myosin

Examples of myosin in the following topics:

• ATP and Muscle Contraction

  • ATP is critical for muscle contractions because it breaks the myosin-actin cross-bridge, freeing the myosin for the next contraction.
  • ATP is critical to prepare myosin for binding and to "recharge" the myosin.
  • ATP first binds to myosin, moving it to a high-energy state.
  • Once myosin binds to the actin, the Pi is released, and the myosin undergoes a conformational change to a lower energy state.
  • ATP then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.

• Regulatory Proteins

  • Tropomyosin and troponin prevent myosin from binding to actin while the muscle is in a resting state.
  • The binding of the myosin heads to the muscle actin is a highly-regulated process.
  • When a muscle is in a resting state, actin and myosin are separated.
  • To keep actin from binding to the active site on myosin, regulatory proteins block the molecular binding sites.
  • Describe how calcium, tropomyosin, and the troponin complex regulate the binding of actin by myosin

• Sliding Filament Model of Contraction

  • Actin myofilaments attach directly to the Z-lines, whereas myosin myofilaments attach via titin molecules.
  • The I-band is spanned by the titin molecule connecting the Z-line with a myosin filament.
  • Titin molecules connect the Z-line with the M-line and provide a scaffold for myosin myofilaments.
  • During contraction myosin ratchets along actin myofilaments compressing the I and H bands.
  • The A-band remains constant throughout as the length of the myosin myofilaments does not change.

• Rigor Mortis

  • Physiologically, rigor mortis is caused a release of calcium facilitating crossbridges in the sarcomeres; the coupling between myosin and actin cannot be broken, creating a constant state of muscle contraction until enzymatic decomposition eventually removes the crossbridges.
  • Diffusion of the calcium through the pumps occurs, facilitation binding of myosin and actin filaments.
  • As part of the process of decomposition, the myosin heads are degraded by the enzymes, allowing the muscle contraction to release and the body to relax.
  • Diagram showing Actin-Myosin filaments in Smooth muscle.
  • When activated they slide over the myosin bundles causing shortening of the cell walls.

• Control of Muscle Tension

  • The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle fiber can produce.
  • Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin.
  • If more cross-bridges are formed, more myosin will pull on actin and more tension will be produced.
  • This results in fewer myosin heads pulling on actin and less muscle tension.
  • Because myosin heads form cross-bridges, actin will not bind to myosin in this zone, reducing the tension produced by the myofiber.

• Microscopic Anatomy

  • The two most important proteins within sarcomeres are myosin, which forms a thick, flexible filament, and actin, which forms the thin, more rigid filament.
  • Myosin has a long, fibrous tail and a globular head, which binds to actin.
  • The myosin head also binds to ATP, which is the source of energy for muscle movement.
  • Together, myosin and actin form myofibrils, the repeating molecular structure of sarcomeres.
  • When ATP binds to myosin, it seperates from the actin of the myofibril, which causes a contraction.

• Skeletal Muscle Fibers

  • Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin.
  • Myofibrils are composed of long myofilaments of actin, myosin, and other associated proteins.
  • Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction.
  • Thick filaments are composed primarily of myosin proteins, the tails of which bind together leaving the heads exposed to the interlaced thin filaments.
  • The molecular model of contraction which describes the interaction between actin and myosin myofilaments is called the cross-bridge cycle.

• Mechanism and Contraction Events of Cardiac Muscle Fibers

  • In the sliding filament model, myosin filaments slide along actin filaments to shorten or lengthen the muscle fiber for contraction and relaxation.
  • This removal of the troponin complex frees the actin to be bound by myosin and initiates contraction.
  • The myosin head binds to ATP and pulls the actin filaments toward the center of the sarcomere, contracting the muscle.
  • This animation shows myosin filaments (red) sliding along the actin filaments (pink) to contract a muscle cell.

• Short-Term Chemical Control

  • The mechanism that leads to vasoconstriction results from the increased concentration of calcium (Ca2+ ions) and phosphorylated myosin within vascular smooth muscle cells.
  • The rise in intracellular calcium complexes with calmodulin, which in turn activates myosin light chain kinase.
  • This enzyme is responsible for phosphorylating the light chain of myosin to stimulate cross bridge cycling.
  • As with vasoconstriction vasodilation is modulated by calcium ion concentration and myosin phosphorylation within vascular smooth muscle cells.
  • Dephosphorylation by myosin light-chain phosphatase and induction of calcium symportersand antiporters that pump calcium ions out of the intracellular compartment both contribute to smooth muscle cell relaxation and therefore vasodilation.

• Microfilaments

  • Actin is powered by ATP to assemble its filamentous form, which serves as a track for the movement of a motor protein called myosin.
  • Actin and myosin are plentiful in muscle cells.
  • When your actin and myosin filaments slide past each other, your muscles contract.