ex phys 1 crossbridge cycle

location of crossbridge

the point at which the myosin head binds to the actin filament

what is the crossbridge the site of?

force generation

__ pulls __

myosin, actin

where is actin pulled towards?

M line

summary of crossbridge cycle

myosin cycles between forming crossbridges by binding to actin, pulling, and then detaching from actin before binding again over and over

first step of crossbridge cycle (5 step cycle)

the active site on actin is exposed as Ca2+ (calcium ion) binds to troponin

2nd step of crossbridge cycle (5 step cycle)

the myosin head forms a cross-bridge with actin

3rd step of crossbridge cycle (5 step cycle)

during the power stroke, the myosin head bends, and ADP and phosphate are released

4th step of crossbridge cycle (5 step cycle)

a new molecule of ATP attaches to the myosin head, causing the crossbridge to detach

5th step of crossbridge cycle (5 step cycle)

ATP hydrolyzes to ADP and phosphate, which returns the myosin to the “cocked” position

energy molecules involved in myosin binding to actin

ATP → ADP + Pi (inorganic phosphate)

crossbridge formation (4 step cycle)

myosin binds to actin (ADP and Pi bound to myosin)

myosin is activated (90 degrees)

power stroke (4 step cycle)

myosin bound to actin

nothing attached to myosin

myosin is deactivated (45 degrees)

detachment (4 step cycle)

myosin not bound to actin

ATP attached to myosin

myosin is deactivated

activaiton (or reactivation) (4 step cycle)

myosin head back to 90 degrees

ATP is broken down (hydrolyzed) to release energy that activates the myosin head

ATP → ADP + Pi + energy

order of steps in 4 step cycle

crossbridge formation, power stroke, detachment, activation

2 criteria for continuous cycling

myosin has access to actin (tropomyosin has to be out of the way)(EC coupling)

ATP supply (metabolism)

statues of myosin at each step

1. activated

2. deactivated

3. deactivated

4. activated