Practice question

Cross bridge cycle parsed by transient kinetics reveals distinct substeps responsible for detachment priming and force generation. Myosin rigor complex actin myosin no nucleotide strongly bound. ATP binding rate 1 to 3 per micromolar per second causes rapid dissociation 500 to 1000 per second. Detached myosin hydrolyzes ATP to ADP plus inorganic phosphate rate about 50 per second inducing recovery stroke moving lever arm into pre power stroke high energy conformation approximately 90 degree rotation storing strain. Myosin ADP Pi weakly binds actin via electrostatic contacts. Isomerization into strongly bound state triggers phosphate release from active site, observed as burst of Pi, accompanied by closure of actin binding cleft and 60 degree rotation of converter domain generating power stroke dragging actin 5 to 10 nanometer generating 2 to 6 piconewton force. ADP remains bound temporarily then released rate limiting step about 20 per second returning to rigor. Thus power stroke specifically driven by phosphate release, not ATP binding which detaches, nor ADP binding which slows, nor actin monomer exchange unrelated to motor mechanism.