BIO HELP on MYOSIN stuff

[pizza1994]

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In the absence of actin, which step in ATP hydrolysis by myosin is prevented, the hydrolysis of ATP or the release of ADP?

Source: TPR

I have no clue about this. I would guess both would be prevented?

 

[Cawolf]

Well actin binding to the myosin head cause the Pi to release, followed by a power stroke and subsequent ADP release.

ATP binds to the site on the myosin head and is hydrolyzed before the actin binds.

So I guess the most correct choice would be ADP release.

 

[pizza1994]

Well actin binding to the myosin head cause the Pi to release, followed by a power stroke and subsequent ADP release.

ATP binds to the site on the myosin head and is hydrolyzed before the actin binds.

So I guess the most correct choice would be ADP release.

but i thought that when the ATP binds to the myosin head ...the myosin head at this point is attached to the actin....cause the whole point of ATP binding is to release the actin....and so I would think that if there is no actin then there will be no ATP binding b/c there is no actin to release...and hence no need for ATP hydrolysis

 

[Cawolf]

Yes - ATP binding causes the actin to be released. Hence rigor mortis with no ATP transport to a muscle cell.

But the question asked about ATP hydrolysis or ADP release?

I don't think it is hydrolysis because this occurs without actin bound - it is the enzyme on the myosin head that causes this.

You only presented two choices, so the ADP release seems most correct - mainly because it is not released until the end of the powerstroke, which involves actin.

 

[pizza1994]

So ATP hydrolysis occurs without actin bound? Why? I don't see the need for it to occur if actin isn't bound ...

 

[Cawolf]

ATP hydrolysis provides the energy for the myosin head to return to it's "baseline" position so it can bind farther down the actin filament and undergo another conformational change - the power stroke.

 

[pizza1994]

ATP hydrolysis provides the energy for the myosin head to return to it's "baseline" position so it can bind farther down the actin filament and undergo another conformational change - the power stroke.

right and so orginally the myosin head is in its "high-energy" conformation. and then it binds actin and it goes to its "low-energy" conformation...so at this point we need ATP hydrolysis so that the myosin head can return to its "baseline."....and it returns to baseline by unbinding actin

so what Im saying is that if there is no actin then you dont ever need ATP hydrolysis cause the myosin head will always be at "baseline."

 

[Czarcasm]

right and so orginally the myosin head is in its "high-energy" conformation. and then it binds actin and it goes to its "low-energy" conformation...so at this point we need ATP hydrolysis so that the myosin head can return to its "baseline."....and it returns to baseline by unbinding actin

so what Im saying is that if there is no actin then you dont ever need ATP hydrolysis cause the myosin head will always be at "baseline."

I'm not really sure what you're saying here. It's a very simple concept, don't over think it:

Myosin head has ATPase activity. Upon binding ATP, it will autocatalyze ATP into ADP + Pi + Energy.

The following involves actin to be present: At this point, myosin head is ready to bind to actin. We say myosin head is in the "cocked" position (high-energy conformation) and is now ready to "grab onto" actin. The myosin head binds to the actin active site (provided tropomyosin isn't blocking) and immediately, phosphate is lost in the process. The removal of phosphate causes an allosteric change in structure, forcing the myosin head to bend 45 degrees. This bend is called the powerstroke and is energy costly, releasing ADP in the process. For myosin head to release, another ATP substrate must bind, bringing it back to the original relaxed position. Provided calcium is still present and ATP is widely available, the process repeats all over, pulling actin closer and closer towards each other, allowing the muscle to contract.

In the absence of actin (with ATP still present), myosin head will still hydrolyze ATP to ADP + Pi + Energy. However, these would remain bound to the myosin head and it would remain in a cocked (high-energy) position. That's all there is to it. It remains in the high-energy conformation. There's no need to consider the "baseline" or "ow-energy conformation" or any subsequent event, because we're specifically told actin is not present.