Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, United Kingdom.
Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, United Kingdom.
Biophys J. 2020 Oct 20;119(8):1501-1512. doi: 10.1016/j.bpj.2020.09.004. Epub 2020 Sep 15.
It is controversial whether the phosphate (Pi) release step in the cross-bridge cycle occurs before or after the first tension-generating step and whether it is fast or slow. We have therefore modified our previous model of the frog cross-bridge cycle by including a Pi release step either before (model A) or after (model B) the first tension-generating step and refined the two models by downhill simplex runs against experimental data for the force-velocity relation and the tension transients after length steps. Pi release step was initially made slow (70 s), but after refinement, it became fast (∼500 s for model A and ∼6000 s for model B). The two models gave similar fits to the experimental tension transients after length steps, but model A gave a better fit to the lengthening limb of the force-velocity relation than model B. 50 mM Pi inhibited the isometric tension of model A by ∼50% but that of model B by only ∼25%. The half-inhibition was at 6.0 mM Pi for model A and at 1.6 mM Pi for model B. The values for model A were consistent with experimental data. We also simulated the effect Pi jump as in caged Pi experiments. For model A, a Pi jump induced a tension fall at a rate similar to the experimental phase II. There was then a small rise in tension to the steady state mimicking the experimental phase III. The initial tension fall was caused by detachment of M⋅ADP⋅Pi myosin heads from actin and reversal of the first tension-generating step. For model B, the fall in tension was more rapid and due to reversal of the first tension-generating step, and phase III was not observed. We conclude that, as in model A, the Pi release step is before the first tension-generating step and is moderately fast.
在横桥循环中，磷酸根（Pi）的释放步骤是发生在第一个产生张力的步骤之前还是之后，以及它是快速还是缓慢的，这一点存在争议。因此，我们通过在第一个产生张力的步骤之前（模型 A）或之后（模型 B）包含 Pi 释放步骤，对我们之前的青蛙横桥循环模型进行了修改，并通过针对力-速度关系和长度步后的张力瞬变的下坡单纯形运行对两个模型进行了细化。Pi 释放步骤最初被设置为缓慢（70 s），但在细化后，它变得很快（模型 A 约为 500 s，模型 B 约为 6000 s）。两个模型对长度步后的实验张力瞬变都有很好的拟合，但模型 A 对力-速度关系的伸长支的拟合比模型 B 更好。50 mM Pi 抑制模型 A 的等长张力约 50%，但抑制模型 B 的张力仅约 25%。模型 A 的半抑制浓度为 6.0 mM Pi，模型 B 的半抑制浓度为 1.6 mM Pi。模型 A 的值与实验数据一致。我们还模拟了 caged Pi 实验中的 Pi 跳跃效应。对于模型 A，Pi 跳跃以类似于实验阶段 II 的速度引起张力下降。然后，张力会略有上升，达到稳态，模拟实验阶段 III。初始张力下降是由于 ADP·Pi 肌球蛋白头部从肌动蛋白上脱离以及第一个产生张力的步骤的逆转引起的。对于模型 B，张力下降更快，这是由于第一个产生张力的步骤的逆转引起的，并且没有观察到阶段 III。我们的结论是，与模型 A 一样，Pi 释放步骤发生在第一个产生张力的步骤之前，并且速度适中。
