Practice question

Force production by myosin II follows an ordered ATPase cycle that tightly couples nucleotide state to actin affinity ensuring efficient energy usage. In rigor nucleotide free myosin binds filamentous actin with high affinity angle about 45 degrees maintaining tension. Rapid binding of Mg ATP to nucleotide pocket causes allosteric opening of actin binding cleft dropping affinity thousand fold and triggering swift detachment within milliseconds even before hydrolysis occurs. While detached intrinsic ATPase hydrolyzes ATP to ADP plus inorganic phosphate providing free energy that reorients lever arm into cocked 90 degree pre stroke conformation. ADP Pi myosin then weakly attaches to new actin monomer Pi release seals cleft and drives power stroke returning lever to post stroke position dragging actin toward M line. Finally ADP dissociates leaving rigor again available for next ATP. Therefore detachment strictly requires ATP binding not hydrolysis; hydrolysis fuels repriming and Pi release triggers force ensuring unidirectional sliding of thin past thick filaments underlying shortening of sarcomeres during contraction.