Inhibition of actomyosin ATPase activity by troponin-tropomyosin without blocking the binding of myosin to actin.
Inhibition of actomyosin ATPase activity by troponin-tropomyosin without blocking the binding of myosin to actin.
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DOI：
10.1016/s0021-9258(18)34942-1
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发表时间：
1982-03
期刊：
影响因子：
--
通讯作者：
J. Chalovich;E. Eisenberg
中科院分区：
文献类型：
--
作者：
J. Chalovich;E. Eisenberg
Relaxation of vertebrate skeletal muscle is thought to occur in the absence of Ca2+ as a result of tropomyosin physically blocking the binding of myosin to actin. This steric blocking model of muscle relaxation predicts that myosin subfragment 1 (S-1) will not bind to actin under conditions where the acto-S-1 ATPase rate is inhibited. Using stopped flow absorbance as a measure of binding, we have previously shown that when the rate of ATP hydrolysis is only 4% of the rate in the presence of Ca2+, S-1·ATP and S-1·ADP·Pi bind to actin-troponin-tropomyosin (regulated actin) with almost the same affinity as in the presence of Ca2+. This result has now been confirmed using sedimentation in an air-driven ultracentrifuge to directly measure the binding at pH 7.0, 25 °C, and μ = 18 mm. In the presence of Ca2+, the rate of ATP hydrolysis is more than 20 times greater than in the absence of Ca2+. In contrast, the association constant of S-1·ATP and S-1·ADP·Pi with regulated actin is virtually the same in the absence of Ca2+ (1.4 × 104 m−1) as in the presence of Ca2+ (1.5 × 104 m−1). Similarly, at 50 mm ionic strength, the ATPase rate is inhibited about 98% in the absence of Ca2+ although the association constant is not significantly changed compared to that in the presence of Ca2+. Finally, it has been shown that, at 18 mm ionic strength, the inhibition of the actin-activated ATPase rate in the absence of Ca2+ is due to a large decrease in the maximum ATPase rate (to 4% of the Ca2+ value) with only a small change in the apparent binding constant of S-1 to actin. These data do not support a simple steric blocking model of muscle relaxation. Rather they suggest that, in the absence of Ca2+, troponin-tropomyosin inhibits a kinetic step, perhaps Pi release, in the cycle of ATP hydrolysis.
