Interaction between Myosin and Actin

When actin combines with energized myosin, the stored energy in the myosin molecule is released. This causes the myosin molecule to return to its resting state, and the globular head pivots about its hinged attachment point to the thick filament. The pivoting motion requires that the thick and thin filaments move longitudinally with respect to each other. This mechanical analog is illustrated schematically in Figure 10-6. The exact nature of the chemical changes in a myosin molecule during the transition from resting to energized state and back is unknown at present; the sliding filament hypothesis, however, requires that there be some chemical equivalent of the hinged arrangement shown in Figure 10-6.

How is the bond between actin and myosin broken so that a new cycle of sliding can be initiated? In the scheme presented so far, each myosin molecule on the thick filament could interact only one time with an actin molecule on the thin filament, and the total excursion of sliding would be restricted to that produced by a single pivoting of the globular head. In order to produce the large movements of the filaments that actually occur, it is necessary that the attachment of the cross-bridges is broken so that the cycle of myosin energization, binding to actin, and movement can be repeated. The full cycle that allows this to occur is summarized in Figure 10-7. When energized myosin binds to actin and releases its stored energy, the ADP bound to the ATPase site of the globular head is released. This allows a new molecule of ATP to bind to the myosin. When this happens, the bond between actin and myosin is broken, possibly because of structural changes in the globular head induced by the interaction between ATP and myosin. The new ATP molecule can then be split by myosin to regenerate the energized form, which is then free to interact with another actin molecule on the chain making up the thin filament. Note that there are two roles for ATP in this scheme: to provide the energy to "cock" myosin for movement, and to break the interaction between actin and myosin after movement has occurred. If there is no ATP present, actin and myosin get stuck together and a rigid muscle results (as in rigor mortis).

Each of the many myosin heads on an individual thick filament independently goes through repetitive cycles of energization by ATP, binding to actin,

Figure 10-6 A schematic representation of the mechanism of filament sliding during contraction of a myofibril. The globular head of energized myosin binds to a specific binding site on actin, and the energy stored in myosin is released. The resulting relaxation of the myosin molecule entails rotation of the globular head, which induces longitudinal sliding of the filaments.

Thin filament

Myosin binding site

Energized myosin

Myosin binding site

Energized myosin

Thick filament

Energized myosin binds to actin

Energized myosin binds to actin

V /

Displacement

Myosin + ATP

-> MyosinATP

Actin

-> MyosinATP

Actin

> Energized myosinADPphosphate + Actin

Actinenergized myosinADPphosphate

> Energized myosinADPphosphate + Actin

Actinenergized myosinADPphosphate

Release stored energy and move filament

Actinmyosin + ADP + phosphate

Figure 10-7 The cycle of cross-bridge formation and dissociation between myosin and actin during filament sliding.

Actinmyosin + ADP + phosphate

Figure 10-8 The mechanism of sarcomere shortening during contraction. For clarity, the myosin heads are shown acting in concert, although in reality they behave independently.

releasing stored energy to produce sliding, and detachment from actin. Each cycle results in the splitting of one molecule of ATP to ADP and inorganic phosphate. Note from Figure 10-4 that the orientation of the myosin heads reverses at the midpoint of the thick filament, the M line. This is the proper orientation to pull both Z lines at the boundary of a sarcomere toward the center (Figure 10-8). The thin filaments attached to the left Z line will be pulled to the right by the cyclical pivoting of the myosin cross-bridges. Similarly, the thin filaments attached to the right Z line will be pulled to the left. Thus, each sarcomere in each myofibril shortens, and the whole muscle shortens.

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Responses

  • gioacchino
    How is the bond between the actin and myosin filaments broken?
    6 years ago

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