Question

1. During one cross-bridging cycle in striated muscle one ATP molecule is hydrolyzed. Explain the two times when the energy from this hydrolysis is used, and what “work” is done in each case.

2. Define what it means for a protein to be a calmodulin. Explain the role of the calmodulin protein that is part of the actin-linked regulatory system in striated muscle.

Answer 1

The two times when the energy released from ATP hydrolysis during muscle contraction used are:

1) During muscle contraction, the ATP is hydrolyzed into ADP and inorganic phosphate (Pi) by the enzyme ATPase. The energy released during ATP hydrolysis changes the angle of the myosin head into a “cocked” position, ready to bind to actin if the sites are available for muscle contraction.

2) After muscle contraction happens,the movement of the myosin head back to its original position is called the recovery stroke. And Resting muscles store energy from ATP hydrolysis in the myosin heads while they wait for another contraction.

The mechanism or work done during this process is:

After Myosin achieves the cocked position, the myosin head is then in a position for further movement, possessing potential energy, but ADP and Pi are still attached. If actin binding sites are covered and unavailable, the myosin will remain in the high energy configuration with ATP hydrolyzed, but still attached.

If the actin binding sites are uncovered, a cross-bridge will form; that is, the myosin head spans the distance between the actin and myosin molecules. Pi is then released, allowing myosin to expend the stored energy as a conformational change. The myosin head moves toward the M line, pulling the actin along with it. As the actin is pulled, the filaments move approximately 10 nm toward the M line. This movement is called the power stroke, as it is the step at which force is produced. As the actin is pulled toward the M line, the sarcomere shortens and the muscle contracts.