Research and Utilization Division, Japan Synchrotron Radiation Research Institute, Hyogo, Japan.
Biophys J. 2010 Jul 7;99(1):193-200. doi: 10.1016/j.bpj.2010.04.021.
The interaction of troponin molecules on the thin filament with Ca(2+) plays a key role in regulating muscle contraction. To characterize the structural changes of troponin caused by Ca(2+) and crossbridge formation, we recorded the small-angle x-ray intensity and the myoplasmic free Ca(2+) concentration using fluo-3 AM in the same frog skeletal muscle during twitch elicited by a single electrical pulse at 16 degrees C. In an overstretched muscle, the intensity of the meridional reflection from troponin at 1/38.5 nm(-1) began to change at 4 ms after the stimulus, reached a peak at 10 ms, and returned to the resting level with a halftime of 25 ms. The concentration of troponin-bound Ca(2+) began to increase at 1-2 ms after the stimulus, reached a peak at 5 ms, and returned to the resting level with a halftime of 40 ms, indicating that troponin begins to change conformation only after a sizable amount of Ca(2+) has bound to it, and returns to the resting structure even when there is still some bound Ca(2+). In a muscle with a filament overlap, crossbridge formation appears to slow down Ca(2+) release from troponin and have a large effect on its conformation.
肌钙蛋白分子与钙离子在细肌丝上的相互作用在调节肌肉收缩中起着关键作用。为了研究钙离子和肌球蛋白横桥形成引起的肌钙蛋白结构变化，我们在 16°C 下，用单脉冲刺激青蛙骨骼肌，在同一根肌纤维上同时记录小角 X 射线强度和肌浆内游离钙离子浓度。在过度拉伸的肌肉中，1/38.5nm^-1 处的肌钙蛋白的子午线反射的强度在刺激后 4ms 开始变化，在 10ms 时达到峰值，然后用 25ms 的半时恢复到静止水平。肌钙蛋白结合的钙离子浓度在刺激后 1-2ms 开始增加，在 5ms 时达到峰值，然后用 40ms 的半时恢复到静止水平，这表明肌钙蛋白只有在结合了相当数量的钙离子后才开始改变构象，即使仍有一些结合的钙离子，它也会恢复到静止结构。在肌节重叠的肌肉中，肌球蛋白横桥的形成似乎会减缓肌钙蛋白释放钙离子，并对其构象产生很大影响。
