Huxley H E, Simmons R M, Faruqi A R, Kress M, Bordas J, Koch M H
J Mol Biol. 1983 Sep 15;169(2):469-506. doi: 10.1016/s0022-2836(83)80062-x.
During normal contractions of vertebrate striated muscle, it is believed that the cross-bridges which produce the sliding force undergo asynchronous cyclical changes in their structure. Thus, an X-ray diffraction diagram from a muscle under these conditions will give structural information averaged over the whole range of cross-bridge states. Such diagrams show characteristic and informative differences from those given by relaxed muscle, but can give little information about changes in the configuration of the cross-bridges at different stages of their working stroke. However, it is possible to effect a partial synchronization of these changes by applying very rapid changes in length, completed in less than one millisecond to an otherwise isometrically contracting muscle. If the amplitude of these length changes is comparable to the length of the cross-bridge stroke (say 100 A per half-sarcomere), then it should bring about a transient but significant redistribution of cross-bridge states, which would show up in the X-ray diagram. We have made use of synchrotron radiation as a high intensity X-ray source in order to record such patterns with the necessary time resolution (1 ms or less) and have found major changes in the intensity of the 143 A meridional reflection accompanying the rapid length changes of the muscle. These changes appear to arise from specific configurational changes in the cross-bridges during the working stroke. A model is suggested in which the 143 A meridional intensity in a contracting muscle arises mainly from attached cross-bridges and is generated by the part of the myosin head near the S1-S2 junction. During normal contraction, cross-bridges go through their structural cycle asynchronously with each other, since they start at different times, but if the S2 changes in length rather little, then the configurational changes in the myosin heads are synchronized with the actin filament movement in such a way that the S1-S2 junction remains relatively fixed in its axial position. In a quick release, it is suggested that bringing many S1 heads simultaneously to the end of their working strokes on actin disrupts the 143 A axial repeat of their distal ends near S2, and brings about the large decrease of the 143 A meridional reflection. This model therefore involves a large change in the position of part of the myosin head structure relative to actin during the working stroke of the cross-bridge.
在脊椎动物横纹肌的正常收缩过程中，人们认为产生滑动力的横桥结构会经历异步周期性变化。因此，在这些条件下肌肉的X射线衍射图将给出在整个横桥状态范围内平均的结构信息。这些图显示出与松弛肌肉的图有特征性且丰富的差异，但对于横桥在其工作行程不同阶段的构型变化几乎无法提供信息。然而，通过对原本等长收缩的肌肉施加在不到一毫秒内完成的非常快速的长度变化，可以实现这些变化的部分同步。如果这些长度变化的幅度与横桥行程的长度相当（比如说每个半肌节100埃），那么它应该会导致横桥状态的短暂但显著的重新分布，这会在X射线图中显现出来。我们利用同步辐射作为高强度X射线源，以便以必要的时间分辨率（1毫秒或更短）记录这样的图案，并发现随着肌肉快速长度变化，143埃子午线反射强度发生了重大变化。这些变化似乎源于横桥在工作行程期间特定的构型变化。提出了一个模型，其中收缩肌肉中143埃子午线强度主要来自附着的横桥，并且由肌球蛋白头部靠近S1 - S2连接处的部分产生。在正常收缩过程中，横桥彼此异步地经历其结构循环，因为它们在不同时间开始，但如果S2长度变化很小，那么肌球蛋白头部的构型变化会与肌动蛋白丝的运动同步，使得S1 - S2连接处的轴向位置保持相对固定。在快速释放过程中，有人提出使许多S1头部同时到达它们在肌动蛋白上工作行程的末端，会破坏它们靠近S2的远端的143埃轴向重复，并导致143埃子午线反射大幅下降。因此，这个模型涉及在横桥工作行程期间肌球蛋白头部结构的一部分相对于肌动蛋白的位置发生很大变化。
