Evaluate and interpret the process in the actin-myosin cross bridge (sliding fil
ID: 3520528 • Letter: E
Question
Evaluate and interpret the process in the actin-myosin cross bridge (sliding filament merchandise) during skeleton muscle contraction and answer the following questions below: 1 what happens to the position of the myosin head when ATP is hydrolyzed and ADP is attached to the myosin head group. 2 what event happens immediately after the myosin head binds to the active site on actin? Describe this process in great detail. 3 what happens to the position of the myosin head when ATP binds to the myosin head group? 4 why does the actin filament only move in one direction? Describe in great detail. Evaluate and interpret the process in the actin-myosin cross bridge (sliding filament merchandise) during skeleton muscle contraction and answer the following questions below: 1 what happens to the position of the myosin head when ATP is hydrolyzed and ADP is attached to the myosin head group. 2 what event happens immediately after the myosin head binds to the active site on actin? Describe this process in great detail. 3 what happens to the position of the myosin head when ATP binds to the myosin head group? 4 why does the actin filament only move in one direction? Describe in great detail. 1 what happens to the position of the myosin head when ATP is hydrolyzed and ADP is attached to the myosin head group. 2 what event happens immediately after the myosin head binds to the active site on actin? Describe this process in great detail. 3 what happens to the position of the myosin head when ATP binds to the myosin head group? 4 why does the actin filament only move in one direction? Describe in great detail.Explanation / Answer
1.
Scientists have demonstrated that the globular end of each myosin protein that is nearest actin, called the S1 region, has multiple hinged segments, which can bend and facilitate contraction . The bending of the myosin S1 region helps explain the way that myosin moves or "walks" along actin. The slimmer and typically longer "tail" region of myosin (S2) also exhibits flexibility, and it rotates in concert with the S1 contraction.
The movements of myosin appear to be a kind of molecular dance. The myosin reaches forward, binds to actin, contracts, releases actin, and then reaches forward again to bind actin in a new cycle. This process is known as myosin-actin cycling. As the myosin S1 segment binds and releases actin, it forms what are called cross bridges, which extend from the thick myosin filaments to the thin actin filaments. The contraction of myosin's S1 region is called the power stroke. The power stroke requires the hydrolysis of ATP, which breaks a high-energy phosphate bond to release energy.
Specifically, this ATP hydrolysis provides the energy for myosin to go through this cycling: to release actin, change its conformation, contract, and repeat the process again (Figure 4). Myosin would remain bound to actin indefinitely — causing the stiffness of rigor mortis — if new ATP molecules were not available.
Two key aspects of myosin-actin cycling use the energy made available by the hydrolysis of ATP. First, the action of the reaching myosin S1 head uses the energy released after the ATP molecule is broken into ADP and phosphate. Myosin binds actin in this extended conformation. Second, the release of the phosphate empowers the contraction of the myosin S1 region.
2.
ATP binding causes myosin to release actin, allowing actin and myosin to detach from each other. After this happens, the newly bound ATP is converted to ADP and inorganic phosphate, Pi. The enzyme at the binding site on myosin is called ATPase. The energy released during ATP hydrolysis changes the angle of the myosin head into a “cocked” position. The myosin head is then in a position for further movement, possessing potential energy, but ADP and Pi 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. Pi 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.
3. When the myosin head is “cocked,” it contains energy and is in a high-energy configuration. 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. 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.
4. All actin filaments are polymerised in one direction so they move only in one direction.
They are pulled by myosin head in same direction only.
Two ends of an actin filament grow at different rates, with monomers being added to the fast-growing end (the plus end) five to ten times faster than to the slow-growing (minus) end. Because ATP-actin dissociates less readily than ADP-actin, this results in a difference in the critical concentration of monomers needed for polymerization at the two ends. This difference can result in the phenomenon known as treadmilling, which illustrates the dynamic behavior of actin filaments . For the system to be at an overall steady state, the concentration of free actin monomers must be intermediate between the critical concentrations required for polymerization at the plus and minus ends of the actin filaments. Under these conditions, there is a net loss of monomers from the minus end, which is balanced by a net addition to the plus end. Treadmilling requires ATP, with ATP-actin polymerizing at the plus end of filaments while ADP-actin dissociates from the minus end. Although the role of treadmilling in the cell is unclear, it may reflect the dynamic assembly and disassembly of actin filaments required for cells to move and change shape.
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