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钙离子浓度如何决定了肌肉收缩. Sal Khan 创建




in the last video we learned how myosin and myosin - in particular when we say myosin - it actually has two of these myosin heads and their tails are inter round with each other how myosin two can use ATP to essentially you can also 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 ATP and it started to ADP and a phosphate group and when that happens that energy is released this 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 conformation change in this protein is enough it generates the powerstroke to actually push on the actin filament and either you can imagine either move the myosin whatever the myosin is connected to to the to the left or whatever the actin is connected to to the right and we're gonna 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 you know this guy had so much effort to pull on this thing right there surprise some tension pulling in the other direction right this is I said this is what happens in muscles so there must be some weight or some other resistance so what happens when this release is you know when when at the the first step when ATP joined and this released wouldn't the act of 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 on this actin you have others all along the chain maybe you have one right there maybe you have one right there and 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 let's 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 you know we have command over our muscles what what can turn on or off the system of the myosin essentially crawling up the actin and to understand that there's two other proteins that come into effect and it's tropomyosin and troponin troponin troppo let me write this down tropomyosin troppo Tropo myosin myosin in a different color I'll write troponin tropone in and so I'm gonna redraw the actin and 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 let's say it looks like this like that well that's close enough and actually these grooves you know it's a kind of a helical scribe all worried too much about that what we drew so far at least in the last video you had you had little you have these little myosin you can view misfeed or head or whatever you that keep attaching to it and then based on where they are on that ATP cycle they can keep getting cranked back up or the spring-loaded and then go to the next one and push back now on top of this actin you actually have this tropomyosin protein in this tropomyosin protein it it for it coils around the actin so let me draw the trope so this is our actin right here this is the mayas this is one of the two heads of the myosin two myosin and then we have our tropomyosin tropomyosin is coiled around it's a very rough sketch but you can imagine it's coiled around then 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 well let me let me let me take a step back it's coiled around it and it's attached to the actin at by another protein called troponin so let's the troponin let me draw so let's say it's attached there and you know this isn't exact but let's say it's attached there and there and there there and there by the troponin so let me write this down this is troponin troponin and then this is tropomyosin TRO for myosin so you can imagine the troponin troponin is kind of like the nails into the actin so it dictates where the tropomyosin is so in when it's when a muscle is not contracting it turns out that these that the tropomyosin is blocking the myosin from being able to and and I've read a bunch of accounts on this and I think this is a still an area of research it's not a hundred percent 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 it sometimes the myosin is attached to the actin but it keeps it from sliding from releasing and sliding up the actin to keep that walking procedure so the bottom line is is that this tropomyosin kind of blocks the myosin blocks the myosin the myosin alright this the myosin head this is the myosin head right there this is the myosin head that I'm talking about from crawling up the actin from crawling from crawling up the actin even by physically Bondy blocking its actual binding site or if it's already bound from keeping it from be able to keep sliding up the actin either way it's blocking it and the only way to make it unblocked is for the troponin to actually change their conformation 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 so if you have calcium ions what they do if you have a high enough concentration these calcium ions are going to bond to the troponin they're going to bond to the troponin and then that changes the conformation of the troponin enough to move the configuration of the tropomyosin so let me write this down so normally tropomyosin blocks well then when you have a high calcium high calcium ion concentration they they bind to troponin tropone in and then the troponin and then the troponin and they change their conformation so it moves the tropomyosin out of the way moves Tropo myosin out of the way out of the way so when it moves out of the way you have a high concentration calcium concentration bond strop and in moose 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 the at or pushing the actin to the right however you want to view it but then if the calcium concentration goes low so a low calcium ion concentration then the calcium's get released from the troponin you have needs to have enough to always hang around you if the concentration becomes really low here these guys will start to leave so then the troponin troponin goes back to I guess standard conformation goes back to the standard conformation and that makes the tropomyosin can block block the myosin again makes Tropo myosin block again so it's actually I mean you know I can't say anything here simple this is this was only discovered maybe 50 or 60 years ago and you could imagine to actually observe these things or to create experiments to make this definitively know what's happening nothing is simple but the idea is simple in the presence and without calcium the tropomyosin is blocking the ability of the myosin to attach where it needs to attach or slide up the actin so that can 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 the tropomyosin that's wound around the actin and it and and when they change their conformation with the calcium ions moves it moves the tropomyosin out of the way so that the myosin can do what it does so you can imagine already where 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