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肌球蛋白和肌动蛋白如何协同产生力量 . Sal Khan 创建




what I want to do in this video is try to understand how two proteins can interact with each other in conjunction with ATP to actually produce mechanical motion and the reason why I want to do this one it's it's it it occurs outside of muscle cells as well but this is really going to be the first video on really how muscles work and then we'll talk about how nerves actually stimulate muscles to work so it'll all build up from this video so what I've done here is I've copied and pasted two images of proteins from Wikipedia this is myosin myosin it's actually myosin 2 because you actually have two strands of the myosin protein they're interwined around each other so you can see it's this very complex looking protein or enzyme however you want to talk about I'll tell you why it's called an enzyme because it actually helps react ATP into ADP and phosphate groups so that's why it's called an ATPase or it's a it's a subclass of the ATPase enzymes this right here is actin this is act and what we're gonna see in this video is how myosin essentially uses the ATP to essentially crawl along you can almost view it as an actin rope and that's what creates mechanical energy so let me draw it I'll actually draw it on this actin right here so let's say we have a myosin one of these myosin heads so when I say a myosin head this is one of the myosin heads right here and then it's connected it's intertwined and it's wolve around this is the other one and it winds around that way now let's just say we're just dealing with one of the myosin heads let's say it's in this position let me see how well I can draw it let's say it starts off in a position that looks like that and then this is its this is kind of the the tail part that connects to some other structure and we'll talk about that in more detail but this is my myosin head right there when in its starting position not doing anything now ATP can come along and bond to this myosin head this enzyme this protein this ATPase enzyme so let me draw some ATP so let's say that that's so ATP comes along and bonds to this guy right here so let's say that's the it's not going to be this big relative to the protein but this is just to give you the idea so as soon as the ATP binds to its appropriate site on this enzyme or protein the enzyme it detaches from the actin so let me write this down so one ATP binds to myosin head myosin head and as soon as that happens that causes that causes the myosin to release myosin releases releases actin so that's step one so I started off with these this guy just touching the actin the ATP comes and it gets released so in the next step so that's so after that step it's going to look something like this and I want to draw it kind of in the same place so after the next step is going to look something like this it will have released it will have release so now let me shade it in with the white so now it looks something like that and you have the ATP attached to it still and it might be a little bit convoluted when I keep writing over the same thing but you have the ATP attached to it now the next step the ATP hydrolyzes the phosphate gets pulled off of it this is this this is an ATPase enzyme that's what it does so let me write that down so step two let me scroll down a little bit step two ATP goes to ADP plus a phosphate group and what that does that releases the energy to [ __ ] this myosin protein into kind of a high energy state so let me do step two so step two this thing you can kind of view it gets hydrolyzed it releases energy we know that ATP is the energy currency of biological systems let me draw it so it releases energy I drop them drawing it it's like a little spark or explosion but you can really imagine it is it's changing the conformation of it kind of spring loads this protein right here to go into a state so it's ready to to kind of crawl along the myosin so in step 2 so plus energy energy and then this you can kind of say it [ __ ] it [ __ ] the myosin protein or enzyme to high energy you can I kind of imagine it kind of wines the spring or loads of the spring to high energy conformation and conformation and for proteins just means shape conformation so the step two what happens is this the the phosphate group gets there still attached but it gets detached from the rest of the ATP so that becomes ADP and then that energy changes the conformation so that this protein now goes into a position that looks like this and let me draw it like so this is this is where we end up at the end of step two let me make sure I do it right so the end of step two it might look something like this it looks something like this trying my best to draw it so the end of step two the protein looks something like this this is kind of in its cocked position it has a lot of energy right now it's kind of wound up in this position you still have your ADP you still have your that's your Edina sign and then let's say you have your two phosphate groups on the ADP and you still have one phosphate group right there now when that phosphate group releases so then let me write this as step 3 so that remember when we started we were just sitting here at the ATP kind of binds on step 1 actually does definitely bind at the end of step 1 it that causes the myosin protein to get released then after step 1 we get well we naturally have step 2 of the the ATP hydrolyzes into adp phosphate that releases energy and that allows the myosin protein to get cocked into this high energy position and kind of attach you can kind of think of it to the next rung to the next run of our actin filament now we're ready to kind of we're in a high energy state when let me write this down and step three in step three the phosphate releases released from myosin the phosphate is released from myosin and step three that's step three right there that's a phosphate group being released and what this does is this releases that energy of that cocked position and it causes it causes this myosin protein to push on the actin this is the power stroke if you imagine it in an engine this is what's causing the mechanical movement so when the phosphate group is actually formed the original release is when you take ATP to ADP and a phosphate that put it in this kind of spring-loaded position when the phosphate releases it this releases the spring releases the spring this is releases the spring and what that does is it pushes on the actin filament it pushes on actin filament pushes on actin so you could view this as the power stroke we're actually creating mechanical energy so depending on which one you want to view is fixed if you view the actin is fixed whatever myosin is attached to would move to the left if thought you imagined the myosin being fixed then whatever the actin and whatever it's attached to would move to the right either way but this is where we fundamentally get the muscle action and then step four and then step four you have the ADP released a ADP released and then we're exactly where we were we're exactly where we were before we did step one except we're just one rung further to the left on the actin on the actin molecule so to me this is pretty amazing we actually are seeing how ATP energy can be used to you know then we're going from chemical energy let me write that down we're going from chemical energy or bond energy in ATP chemical energy to mechanical energy mechanical energy and I'm for me that's amazing because when I first learned about 80 people say oh they use ATP to do everything in your cells and contract muscles well gee how do you go from bond energy to actually contracting things to actually doing you know what we see in our everyday world is mechanical energy and this is really where it all occurs this is really the core issue that's going on here and you have to say well gee how does this thing change shape and all that and and you have to remember these proteins based on what's bonded to and what's not bonded to it they change shape and some of those shapes take more energy to attain and then if you kind of do the right things that energy can be released and then it can push another protein but I find this just fascinating and now we can build up from this actin and myosin interactions to understand how muscles actually work