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How tropomyosin and troponin regulate muscle contraction

How calcium ion concentration dictates whether a muscle contracts or not. Created by Sal Khan.
Video transcript
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 spring-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.