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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

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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.


Actin-activated myosin ATPase rate and the enthalpy production rate of myosin head calculated by the revised Hill’s formulae. Both are plotted against the normalized sliding velocity. The circles show the actin-activated myosin ATPase rate derived by analyzing Harada et al.’s experimental data in the case of double-headed myosin (Fig. 4 of Harada et al. 1987) using ATPase concentration as a parameter. The solid line represents the enthalpy production rate of the myosin head calculated by the revised Hill’s formulae based on the normalized load-velocity relationship shown in Figure 6.
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f8-5_11: Actin-activated myosin ATPase rate and the enthalpy production rate of myosin head calculated by the revised Hill’s formulae. Both are plotted against the normalized sliding velocity. The circles show the actin-activated myosin ATPase rate derived by analyzing Harada et al.’s experimental data in the case of double-headed myosin (Fig. 4 of Harada et al. 1987) using ATPase concentration as a parameter. The solid line represents the enthalpy production rate of the myosin head calculated by the revised Hill’s formulae based on the normalized load-velocity relationship shown in Figure 6.

Mentions: Harada et al. measured the sliding velocities of actin filaments on a myosin-coated glass surface and the ATPase rate of myosin heads coating the glass surface during the sliding of actin filaments by varying the ATP concentration in the medium4,5. From their results (Fig. 4 of Harada et al., 1987), the relationship between the ATPase rate of the myosin head and the sliding velocity of the actin filament can be derived, as shown by the circles in Figure 8. The ATPase rate exhibits a saturation-like profile with the increase of sliding velocity of the actin filament. This result indicates the molecular nature of actomyosin ATPase; i.e., the ATPase rate of a myosin head is virtually independent of sliding velocity above 0.3 Vmax, and it decreases with the decrease in sliding velocity below 0.3 Vmax. In shortening muscle, the maximum rate of enthalpy production is observed in the region of 0.4 Vmax, and the rate observed at Vmax is only slightly decreased19. The ATPase rate derived above shows a similar tendency.


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
Actin-activated myosin ATPase rate and the enthalpy production rate of myosin head calculated by the revised Hill’s formulae. Both are plotted against the normalized sliding velocity. The circles show the actin-activated myosin ATPase rate derived by analyzing Harada et al.’s experimental data in the case of double-headed myosin (Fig. 4 of Harada et al. 1987) using ATPase concentration as a parameter. The solid line represents the enthalpy production rate of the myosin head calculated by the revised Hill’s formulae based on the normalized load-velocity relationship shown in Figure 6.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036636&req=5

f8-5_11: Actin-activated myosin ATPase rate and the enthalpy production rate of myosin head calculated by the revised Hill’s formulae. Both are plotted against the normalized sliding velocity. The circles show the actin-activated myosin ATPase rate derived by analyzing Harada et al.’s experimental data in the case of double-headed myosin (Fig. 4 of Harada et al. 1987) using ATPase concentration as a parameter. The solid line represents the enthalpy production rate of the myosin head calculated by the revised Hill’s formulae based on the normalized load-velocity relationship shown in Figure 6.
Mentions: Harada et al. measured the sliding velocities of actin filaments on a myosin-coated glass surface and the ATPase rate of myosin heads coating the glass surface during the sliding of actin filaments by varying the ATP concentration in the medium4,5. From their results (Fig. 4 of Harada et al., 1987), the relationship between the ATPase rate of the myosin head and the sliding velocity of the actin filament can be derived, as shown by the circles in Figure 8. The ATPase rate exhibits a saturation-like profile with the increase of sliding velocity of the actin filament. This result indicates the molecular nature of actomyosin ATPase; i.e., the ATPase rate of a myosin head is virtually independent of sliding velocity above 0.3 Vmax, and it decreases with the decrease in sliding velocity below 0.3 Vmax. In shortening muscle, the maximum rate of enthalpy production is observed in the region of 0.4 Vmax, and the rate observed at Vmax is only slightly decreased19. The ATPase rate derived above shows a similar tendency.

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.