Energized Myosin Heads Bind to Actin Only When ____ Binds

Question

Energized myosin heads bind to actin only when ____ binds to troponin.  
A) myosin
B) AMP
C) calcium
D) ATP
E) sodium

Accepted Answer

Calcium is necessary for muscle contraction because it binds to troponin, causing a conformational change in tropomyosin so that myosin heads can bind to actin. ATP is also required for myosin heads to detach from actin, but it is not necessary for the initial binding. AMP and sodium are not involved in muscle contraction.