MECHANISM OF REGULATION OF THE ACTOMYOSIN ATPASE

在我们的肌肉调节模型中，受调控的肌动蛋白可以存在于 打开Form，完全激活肌球蛋白S-1 ATPase活性，或 关闭的表单，它显示的激活很少。缺乏 被关闭的表单激活假定是由于 托品-原肌球蛋白抑制Acto.S-1 ATPase中PI的释放 循环，而不是通过阻断S-1.ATP(和S-1.ADP.PI)与 肌动蛋白，如空间位阻模型所提示的。我们测试了几个 我们模型的各个方面。首先，我们的模型预测S-1，ATP和 S-1.ATP类似物pPDM.S-1不应开启调节的Acto.S-1 ATPase 在无Ca~(2+)条件下的活性。与我们的模型一致，我们发现 与最大启动率相比，无论是S-1.ATP还是pPDM.S-1 显著开启受调控的acto.S-1 ATPase活性。第二，我们的 模型预测，这些S一号物种应该会显著开启 在Ca~(2+)存在下调节acto.S-1 ATPase活性 S-1.ATP和pPDM.S-1对打开的形式的结合略强于对 受调控的肌动蛋白的关闭形式。我们发现，在下列条件下 哪个pPDM.S-1广泛地与受调控的肌动蛋白结合，它确实完全打开 Ca~(2+)对Acto.S-1 ATPase活性的调节作用。这些 数据与我们最初的模型是一致的，在这个模型中 在开启和关闭形式的受调控肌动蛋白之间 Ca~(2+)使其向开启状态移动。然而，它确实排除了 我们的替代模型中，受调控的肌动蛋白可以存在于 表单，但在任何给定条件下，这些表单中只有一个在 存在。最后，我们的模型预测，在钙离子中，细丝是 仅部分打开，而需要绑定严格的桥 到细丝上以完全打开它。同意这一点 预测发现，受调控的acto.S-1的ATPase活性为 远低于完全开启的速率。

In our model of muscle regulation, regulated actin can exist in either the turned on form, which fully activates the myosin S-1 ATPase activity, or the turned off form, which shows very little activation. The lack of activation by the turned off form is postulated to be due to tropinin-tropomyosin inhibiting the release of Pi in the acto.S-1 ATPase cycle, rather than by blocking the binding of S-1.ATP (and S-1.ADP.Pi) to actin, as was suggested by the steric blocking model. We tested several aspects of our model. First our model predicts that S-1.ATP and the S-1.ATP analog, pPDM.S-1, should not turn on the regulated acto.S-1 ATPase activity in the absence of Ca-2+. In agreement with our model, we found that compared to the maximal turned on rate, neither S-1.ATP nor pPDM.S-1 significantly turns on the regulated acto.S-1 ATPase activity. Second, our model predicts that these S-1 species should significantly turn on the regulated acto.S-1 ATPase activity in the presence of Ca-2+ provided that S-1.ATP and pPDM.S-1 bind slightly stronger to the turned on form than to the turned off form of regulated actin. We find that under conditions in which pPDM.S-1 binds extensively to regulated actin, it does fully turn on the regulated acto.S-1 ATPase activity in the presence of Ca-2+. These data are consistent with our original model in which the equilibrium between the turned on and turned off forms of regulated actin is partially shifted towards the turned on form by Ca-2+. It does, however, rule out our alternate model in which regulated actin can exist in a continuum of forms, but under any given conditions, only one of these forms are in existence. Lastly our model predicts that in Ca-2+, the thin filament is only partially turned on, while it is necessary to have rigor bridges bound to the thin filament to completely turn it on. In agreement with this prediction, we found that the ATPase activity of regulated acto.S-1 was much less than the fully turned on rate.