University of Bristol, Bristol, BS8 1TD, UK.
J Muscle Res Cell Motil. 2010 Dec;31(4):245-65. doi: 10.1007/s10974-010-9232-7. Epub 2010 Dec 4.
The stiffness of myosin heads attached to actin is a crucial parameter in determining the kinetics and mechanics of the crossbridge cycle. It has been claimed that the stiffness of myosin heads in the anterior tibialis muscle of the common frog (Rana temporaria) is as high as 3.3 pN/nm, substantially higher than its value in rabbit muscle (~1.7 pN/nm). However, the crossbridge stiffness measurement has a large error since the contribution of crossbridges to half-sarcomere compliance is obtained by subtracting from the half-sarcomere compliance the contributions of the thick and thin filaments, each with a substantial error. Calculation of its value for isometric contraction also depends on the fraction of heads that are attached, for which there is no consensus. Surprisingly, the stiffness of the myosin head from the edible frog, Rana esculenta, determined in the same manner, is only 60% of that in Rana temporaria. In our view it is unlikely that the value of such a crucial parameter could differ so substantially between two frog species. Since the means of the myosin head stiffness in these two species are not significantly different, we suggest that the best estimate of the stiffness of the myosin heads for frog muscle is the average of these data, a value similar to that for rabbit muscle. This would allow both frog and rabbit muscles to operate the same low-cooperativity mechanism for the crossbridge cycle with only one or two tension-generating steps. We review evidence that much of the compliance of the myosin head is located in the pliant region where the lever arm emerges from the converter and propose that tension generation ("tensing") caused by the rotation and movement of the converter is a separate event from the passive swinging of the lever arm in its working stroke in which the strain energy stored in the pliant region is used to do work.
肌球蛋白头部与肌动蛋白的结合刚性是决定交联桥循环动力学和力学性质的关键参数。有人声称，普通青蛙（Rana temporaria）前胫骨肌的肌球蛋白头部的刚性高达 3.3 pN/nm，远高于兔肌 (~1.7 pN/nm) 的值。然而，交联桥刚度的测量存在很大的误差，因为半肌节顺应性的交联桥贡献是通过从半肌节顺应性中减去厚丝和细丝的贡献来获得的，而这两者都有很大的误差。等长收缩时其值的计算还取决于附着的头部的分数，而对于这一点，没有共识。令人惊讶的是，以相同方式确定的食用青蛙（Rana esculenta）肌球蛋白头部的刚性仅为 Rana temporaria 的 60%。在我们看来，如此关键的参数值在两种青蛙物种之间如此显著地不同是不太可能的。由于这两种物种的肌球蛋白头部刚性的平均值没有显著差异，我们建议，青蛙肌肉肌球蛋白头部刚性的最佳估计值是这些数据的平均值，这与兔肌的刚性值相似。这将允许青蛙和兔肌肉使用相同的低协同机制来运行交联桥循环，只需一个或两个张力产生步骤。我们回顾了证据表明，肌球蛋白头部的大部分顺应性位于柔韧区域，其中杠杆臂从转换器伸出，并提出由转换器的旋转和运动引起的张力产生（“紧张”）是与被动摆动的单独事件在其工作冲程中，储存在柔韧区域的应变能用于做功。
