Question

what is the relevance of ADP and pi to the cycle of the myosin head ....

explain in a paragraph

Answer 1

Answer: Myosin has a binding site for actin as well as for ATP. There is binding of ATP to myosin head and then the enzyme myosin ATPase breaks down ATP into ADP and inorganic phosphate. The energy released takes part in cross bridge formation. Whereas the ADP and iP remains attached and the myosin is in high energy state. After the power stroke, the ADP is released but the cross bridge remains at its place. Now again the ATP binds to myosin resulting in release of actin from the myosin head, thus freeing the active sites on the myosin head for the next cycle and hence causing relaxation.

Since ATP is needed for relaxation, therefore in the absence of ATP, the muscle remains in contacted state.

The relevance of ADP is that it helps the myosin to stay in high energy state.