How does myosin work

Myosin binds to actin at a binding site on the globular actin protein. ATP binding causes myosin to release actin, allowing actin and myosin to detach from each other. The enzyme at the binding site on myosin is called ATPase. The myosin head is then in a position for further movement, possessing potential energy, but ADP and P i are still attached.

If actin binding sites are covered and unavailable, the myosin will remain in the high energy configuration with ATP hydrolyzed, but still attached. If the actin binding sites are uncovered, a cross-bridge will form; that is, the myosin head spans the distance between the actin and myosin molecules. P i is then released, allowing myosin to expend the stored energy as a conformational change.

The myosin head moves toward the M line, pulling the actin along with it. As the actin is pulled, the filaments move approximately 10 nm toward the M line. This movement is called the power stroke, as it is the step at which force is produced. As the actin is pulled toward the M line, the sarcomere shortens and the muscle contracts. This energy is expended as the myosin head moves through the power stroke; at the end of the power stroke, the myosin head is in a low-energy position.

After the power stroke, ADP is released; however, the cross-bridge formed is still in place, and actin and myosin are bound together. ATP can then attach to myosin, which allows the cross-bridge cycle to start again and further muscle contraction can occur Figure 1. The movement of the myosin head back to its original position is called the recovery stroke. Resting muscles store energy from ATP in the myosin heads while they wait for another contraction.

Figure 1. With each contraction cycle, actin moves relative to myosin. 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. AF: actin filament; MF myosin filament. Modified from Goody The missing link in the muscle cross-bridge cycle. Nature Structural Biology 10, Calcium and ATP are cofactors nonprotein components of enzymes required for the contraction of muscle cells.

ATP supplies the energy, as described above, but what does calcium do? Calcium is required by two proteins, troponin and tropomyosin, that regulate muscle contraction by blocking the binding of myosin to filamentous actin.

In a resting sarcomere, tropomyosin blocks the binding of myosin to actin. In the above analogy of pulling shelves, tropomyosin would get in the way of your hand as it tried to hold the actin rope. For myosin to bind actin, tropomyosin must rotate around the actin filaments to expose the myosin-binding sites. By comparing the action of troponin and tropomyosin under these two conditions, they found that the presence of calcium is essential for the contraction mechanism.

Specifically, troponin the smaller protein shifts the position of tropomyosin and moves it away from the myosin-binding sites on actin, effectively unblocking the binding site Figure 5. Once the myosin-binding sites are exposed, and if sufficient ATP is present, myosin binds to actin to begin cross-bridge cycling.

Then the sarcomere shortens and the muscle contracts. In the absence of calcium, this binding does not occur, so the presence of free calcium is an important regulator of muscle contraction. Figure 5: Troponin and tropomyosin regulate contraction via calcium binding Simplified schematic of actin backbones, shown as gray chains of actin molecules balls , covered with smooth tropomyosin filaments.

Troponin is shown in red subunits not distinguished. Upon binding calcium, troponin moves tropomyosin away from the myosin-binding sites on actin bottom , effectively unblocking it.

Modified from Lehman et al. Is muscle contraction completely understood? Scientists are still curious about several proteins that clearly influence muscle contraction, and these proteins are interesting because they are well conserved across animal species. For example, molecules such as titin, an unusually long and "springy" protein spanning sarcomeres in vertebrates, appears to bind to actin, but it is not well understood. In addition, scientists have made many observations of muscle cells that behave in ways that do not match our current understanding of them.

For example, some muscles in mollusks and arthropods generate force for long periods, a poorly understood phenomenon sometimes called "catch-tension" or force hysteresis Hoyle Studying these and other examples of muscle changes plasticity are exciting avenues for biologists to explore.

Ultimately, this research can help us better understand and treat neuromuscular systems and better understand the diversity of this mechanism in our natural world. Clark, M. Milestone 3 : Sliding filament model for muscle contraction. Muscle sliding filaments. Nature Reviews Molecular Cell Biology 9 , s6—s7 doi Goody, R. Nature Structural Molecular Biology 10 , — doi Hoyle, G. Comparative aspects of muscle. Annual Review of Physiology 31 , 43—82 doi Huxley, H.

Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation.

Nature , — doi Huxley, A. Structural changes in muscle during contraction: Interference microscopy of living muscle fibres. Hynes, T. Movement of myosin fragments in vitro: Domains involved in force production. Cell 48 , — Doi Lehman, W. Nature , 65—67 doi Lorand, L. Spudich, J. Nature Reviews Molecular Cell Biology 2 , — doi What Is a Cell? Eukaryotic Cells. Cell Energy and Cell Functions. Photosynthetic Cells.

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Krans, Ph. Citation: Krans, J. Nature Education 3 9 How do muscles contract? What molecules are necessary for a tissue to change its shape? Aa Aa Aa. Muscle is a specialized contractile tissue that is a distinguishing characteristic of animals. Changes in muscle length support an exquisite array of animal movements, from the dexterity of octopus tentacles and peristaltic waves of Aplysia feet to the precise coordination of linebackers and ballerinas.

What molecular mechanisms give rise to muscle contraction? The process of contraction has several key steps, which have been conserved during evolution across the majority of animals. What Is a Sarcomere? Figure 1: A gastrocnemius muscle calf with striped pattern of sarcomeres. The view of a mouse gastrocnemius calf muscle under a microscope. The Sliding Filament Theory. Figure 2: Comparison of a relaxed and contracted sarcomere.

A The basic organization of a sarcomere subregion, showing the centralized location of myosin A band. Figure 3: The power stroke of the swinging cross-bridge model, via myosin-actin cycling. Actin red interacts with myosin, shown in globular form pink and a filament form black line. Figure 4: Illustration of the cycle of changes in myosin shape during cross-bridge cycling 1, 2, 3, and 4.

ATP hydrolysis releases the energy required for myosin to do its job. What Regulates Sarcomere Shortening? Figure 5: Troponin and tropomyosin regulate contraction via calcium binding. Simplified schematic of actin backbones, shown as gray chains of actin molecules balls , covered with smooth tropomyosin filaments. Unresolved Questions. Muscle contraction provides animals with great flexibility, allowing them to move in exquisite ways.

The molecular changes that result in muscle contraction have been conserved across evolution in the majority of animals. By studying sarcomeres, the basic unit controlling changes in muscle length, scientists proposed the sliding filament theory to explain the molecular mechanisms behind muscle contraction. Within the sarcomere, myosin slides along actin to contract the muscle fiber in a process that requires ATP. Scientists have also identified many of the molecules involved in regulating muscle contractions and motor behaviors, including calcium, troponin, and tropomyosin.

This research helped us learn how muscles can change their shapes to produce movements. References and Recommended Reading Clark, M. Article History Close. Share Cancel.