Anderson M L, Schoenberg M
Laboratory of Physical Biology, National Institute of Arthritis, Bethesda, Maryland 20892.
Biophys J. 1987 Dec;52(6):1077-82. doi: 10.1016/S0006-3495(87)83302-7.
When rabbit psoas muscle fibers bathed in solutions containing the ATP analogue magnesium pyrophosphate (MgPPi) are first stretched rapidly and then held isometric, a force is generated during the stretch which decays during the subsequent isometric period (Schoenberg, M., and E. Eisenberg. 1985. Biophys. J. 48:863-871). Previously we showed that the force decay is due to crossbridge heads detaching and reattaching in positions of lesser strain, the rate of decay of force reflecting the crossbridge detachment rate constants. Since the crossbridge detachment rate constants with MgPPi bound to the active site are so much faster than without analogue bound, at subsaturating concentrations of analogue, if the heads act independently and nucleotide association and dissociation is rapid, the rate of force decay should simply be proportional to the number of heads with bound analogue. That is, the analogue concentration dependence of the rate of force decay should have the same form as the Michaelis-Menten equation. Here we report that the concentration dependence of the rate of force decay is not described by the Michaelis equation, but is instead sigmoidal. This suggests possible cooperativity in the detachment of the crossbridge heads, the amount of cooperativity being described by an interaction coefficient of approximately 2. One idea put forward to explain the data is that both of the heads of a crossbridge may need to bind analogue before the crossbridge can relax a substantial fraction of the tension it supports.
当浸浴在含有ATP类似物焦磷酸镁（MgPPi）溶液中的兔腰大肌纤维先快速拉伸然后保持等长收缩时，拉伸过程中会产生一种力，该力在随后的等长收缩期间会衰减（舍恩伯格，M.，和E. 艾森伯格。1985年。《生物物理杂志》。48:863 - 871）。之前我们表明，力的衰减是由于横桥头部在应变较小的位置分离并重新附着，力的衰减速率反映了横桥分离速率常数。由于与活性位点结合MgPPi时横桥分离速率常数比未结合类似物时快得多，在类似物亚饱和浓度下，如果头部独立起作用且核苷酸结合和解离迅速，力的衰减速率应该仅仅与结合类似物的头部数量成正比。也就是说，力衰减速率的类似物浓度依赖性应该具有与米氏方程相同的形式。在此我们报告，力衰减速率的浓度依赖性并非由米氏方程描述，而是呈S形。这表明横桥头部分离过程中可能存在协同作用，协同作用的程度由约为2的相互作用系数描述。为解释这些数据而提出的一个观点是，在横桥能够松弛其所支撑张力的很大一部分之前，横桥的两个头部可能都需要结合类似物。
