for 1 g of tissue) – enough for 2-3 s Kreatinphosphate – till 10 s Glycolysis Oxidative phosphorylation

RED AND WHITE MUSCLES Res fibers Lot of myoglobin and mitochondria Oxidative phosphorylation is active Are contracted slowly, for a long time, no tiredness for long time White fibers Little hemoglobin and mitochondria More glycogen Glycolisis is specific Are contracted fast, fast tiredness

Skeletal muscle functions under both aerobic (resting) and anaerobic (eg, sprinting) conditions, so both aerobic and anaerobic glycolysis operate, depending on conditions. Skeletal muscle contains myoglobin as a reservoir of oxygen. Skeletal muscle contains different types of fibers primarily suited to anaerobic (fast twitch fibers) or aerobic (slow twitch fibers) conditions. Actin, myosin, tropomyosin, troponin complex (TpT, Tpl, and TpC), ATP, and Ca2+ are key constituents in relation to contraction. The Ca2+ ATPase, the Ca2+ release channel, and calsequestrin are proteins involved in various aspects of Ca2+ metabolism in muscle. Summary of major features of the biochemistry of skeletal muscle related to its metabolism

Insulin acts on skeletal muscle to increase uptake of glucose. In the fed state, most glucose is used to synthesize glycogen, which acts as a store of glucose for use in exercise; “preloading” with glucose is used by some long-distance athletes to build up stores of glycogen. Epinephrine stimulates glycogenolysis in skeletal muscle, whereas glucagon does not because of absence of its receptors. Skeletal muscle cannot contribute directly to blood glucose because it does not contain glucose-6-phosphatase. Lactate produced by anaerobic metabolism in skeletal muscle passes to liver, which uses it to synthesize glucose, which can then return to muscle (the Cori cycle). Summary of major features of the biochemistry of skeletal muscle related to its metabolism

Skeletal muscle contains phosphocreatine, which acts as an energy store for short-term (seconds) demands. Free fatty acids in plasma are a major source of energy, particularly under marathon conditions and in prolonged starvation. Skeletal muscle can utilize ketone bodies during starvation. Skeletal muscle is the principal site of metabolism of branched-chain amino acids, which are used as an energy source. Proteolysis of muscle during starvation supplies amino acids for gluconeogenesis. Major amino acids emanating from muscle are alanine (destined mainly for gluconeogenesis in liver and forming part of the glucose-alanine cycle) and glutamine (destined mainly for the gut and kidneys). Summary of major features of the biochemistry of skeletal muscle related to its metabolism