Veigel Claudia, Molloy Justin E, Schmitz Stephan, Kendrick-Jones John
Division of Physical Biochemistry, NIMR, The Ridgeway Mill Hill, London NW7 1AA, UK.
Nat Cell Biol. 2003 Nov;5(11):980-6. doi: 10.1038/ncb1060. Epub 2003 Oct 26.
Muscle contraction is driven by the cyclical interaction of myosin with actin, coupled with ATP hydrolysis. Myosin attaches to actin, forming a crossbridge that produces force and movement as it tilts or rocks into subsequent bound states before finally detaching. It has been hypothesized that the kinetics of one or more of these mechanical transitions are dependent on load, allowing muscle to shorten quickly under low load, but to sustain tension economically, with slowly cycling crossbridges under high load conditions. The idea that muscle biochemistry depends on mechanical output is termed the 'Fenn effect'. However, the molecular details of how load affects the kinetics of a single crossbridge are unknown. Here, we describe a new technique based on optical tweezers to rapidly apply force to a single smooth muscle myosin crossbridge. The crossbridge produced movement in two phases that contribute 4 nm + 2 nm of displacement. Duration of the first phase depended in an exponential manner on the amplitude of applied load. Duration of the second phase was much less affected by load, but was significantly shorter at high ATP concentration. The effect of load on the lifetime of the bound crossbridge is to prolong binding when load is high, but to accelerate release when load is low or negative.
肌肉收缩是由肌球蛋白与肌动蛋白的周期性相互作用驱动的，并伴有ATP水解。肌球蛋白附着于肌动蛋白，形成一个横桥，在其倾斜或摆动到后续结合状态并最终分离之前，该横桥会产生力和运动。据推测，这些机械转变中一个或多个的动力学取决于负荷，这使得肌肉在低负荷下能够快速缩短，但在高负荷条件下，通过缓慢循环的横桥，能够经济地维持张力。肌肉生物化学取决于机械输出这一观点被称为“芬恩效应”。然而，负荷如何影响单个横桥动力学的分子细节尚不清楚。在此，我们描述了一种基于光镊的新技术，用于快速向单个平滑肌肌球蛋白横桥施加力。该横桥产生的运动分两个阶段，位移为4纳米 + 2纳米。第一阶段的持续时间以指数方式取决于施加负荷的幅度。第二阶段的持续时间受负荷影响较小，但在高ATP浓度下明显更短。负荷对结合横桥寿命的影响是，当负荷高时延长结合时间，但当负荷低或为负时加速释放。
