Structure and Function of the Two Heads of the Myosin Molecule

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Abstract
1. The myosin content of myofibrils was found to be 51% by SDS-gel electrophoresis. 2. The initial burst of P1 liberation of the ATPase (EC 3.6.1.3) of a solution of myofibrils in 1 M KCI was measured in 0.5 M KCI, and found to be 0.93 mole/mole of myosin. 3. The amount of ADP bound to myofibrils during the ATPase reaction and the ATPase activity were measured by coupling the myofibrillar ATPase reaction with sufficient amounts of pyruvate kinase (EC 2.7.1.40) and PEP to regenerate ATP. The maximum amount of ADP bound to myofibrils in 0.05M KCI and in the relaxed state was about 1.5 mole/mole of myosin. On the other hand, the ATPase activity exhibited substrate inhibition, and the amount of ATP required for a constant level of ATPase activity was smaller than that required for the maximum binding of ADP to myofibrils. 4. The maximum amount of ADP bound to myofibrils in 0.5 M KCI was about 1.9 mole/mole of myosin. When about one mole of ADP was bound to 1 mole of myosin in myofibrils, the myofibrillar ATPase activity reached the saturated level, and with further increase in the concentration of ATP one more mole of ADP was bound per mole of myosin.