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Mechanistic role of movement and strain sensitivity in muscle contraction

机译：运动和应变敏感性在肌肉收缩中的机械作用

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Tension generation can be studied by applying step perturbations to contracting muscle fibers and subdividing the mechanical response into exponential phases. The de novo tension-generating isomerization is associated with one of these phases. Earlier work has shown that a temperature jump perturbs the equilibrium constant directly to increase tension. Here, we show that a length jump functions quite differently. A step release (relative movement of thick and thin filaments) appears to release a steric constraint on an ensemble of noncompetent postphosphate release actomyosin cross-bridges, enabling them to generate tension, a concentration jump in effect. Structural studies [Taylor KA, et al. (1999) Tomo-graphic 3D reconstruction of quick-frozen, Ca~(2+)-activated contracting insect flight muscle. Cell 99:421-431] that map to these kinetics indicate that both catalytic and lever arm domains of noncompetent myosin heads change angle on actin, whereas lever arm' ' movement alone mediates the power stroke. Together, these kinetic and structural observations show a 13-nm overall interaction distance of myosin with actin, including a final 4- to 6-nm power stroke when the catalytic domain is fixed on actin. Raising fiber temperature with both perturbation techniques accelerates the forward, but slows the reverse rate constant of tension generation, kinetics akin to the unfolding/folding of small proteins. Decreasing strain, however, causes both forward and reverse rate constants to increase. Despite these changes in rate, the equilibrium constant is strain-insensitive. Activation enthalpy and entropy data show this invariance to be the result of enthalpy-entropy compensation. Reaction amplitudes confirm a strain-invariant equilibrium constant and thus a strain-insensitive ratio of pretension- to tension-generating states as work is done.
机译：可以通过将逐步扰动应用于收缩肌肉纤维并将机械响应细分为指数阶段来研究张力的产生。从头产生张力的异构化与这些相之一有关。早期的工作表明，温度跃变直接扰乱了平衡常数，从而增加了张力。在这里，我们显示了长度跳变的功能完全不同。步进释放（粗细丝的相对运动）似乎释放了对非功能性磷酸盐释放放线肌球蛋白跨桥的空间约束，使它们能够产生张力，实际上是浓度的跳跃。结构研究[Taylor KA等。 （1999）快速冻结，Ca〜（2+）激活的昆虫昆虫飞行肌肉的层析3D重建。映射到这些动力学的细胞[99：421-431]表明，不称职的肌球蛋白头的催化域和杠杆臂域都改变了肌动蛋白的角度，而杠杆臂的运动仅介导了动力冲程。总之，这些动力学和结构观察结果表明，肌球蛋白与肌动蛋白之间的相互作用总距离为13 nm，包括将催化域固定在肌动蛋白上时的最终4至6 nm功率冲程。通过两种扰动技术提高纤维温度可加快前进速度，但会降低张力生成的反向速率常数，其动力学类似于小蛋白的展开/折叠。但是，应变降低会导致正向和反向速率常数都增加。尽管速率有这些变化，但平衡常数对应变不敏感。激活焓和熵数据表明这种不变性是焓-熵补偿的结果。反应幅度确定了应变不变的平衡常数，因此确定了工作完成后预应力与张力产生状态的应变不敏感比率。

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