Inhibition of actomyosin ATPase activity by troponin-tropomyosin without blocking the binding of myosin to actin.

TL;DRAbstract

Relaxation of vertebrate skeletal muscle is thought to occur in the absence of Ca(2+) 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 Ca(2+), S-1·ATP and S-1·ADP·P(i) bind to actin-troponin-tropomyosin (regulated actin) with almost the same affinity as in the presence of Ca(2+). 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 Ca(2+), the rate of ATP hydrolysis is more than 20 times greater than in the absence of Ca(2+). In contrast, the association constant of S-1·ATP and S-1·ADP·P(i) with regulated actin i

Chat with Paper

AI Agents for this Paper