G1SG2

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May 2, 2008
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According to TPR:

The steps of the contractile cycle are:

1.Binding of the myosin head to a myosin binding site on actin, also known as cross bridge formation. At this stage, myosin has ADP and Pi bound
2.The power stroke, in which the myosin head moves to a low-energy conformation, and pulls the actin chain towards the center of the sarcomere. ADP is released.
3.Binding of a new ATP molecule is necessary for RELEASE of actin by the myosin head
4.ATP hydrolysis occurs immediately and the myosin head is cocked (set in a high-energy conformation, like the hammer of a gun). Another cycle begins when the myosin head binds to a new binding site on the thin filament.

From my understanding, myosin hydrolyzes ATP (hence myosin ATPase) to ADP and Pi when it attaches to actin, and at that point, is in a "high energy confirmation". It needs an ATP molecule to attach to it and release actin, after which it sets in a "low energy confirmation". So, the high energy confirmation would be when myosin is attached to actin, and has ADP and Pi, and low energy confirmation would be when myosin has ATP bound, and is not bound to actin. That's correct, yes? However, wikipedia says something that confuses me:

"
At rest, the myosin head is bound to an ATP molecule in a low-energy configuration and is unable to access the cross bridge binding sites on the actin. However, the myosin head can hydrolyze ATP into ADP and an inorganic phosphate ion. A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration. Through the process of binding to the actin, the myosin head releases ADP and an inorganic phosphate ion, changing its configuration back to one of low energy. The myosin remains attached to actin in a state known as Rigor, until a new ATP binds the myosin head. This binding of ATP to myosin releases the actin by cross-bridge dissociation. The ATP associated myosin is ready for another cycle, beginning with hydrolysis of the ATP."

Wikipedia says that myosin changes into a LOW ENERGY CONFIRMATION WHILE REMAINING ATTACHED TO ACTIN, and that too when it has ADP and Pi????? I thought that was when it had the high energy confirmation? :confused: Or does the myosin start with the low energy confirmation with ATP, undergoes ATP hydrolysis to form the high energy confirmation WHILE BOUND to actin, and then WHILE its BOUND to actin it then switches to the low energy confirmation BEFORE ATP is added, as stated in step 2?
 
May 8, 2009
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1. Cross Bridge formation
____ (actin)
|___ = myosin head in high energy conformation (adp + pi bound)
Binds to actin

2. Power Stroke
____ (actin)
/_____ = myosin low energy conformation (power stroke)

3. ATP present myosin can release

4. Myosin binds ATP and cleaves it to go from
/____ (low energy) ---> |____ (high energy)



The myosin is starting with high-energy conformation.
It moves to low energy conformation while bound to actin.
It release actin due to ATP presence.
It binds atp and cleaves it to go back to high energy.

Recycle.
 
OP
G

G1SG2

10+ Year Member
5+ Year Member
May 2, 2008
1,454
2
Status
Pre-Medical
1. Cross Bridge formation
____ (actin)
|___ = myosin head in high energy conformation (adp + pi bound)
Binds to actin

2. Power Stroke
____ (actin)
/_____ = myosin low energy conformation (power stroke)

3. ATP present myosin can release

4. Myosin binds ATP and cleaves it to go from
/____ (low energy) ---> |____ (high energy)



The myosin is starting with high-energy conformation.
It moves to low energy conformation while bound to actin.
It release actin due to ATP presence.
It binds atp and cleaves it to go back to high energy.

Recycle.
Simple and perfect. I love your little summaries at the end :thumbup:
 

capostat

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Apr 8, 2008
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you know what I thought...

I thought that it was the binding to ATP that caused myosin to release--and it was that release which caused actin to be dragged to the right (or opposite direction)

edit: i managed to figure out EK's confusing wording--and i guess i missed the key part, which is that ADP without it's partner "little-P" for support, will bend, and that's what causes the actin to be dragged. And it was the myosin head that shot away that phosphate.
 
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