Myosin head power stroke does not obey predictions based on the swinging lever arem mechanism of muscle contraction
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Although more than 50 yeares have passed since the monumental discovery of sliding filament mechanism in muscle contraction, the moleculare mechanism of myosin head movement, coupled with ATP hydrolysis, is still a matter for debate and speculation. A most straightforwared way to study myosin head movement, producing myofilament sliding, may directly record ATP-induced myosin head movement in hydrated, living myosin filaments using the gas Environmental Chamber (EC) attached to an electron microscope. While the EC has long been used by material scientists for the in situ observation of chemical reaction of inorganic compounds, we aree the only group successfully using the EC to record myosin head movement in living myosin filaments. We position-marek individual myosin heads by attaching gold pareticles (diameter, 20 nm) via three different monoclonal antibodies, attaching to at the distal region of myosin head Catalytic Domain (CAD), at the myosin head Converter Domain(COD) and at the myosin head Lever arem Domain (LD). First, we recoded ATPinduced myosin head movement in the absence of actin filaments and found that myosin heads moved away from, but not towareds the central baree region of myosin filaments. We also succeeded in recording ATP-induced myosin head power stroke in actin-myosin filament mixture. Since only a limited proportion of myosin heads can be activated by a limited amount of ATP applied, myosin heads only move by stretching adjacent sarecomere structures. As shown in Figure-1, myosin head CAD did not move pareallel to the filament axis in the standared ionic strength (B), while it moved pareallel to the filament axis (C). These results indicate that myosin head movement does not necessareily obey predictions of the swinging lever arem hypothesis appeareing in every textbook as an established fact.Keywords:
Meromyosin
Myofilament
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The mechanism of movement of the myosin heads along the actin filament is described. Based on the assumption that in the excited state of the enzyme (achieved when interacting with ATP), the probability of displacement of one head of myosin is less than the probability of displacement of the other head of myosin, a simplified theoretical model of the movement of myosin with two heads along the actin filament due to ATP hydrolysis was constructed. The expression for the flux of the ATP hydrolysis reaction during the movement of myosin is obtained, which explicitly includes the driving forces of the reaction.
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Meromyosin
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Heavy meromyosin
Meromyosin
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Muscle contraction is driven by a change in the structure of the head domain of myosin, the “working stroke” that pulls the actin filaments toward the midpoint of the myosin filaments. This movement of the myosin heads can be measured very precisely in intact muscle cells by X-ray interference, but until now this technique has not been applied to physiological activation and force generation following electrical stimulation of muscle cells. By using this approach, we show that the long axes of the myosin head domains are roughly parallel to the filaments in resting muscle, with their center of mass offset by approximately 7 nm from the C terminus of the head domain. The observed mass distribution matches that seen in electron micrographs of isolated myosin filaments in which the heads are folded back toward the filament midpoint. Following electrical stimulation, the heads move by approximately 10 nm away from the filament midpoint, in the opposite direction to the working stroke. The time course of this motion matches that of force generation, but is slower than the other structural changes in the myosin filaments on activation, including the loss of helical and axial order of the myosin heads and the change in periodicity of the filament backbone. The rate of force development is limited by that of attachment of myosin heads to actin in a conformation that is the same as that during steady-state isometric contraction; force generation in the actin-attached head is fast compared with the attachment step.
Myofibril
Meromyosin
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Lever
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Abstract Unidirectional sliding of myosin filaments along F‐actin bundles was produced with purified muscle actin and myosin in the presence of ATP. The velocity of myosin filament sliding was independent of myosin filament length. This result supports a recent hypothesis that long distance movement of myosin cross‐bridge can be induced by splitting of one ATP molecule [Yanagida, Arata, and Oosawa, 1985. Nature. 316:366–369; Higashi‐Fujime. 1985. J. Cell Biol., 101:2335–2344].
Meromyosin
Actina
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The most straightforward way to get information on the performance of individual myosin heads producing muscle contraction may be to record their movement, coupled with ATP hydrolysis, electron-microscopically using the gas environmental chamber (EC). The EC enables us to visualize and record ATP-induced myosin head movement in hydrated skeletal muscle myosin filaments. When actin filaments are absent, myosin heads fluctuate around a definite neutral position, so that their time-averaged mean position remains unchanged. On application of ATP, myosin heads are found to move away from, but not towards, the bare region, indicating that myosin heads perform a recovery stroke (average amplitude, 6 nm). After exhaustion of ATP, myosin heads return to their neutral position. In the actin–myosin filament mixture, myosin heads form rigor actin myosin linkages, and on application of ATP, they perform a power stroke by stretching adjacent elastic structures because of a limited amount of applied ATP ≤ 10 µM. The average amplitude of the power stroke is 3.3 nm and 2.5 nm at the distal and the proximal regions of the myosin head catalytic domain (CAD), respectively. The power stroke amplitude increases appreciably at low ionic strength, which is known to enhance Ca2+-activated force in muscle. In both the power and recovery strokes, myosin heads return to their neutral position after exhaustion of ATP.
Adenosine triphosphate
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CrossBridge
Meromyosin
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