Biochem: protein function

Question 1

What is the structure of myosin?

Answer 1

6 subunits: 2 heavy chains (meromyosin) and 2 light chains. the heavy chains form a left handed coiled coil in their c terminus and the head if the protein in the amino terminus. the light chains sit inside the head and form the active zone (binding domain). In the presence of papain or chymotrypsin the chains dissolve into sub fragment (S1, the head) and the meromyosin.

Question 2

What is the cycle of actin-myosin binding?

Answer 2

When ATP is not bound to myosin, a face on the myosin head group binds tightly to actin, and releases the F-actin to bind another one down the thin filament, thereby slide along actin filaments:

1. ATP binds to myosin: opens a cleft in the myosin that disrupts the myosin interaction, to release the bound actin.
2. ATP hydrolysis: results in a conformational change in myosin to high-energy state that moves the myosin head and changes its orientation according to the f-actin. myosin loosely binds F actin closer to the Z-disk than the one that was just released. next phosphate from ATP hydrolysis is released.
3. another conformational change to close myosin cleft and myosin-actin binding is strengthened
4. Power stroke brings back the conformation of the myosin head to resting state, release of ADP.
   The process generates ~ 4 pN of the force. and moves the thick filament 5 to 10 nm relative to the thin filament.