Ca2+ release and cross bridge cycle

Question 1

cross bridge cycle 4 phases:

Answer 1

1. In the stage of cross bridge formation, the activated myosin head binds to actin. Inorganic phosphate and ADP get released.
2. Activated myosin pivots, sliding the thin myofilament towards the center of the sarcomere.
3. In cross bridge detachments: another molecule of ATP attaches to the myosin head weakening the bond between actin and myosin. (uncorked).
4. Reactivation of myosin head, ATP is hydrolyzed to ADP and inorganic phosphate, the energy released during hydrolysis reactivates the myosin head and returns it to a cocked position.

Question 2

In cross bridge detachments: another molecule of ATP attaches to the myosin head weakening the bond between actin and myosin. (corked).
What stage is this? explain why.

Answer 2

When skeletal muscle is at its relaxed state, there is a high amount of ATP, myosin head is cocked, Ca2+ conc is low in the intracellular cytosol, which leads to a weak binding of actin and myosin, because tropomyosin is partially blocking the myosin head on the actin myosin binding site on the actin thin filaments.

Question 3

What is the rigor state?

Answer 3

This a temporary state, where myosin and actin are strongly bonded due to lack of ATP. Muscles are stuck in contracted phase.

Question 4

What are some roles of ATP in the skeletal muscle contraction?

Answer 4

1. ATP releases the strong bonds between actin and myosin, by binding to myosin. (Releasing rigor state)
2. Allows myosin head to swivel and adapt to high energy state in the (relaxed muscle)
3. Powers the Na/K+ ATPase
4. ATP actively transports Ca2+ back into the Sarcoplasmic reticulum to stop contraction: Ca2+ unbinds from Troponin, tropomyosin glides over the actin filament and blocks the actin myosin binding site, allowing muscle relaxation.