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Load-dependent sliding direction change of a myosin head on an actin molecule and its energetic aspects: Energy borrowing model of a cross-bridge cycle

View Article: PubMed Central - PubMed

ABSTRACT

A model of muscle contraction is proposed, assuming loose coupling between power strokes and ATP hydrolysis of a myosin head. The energy borrowing mechanism is introduced in a cross-bridge cycle that borrows energy from the environment to cover the necessary energy for enthalpy production during sliding movement. Important premises for modeling are as follows: 1) the interaction area where a myosin head slides is supposed to be on an actin molecule; 2) the actomyosin complex is assumed to generate force F(θ), which slides the myosin head M* in the interaction area; 3) the direction of the force F(θ) varies in proportion to the load P; 4) the energy supplied by ATP hydrolysis is used to retain the myosin head in the high-energy state M*, and is not used for enthalpy production; 5) the myosin head enters a hydration state and dehydration state repeatedly during the cross-bridge cycle. The dehydrated myosin head recovers its hydrated state by hydration in the surrounding medium; 6) the energy source for work and heat production liberated by the AM* complex is of external origin. On the basis of these premises, the model adequately explains the experimental results observed at various levels in muscular samples: 1) twist in actin filaments observed in shortening muscle fibers; 2) the load-velocity relationship in single muscle fiber; 3) energy balance among enthalpy production, the borrowed energy and the energy supplied by ATP hydrolysis during muscle contraction. Force F(θ) acting on the myosin head is depicted.

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Direction of force acting at a cross-bridge in muscle. (A) Direction of force F and counteraction C acting on an actin molecule under rapid-sliding conditions. This counteraction causes a right-handed super-coil of the actin filament in in vitro motility assay systems in which the pointed end of the actin filament is fixed to a glass surface. (B) Direction of force F and counteraction C acting under isometric conditions. This counteraction causes left-handed super-coil of the actin filament in in vitro motility assay systems. Note that the direction of the torque component changes with shortening conditions. Red balls represent the actin molecules included in a particular interaction unit.
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f3-5_11: Direction of force acting at a cross-bridge in muscle. (A) Direction of force F and counteraction C acting on an actin molecule under rapid-sliding conditions. This counteraction causes a right-handed super-coil of the actin filament in in vitro motility assay systems in which the pointed end of the actin filament is fixed to a glass surface. (B) Direction of force F and counteraction C acting under isometric conditions. This counteraction causes left-handed super-coil of the actin filament in in vitro motility assay systems. Note that the direction of the torque component changes with shortening conditions. Red balls represent the actin molecules included in a particular interaction unit.

Mentions: In this section we analyze the observations cited above and clarify the relationship between the direction of the force and the load acting at the cross-bridge. It is likely that myosin heads are randomly oriented on the myosin-coated glass surface, while myosin heads in the myosin track seem to be highly oriented. Thus, the number of myosin heads effectively binding to the actin filament to promote the sliding movement is larger on the myosin track than on the myosin-coated glass surface. As a result, the load shared by each myosin head during the sliding movement of the actin filament will be smaller on the myosin track than on the myosin-coated glass surface. It is supposed that the experiment on the myosin track that caused the left-handed super-coil of an actin filament (Fig. 2C) simulated the sliding movement of the myosin head on the actin filament under no-load or light-load conditions (Fig. 3A). In contrast, the force that caused the right-handed super-coil of an actin filament (Fig. 2A) on the myosin-coated glass surface corresponded to the force acting under heavy-load conditions (Fig. 3B). An actin filament (Fig. 2B) buckles when the myosin head slides parallel to the axis of the actin filament, because the torque component is not generated under this condition.


Load-dependent sliding direction change of a myosin head on an actin molecule and its energetic aspects: Energy borrowing model of a cross-bridge cycle
Direction of force acting at a cross-bridge in muscle. (A) Direction of force F and counteraction C acting on an actin molecule under rapid-sliding conditions. This counteraction causes a right-handed super-coil of the actin filament in in vitro motility assay systems in which the pointed end of the actin filament is fixed to a glass surface. (B) Direction of force F and counteraction C acting under isometric conditions. This counteraction causes left-handed super-coil of the actin filament in in vitro motility assay systems. Note that the direction of the torque component changes with shortening conditions. Red balls represent the actin molecules included in a particular interaction unit.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036636&req=5

f3-5_11: Direction of force acting at a cross-bridge in muscle. (A) Direction of force F and counteraction C acting on an actin molecule under rapid-sliding conditions. This counteraction causes a right-handed super-coil of the actin filament in in vitro motility assay systems in which the pointed end of the actin filament is fixed to a glass surface. (B) Direction of force F and counteraction C acting under isometric conditions. This counteraction causes left-handed super-coil of the actin filament in in vitro motility assay systems. Note that the direction of the torque component changes with shortening conditions. Red balls represent the actin molecules included in a particular interaction unit.
Mentions: In this section we analyze the observations cited above and clarify the relationship between the direction of the force and the load acting at the cross-bridge. It is likely that myosin heads are randomly oriented on the myosin-coated glass surface, while myosin heads in the myosin track seem to be highly oriented. Thus, the number of myosin heads effectively binding to the actin filament to promote the sliding movement is larger on the myosin track than on the myosin-coated glass surface. As a result, the load shared by each myosin head during the sliding movement of the actin filament will be smaller on the myosin track than on the myosin-coated glass surface. It is supposed that the experiment on the myosin track that caused the left-handed super-coil of an actin filament (Fig. 2C) simulated the sliding movement of the myosin head on the actin filament under no-load or light-load conditions (Fig. 3A). In contrast, the force that caused the right-handed super-coil of an actin filament (Fig. 2A) on the myosin-coated glass surface corresponded to the force acting under heavy-load conditions (Fig. 3B). An actin filament (Fig. 2B) buckles when the myosin head slides parallel to the axis of the actin filament, because the torque component is not generated under this condition.

View Article: PubMed Central - PubMed

ABSTRACT

A model of muscle contraction is proposed, assuming loose coupling between power strokes and ATP hydrolysis of a myosin head. The energy borrowing mechanism is introduced in a cross-bridge cycle that borrows energy from the environment to cover the necessary energy for enthalpy production during sliding movement. Important premises for modeling are as follows: 1) the interaction area where a myosin head slides is supposed to be on an actin molecule; 2) the actomyosin complex is assumed to generate force F(θ), which slides the myosin head M* in the interaction area; 3) the direction of the force F(θ) varies in proportion to the load P; 4) the energy supplied by ATP hydrolysis is used to retain the myosin head in the high-energy state M*, and is not used for enthalpy production; 5) the myosin head enters a hydration state and dehydration state repeatedly during the cross-bridge cycle. The dehydrated myosin head recovers its hydrated state by hydration in the surrounding medium; 6) the energy source for work and heat production liberated by the AM* complex is of external origin. On the basis of these premises, the model adequately explains the experimental results observed at various levels in muscular samples: 1) twist in actin filaments observed in shortening muscle fibers; 2) the load-velocity relationship in single muscle fiber; 3) energy balance among enthalpy production, the borrowed energy and the energy supplied by ATP hydrolysis during muscle contraction. Force F(θ) acting on the myosin head is depicted.

No MeSH data available.


Related in: MedlinePlus