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  • In the last video, we learned how myosin-- and myosin II in

  • particular-- when we say myosin II it actually has two

  • of these myosin heads and their tails are inter-wound

  • with each other-- how myosin II can use ATP to

  • essentially-- you can almost imagine either pulling an

  • actin filament or walking up an actin filament.

  • It starts attached.

  • ATP comes and bonds onto it.

  • That causes it to be released.

  • Then the ATP hydrolyzes into ADP and a phosphate group.

  • And when that happens, that energy's released.

  • It puts this into a higher energy state.

  • It kind of spring-loads the protein and then it attaches

  • up another notch on the actual actin filament and then the

  • phosphate group leaves and that's where the confirmation

  • change in this protein is enough.

  • It generates the power stroke to actually push on the actin

  • filament-- and you could imagine, either move the

  • myosin-- whatever the myosin is connected to-- to the left

  • or whatever the actin is connected to to the right.

  • We're going to talk a lot more about what they're connected

  • to in future videos.

  • Now, a couple of questions might have been

  • raising in your head.

  • This guy had so much effort to pull on this thing, right?

  • There's some tension pulling in the other direction, right?

  • I said this is what happens in muscles, so there must be some

  • weight or some other resistance.

  • So what happens when this releases?

  • At the first step when ATP joined and this released,

  • wouldn't the actin filament just go back to

  • where it was before?

  • Especially if there's some tension on it

  • going in that direction.

  • And the simple answer to that is, this isn't the only myosin

  • protein that's acting on this actin.

  • You have others all along the chain.

  • Maybe you have one right there.

  • Maybe you have one right there.

  • They're all working at their own pace at different times.

  • So you have so many of these that when one of them is

  • disengaged, another one of them might be in their power

  • stroke or another one might be engaged.

  • So it's not like you have this notion of, if all of a sudden

  • one lets go, that the actin filament will recoil back to

  • where it was.

  • Now the next question that you might be thinking is, how do I

  • turn on and off this situation?

  • We have command over our muscles.

  • What can turn on or off this system of the myosin

  • essentially crawling up the actin?

  • And to understand that, there's two other proteins

  • that come into effect.

  • That's tropomyosin and troponin.

  • And so I'm going to redraw the actin-- I'll do a very rough

  • drawing of the actin filament.

  • Let's say that that's my actin filament right there with its

  • little grooves.

  • It's actually a helical structure.

  • And actually, these grooves-- it's kind of a helical-- but

  • we won't worry too much about that.

  • What we drew so far, at least in the last video, you had

  • these little myosin.

  • You can view them as feet or head or whatever that keep

  • attaching to it and then based on where they are in that ATP

  • cycle, they can keep getting cranked back up or

  • sprinr-loaded and go to the next one and push back.

  • Now, on top of this actin, you actually have

  • this tropomyosin protein.

  • And this tropomyosin protein, it coils around the actin.

  • So this is our actin right here.

  • This is one of the two heads of the myosin II.

  • And then we have our tropomyosin.

  • Tropomyosin is coiled around.

  • It's a very rough sketch, but you can imagine it's coiled

  • around and it goes back behind it, then it goes like that,

  • and then it goes back behind it, then it goes like that.

  • So it's coiled around it and the important thing about it

  • is, if there's-- let me take a step back.

  • It's coiled around and it's attached to the actin by

  • another protein called troponin.

  • Let's say it's attached there and-- this isn't exact, but

  • let's say it's attached there, and there, and there, and

  • there, and there by the troponin.

  • So let me write this down.

  • So you can imagine, the troponin is kind of like the

  • nails into the actin.

  • So it dictates where the tropomyosin is.

  • So when a muscle is not contracting, it turns out that

  • the tropomyosin is blocking the myosin from being able

  • to-- and I've read a bunch of accounts on this and I think

  • this is still an area of research.

  • It's not 100% clear one way or the other.

  • Tropomyosin is-- or maybe both-- blocking the myosin

  • from being able to attach to the actin where it normally

  • attaches so it won't be able to crawl up the actin-- or

  • sometimes the myosin is attached to the actin, but it

  • keeps it from releasing and sliding up the actin to keep

  • that walking procedure.

  • So the bottom line is that this tropomyosin kind of

  • blocks the myosin head-- this is the myosin head right

  • there-- from crawling up the actin, either by physically

  • blocking its actual binding site or if it's already bound,

  • keeping it from being able to keep sliding up the actin.

  • Either way, it's blocking it and the only way to make it

  • unblocked is for the troponins to actually change their

  • confirmation, for them to change their shape.

  • And the only way for them to change their shape is if we

  • have a high calcium ion concentration.

  • So if you have a bunch of calcium ions, if you have a

  • high enough concentration, these calcium ions are going

  • to bond to the troponin and then that changes the

  • confirmation of the troponin enough to move the

  • configuration of the tropomyosin.

  • So let me write this down.

  • So normally, tropomyosin blocks, but then when you have

  • a high calcium ion concentration, they bind to

  • troponin and then the troponin, they change their

  • confirmation so it moves the tropomyosin out of the way.

  • So when it moves out of the way, you have a high calcium

  • concentration, bonds troponin, moves tropomyosin out of the

  • way, then all of a sudden what we talked about in the last

  • video-- these guys can start walking up the actin or

  • pushing the actin to the right, however you

  • want to view it.

  • But then if the calcium concentration goes low, then

  • the calciums get released from the troponin.

  • You need to have enough to always hang around here.

  • If the concentration becomes really low here, these guys

  • will start to leave. So then the troponin goes back to, I

  • guess, standard confirmation.

  • That makes the tropomyosin block the myosin again.

  • So it's actually-- I mean, I can't say

  • anything here is simple.

  • This was only discovered maybe 50 or 60 years ago and you can

  • imagine to actually observe these things or to create

  • experiments to definitively know what's happening--

  • nothing is simple, but the idea is simple.

  • Without calcium, the tropomyosin is blocking the

  • ability of the myosin to attach where it needs to

  • attach or slide up the actin so it can keep pushing on it.

  • But if the calcium concentration is high enough,

  • they will bond to the troponin-- which essentially

  • nails down the tropomyosin that's wound around the actin

  • and when they change their confirmation with the calcium

  • ions, it moves the tropomyosin out of the way so that the

  • myosin can do what it does.

  • So you can imagine already, we're building up a way for--

  • one, for muscles to contract, but even better, for us to

  • control muscles to contract.

  • So if we have a high calcium concentration within the cell,

  • the muscle will contract.

  • If we have a low calcium concentration again, then all

  • of a sudden, these will release.

  • They'll be blocked, and then the muscle will relax again.

In the last video, we learned how myosin-- and myosin II in

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