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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.


Related in: MedlinePlus

Schematic presentations of reaction pathways and energetic events included in the cross-bridge cycle. The cross-bridge cycle includes two reaction pathways. Reaction pathway 1 occurs under hydrophobic conditions and reaction pathway 2 occurs under hydrophilic conditions. ‘Gross work on actin molecule’ includes the energy necessary for the generation of M*after and enthalpy production. Pathways 1 and 3 show a schematic illustration of energy balance in the cross-bridge cycle. Pathway 1 includes the generation process of M*after from M*before and the process of enthalpy production. The potential of the reaction product of two pathways at the goal is the same; therefore, the potential difference generated by pathway 2 covers gross work production on the actin molecule in addition to the energy required for the generation of M*after (see text for details).
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f13-5_11: Schematic presentations of reaction pathways and energetic events included in the cross-bridge cycle. The cross-bridge cycle includes two reaction pathways. Reaction pathway 1 occurs under hydrophobic conditions and reaction pathway 2 occurs under hydrophilic conditions. ‘Gross work on actin molecule’ includes the energy necessary for the generation of M*after and enthalpy production. Pathways 1 and 3 show a schematic illustration of energy balance in the cross-bridge cycle. Pathway 1 includes the generation process of M*after from M*before and the process of enthalpy production. The potential of the reaction product of two pathways at the goal is the same; therefore, the potential difference generated by pathway 2 covers gross work production on the actin molecule in addition to the energy required for the generation of M*after (see text for details).

Mentions: The energy processes included in the cross-bridge cycle are summarized in Figure 13. These are categorized into two reaction pathways; reaction pathway 1 and reaction pathway 2. Reaction pathway 1 starts at M*before under hydrophilic conditions and it continues under hydrophobic conditions in the AM* complex, including dissociation of the AM* complex, as shown in Figure 12, and reaches M*after under hydrophilic conditions; therefore, reaction pathway 1 includes two energy events, i.e., generation of M*after and generation of enthalpy production. Both of these energy events are generated by the gross work performed on the actin molecule. It should be noted that the gross work is defined from the interaction unit by Equation 9; therefore, the ‘gross work on actin molecule’ shown in Figure 13 should be regarded as a product of the force and the sliding distance of the myosin head on the action molecule. Reaction pathway 2 starts at M*after in hydrophilic conditions and it reaches M*before under hydrophilic conditions. It corresponds to hydration step (d) of M*after for the reformation of M*before, as shown in Figure 12.


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
Schematic presentations of reaction pathways and energetic events included in the cross-bridge cycle. The cross-bridge cycle includes two reaction pathways. Reaction pathway 1 occurs under hydrophobic conditions and reaction pathway 2 occurs under hydrophilic conditions. ‘Gross work on actin molecule’ includes the energy necessary for the generation of M*after and enthalpy production. Pathways 1 and 3 show a schematic illustration of energy balance in the cross-bridge cycle. Pathway 1 includes the generation process of M*after from M*before and the process of enthalpy production. The potential of the reaction product of two pathways at the goal is the same; therefore, the potential difference generated by pathway 2 covers gross work production on the actin molecule in addition to the energy required for the generation of M*after (see text for details).
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036636&req=5

f13-5_11: Schematic presentations of reaction pathways and energetic events included in the cross-bridge cycle. The cross-bridge cycle includes two reaction pathways. Reaction pathway 1 occurs under hydrophobic conditions and reaction pathway 2 occurs under hydrophilic conditions. ‘Gross work on actin molecule’ includes the energy necessary for the generation of M*after and enthalpy production. Pathways 1 and 3 show a schematic illustration of energy balance in the cross-bridge cycle. Pathway 1 includes the generation process of M*after from M*before and the process of enthalpy production. The potential of the reaction product of two pathways at the goal is the same; therefore, the potential difference generated by pathway 2 covers gross work production on the actin molecule in addition to the energy required for the generation of M*after (see text for details).
Mentions: The energy processes included in the cross-bridge cycle are summarized in Figure 13. These are categorized into two reaction pathways; reaction pathway 1 and reaction pathway 2. Reaction pathway 1 starts at M*before under hydrophilic conditions and it continues under hydrophobic conditions in the AM* complex, including dissociation of the AM* complex, as shown in Figure 12, and reaches M*after under hydrophilic conditions; therefore, reaction pathway 1 includes two energy events, i.e., generation of M*after and generation of enthalpy production. Both of these energy events are generated by the gross work performed on the actin molecule. It should be noted that the gross work is defined from the interaction unit by Equation 9; therefore, the ‘gross work on actin molecule’ shown in Figure 13 should be regarded as a product of the force and the sliding distance of the myosin head on the action molecule. Reaction pathway 2 starts at M*after in hydrophilic conditions and it reaches M*before under hydrophilic conditions. It corresponds to hydration step (d) of M*after for the reformation of M*before, as shown in Figure 12.

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