Smith D A, Geeves M A
Max-Planck Institute for Molecular Physiology, Dortmund, Germany.
Biophys J. 1995 Aug;69(2):538-52. doi: 10.1016/S0006-3495(95)79927-1.
Quantitative predictions of steady-state muscle properties from the strain-dependent cross-bridge for muscle are presented. With a stiffness of 5.4 x 10(-4) N/m per head, a throw distance of 11 nm, and three allowed actin sites/head, isometric properties and their dependence on phosphate and nucleotide levels are well described if the tension-generating step occurs before phosphate release. At very low ATP levels, rigorlike states with negative strain are predicted. The rate-limiting step for cycling and ATP consumption is strain-blocked ADP release for isometric and slowly shortening muscle. Under rapid shortening, ATP hydrolysis on detached heads is the rate-limiting step, and the ratio of bound ATP to bound ADP.Pi increases by a factor of 7. At large positive strains, bound heads must be forcibly detached from actin to account for tension in rapid extension, but forced detachment in shortening has no effect without destroying isometric attached states. Strain-blocked phosphate release as proposed produces modest inhibition of the ATPase rate under rapid shortening, sufficient to give a maximum for one actin site per helix turn. Alternative cross-bridge models are discussed in the light of these predictions.
本文给出了基于应变依赖型横桥模型对肌肉稳态特性的定量预测。若每个横桥头部的刚度为5.4×10⁻⁴N/m，摆动距离为11nm，且每个头部有三个可利用的肌动蛋白位点，那么如果张力产生步骤发生在磷酸释放之前，等长特性及其对磷酸盐和核苷酸水平的依赖性就能得到很好的描述。在非常低的ATP水平下，预测会出现具有负应变的类似僵直状态。对于等长收缩和缓慢缩短的肌肉，循环和ATP消耗的限速步骤是应变阻碍的ADP释放。在快速缩短过程中，游离头部的ATP水解是限速步骤，且结合的ATP与结合的ADP·Pi的比率增加了7倍。在大的正应变下，为了解释快速伸展时的张力，结合的头部必须从肌动蛋白上强行分离，但在缩短过程中的强行分离如果不破坏等长附着状态则没有影响。所提出的应变阻碍的磷酸盐释放会在快速缩短过程中对ATP酶速率产生适度抑制，足以使每螺旋圈的一个肌动蛋白位点达到最大值。根据这些预测对其他横桥模型进行了讨论。
