VPB 121: Veterinary Physiology - II (2+1)

MYOSIN AND ACTIN FILAMENT

Myosin Filament (View animation)

• It is composed of multiple myosin molecules, each molecule having a molecular weight of 480,000. It has two polymerized portions, Heavy meromyosin and Light meromyosin . Heavy meromyosin is formed by HMM – S₁ and HMM – S₂ molecules.
• The myosin molecule is made up of six polypeptide chains: 2 heavy chains and 4 light chains. The two heavy chains wrap spirally around each other forming a double helix. At one end of these chains get folded into a globular structure called myosin head. Thus each myosin molecule has two free heads lying side by side at one end of the myosin molecule. Myosin head is made up of HMM – S₁ molecule, has ATPase activity and readily binds with the “G” actin. The elongated coiled portion of the myosin molecule is called tail formed by HMM – S₂ molecule has no ATPase activity and binding property with “G” actin. Out of the 4 light chains, outer two are folded and form part of the myosin heads whereas the inner two form the tail and body.
• The heads of the myosin molecules hang outward to the sides of the body. Along with the head, “arm” extends outward from the body of the myosin filament. The flexible points of the arm are hinges which allow the head to be extended outward or brought closer to the body of myosin filament. The protruding arms and heads together are called “cross-bridges”. There are no cross-bridges at the centre of the myosin filament.
• The myosin head possess ATPase enzyme, which helps to cleave the ATP to release energy for the contraction process. The binding ability of the myosin head with the binding site G actin is regulates by ATPase. Only in the presence of actin, Mg⁺⁺ ions enhance the ATPase activity of the myosin head.

Actin Filament

• It is composed of three proteins; actin, tropomyosin and troponin
• Actin
  • It is a double helix structure formed by the basic unit “F” actin molecule. Each strand of F-actin helix has hundreds of “G” actin molecules which have ADP molecule. G-actin provides the active binding site to myosin head. Tropomyosin and Troponin are the polymer of actin.
  • Tropomyosin
    • During resting stage, it covers “G” actin and prevents the binding of myosin head with the binding site.
• Troponin
  • It is a complex made up of 3 proteins namely Troponin “T”, Troponin “I”, and Troponin “C”.
    • Troponin “T” has high affinity to tropomyosin and binds the troponin complex with tropomyosin.
    • Troponin “I” readily binds with actin and tropomyosin. In resting muscles, they inhibit ATPase activity in myosin head.
    • Troponin “C” has a very high affinity for calcium ions. Each molecule can bind up to 4 Ca⁺⁺ ions. The combination of Ca⁺⁺ ions with troponin is the triggering factor to initiate muscular contraction.
  • The presence of troponin and tropomyosin (tt-complex) in actin filament inhibits the binding reaction of actin with myosin. In a relaxed muscle fibre, the tt - complex physically covers the “G” actin for binding with myosin.
• When a muscle is stimulated, a high concentration of Ca⁺⁺ ions is released from the cisternae into the sarcoplasm. The Ca⁺⁺ ions readily combine with the troponin `C' of the TT – complex causes conformational change in the troponin complex and moves the tropomyosin molecule deeper into the groove between two actin strands. This exposes the “G” actin filament to myosin to initiate the contractions of the myofibrils. Only the skeletal muscle and cardiac muscle require troponin and tropomyosin, for execution of contraction coupling.
• In cardiac muscle during excitation-contraction coupling, Ca⁺⁺ ion is released not only from the cisterna of ‘L’ tubules, but also from ECF through “T” tubules and the strength of cardiac fibre contraction depends to a great extent on the ECF Ca⁺⁺ concentration.
• In smooth muscle instead of troponin another regulatory protein “calmodulin” reacts with four Ca⁺⁺ ions. The calmodulin-Ca⁺⁺ complex activates myosin kinase and Ca⁺⁺ dependent phosphorylation of light chain myosin molecule to activate myosin cross-bridge reactions.

Sarcoplasmic reticulum - Tubular system

• Muscle cells also contain a special structure called sarcotubular system which has two sets of tubules. Longitudinal tubules (L-Tubules) and transverse tubules (T-Tubules).
• `L' Tubule system
  • Present parallel to myofibrils. At both the ends of the tubule, bulbous structures known as terminal cisternae function as a storage place for the Ca<sup>++</sup> ions and play a key role in muscle contraction.
• `T' Tubule System
  • The T system consist of set of tubules formed by the invaginations of the plasmalemma. This tubule run perpendicular to the long axis of the muscle cell and lie between the cisternae of two successive “L” tubules.
  • The “T”’ tubules are located at the junction of “A” and “I” bands which pass through the fibres, open into the inter-fibre space, permits the flow of extracellular fluid.
  • For every sarcomere 1 or 2 T-tubules are observed (preferably at Z disc region). Lumen of T-tubule is actually the extra cellular space. T - Tubule brings action potential from sarcolemma to the interior of the muscle cells. As sarcoplasmic reticular membranes approach the T-tubule, they coalesce to from 2 large sacs called the lateral cisternae, one on each side.

Triad

• It is a paired cisternae and a “T” tubule in between the cisternae. One T- tubule and 2 lateral cisternae associated with it forms a ‘Triad’. They have remarkable ability to accumulate Ca²⁺ against a concentration gradient. Calcium pump an energy dependant - operating in the L-tubules which can transport 2 Ca²⁺ with an expense of one ATP from the sarcoplasm to the lumen of sarcoplamic reticulum.
• The ATPase enzyme found associated with ca – pump, keeps free of Ca²⁺ concentration in the sarcoplasm of a resting muscle at a very low level (10^-8M) and the most of Ca²⁺ found in the lateral cisternae bound to a protein called calsequestrin. In the cytoplasm, the Ca²⁺ is bound with calmodulin, which permits the transports cytoplasmic Ca²⁺ to its site of action.
• When an action potential pass along with T-tubule, Ca²⁺ from the lateral cisternae become disgorged go to the sarcoplasm thereby rising the free Ca²⁺ concentration from (10^-8M) to (10^-5 to 10^-6M) and this intracellular Ca²⁺ triggers muscle contraction.

Proximate composition of mammalian skeletal muscle

• Water: 55 – 78% by wt: Water content varies inversely with lipid content.
• Protein: 15-23%
• Lipid: 1 to 20%
• Carbohydrate: 1-2%: mostly glycogen or lactic acid
• Ash: 1% mostly K⁺, less Na⁺, Cl^- and Mg 2⁺
• Nucleic acid: < 1% - 25 – 30 mg DNA/100g and 100mg RNA/100g.
• Other soluble organic composition: 1% - 8-5 mM ATP, 20mM phosohcreatine, 350 mg carnosine /100g.