Myosin-actin interaction dissociation-association

The rapid decrease in the level of ATP following death has two consequences. First, the cytosolic level of calcium rises rapidly because the Ca i-ATPase pumps in the plasma membrane and sarcoplasmic reticulum membrane no longer operate. High Ca, through troponin and tropomyosin, enables myosin to interact with actin. Second, a large proportion of SI heads -will be associated with actin. Recall that ATP is required to dissociate the actomyosin complex. In the absence of ATP, skeletal muscle is locked in the contracted (rigor) state. 

Fig. 12.6. Direct determination of mechano-chemical coupling of myosin, (a) An experimental arrangement for simultaneous measurement. Single-headed myosin in the co-filament with an excess myosin rod is immobilized on the pedestal of the slide glass. An actin filament is manipulated by a trapped laser through two beads attached at both ends to interact with a myosin head in the correct arrangement. The ATP turnover is measured by monitoring the fluorescence from Cy3-ATP (Cy3-ADP) associated to and dissociated from the myosin head using TIRF microscopy, (b) Time trajectory of the displacement of the myosin head and the ATP turnover. The upper, middle, and bottom trace show the time course of displacements, changes in stiffness, which was calculated from the variance of the thermal motion of the beads, and changes in the fluorescence intensity of Cy3-nucleotide at the position of the myosin head...

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