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Myosin and Actin : How myosin and actin interact to produce mechanical force.
相關課程
- 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 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 copy and pasted two images of
- proteins from Wikipedia.
- This is myosin.
- It's actually myosin II because you actually have two
- strands of the myosin protein.
- They're interwound around each other so you can see it's this
- very complex looking protein or enzyme, however you want to
- talk about it.
- 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.
- It's a subclass of the ATPase enzymes.
- This right here is actin.
- What we're going to 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 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 interwound,
- it's woven 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 kind of the tail part that connects to
- some other structural and we'll talk about that in more
- detail, but this is my myosin head right there 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 ATP comes along and bonds to this guy right here.
- Let's say that's the-- and it's not going to be this big
- relative to the protein, but this is just to
- give you the idea.
- So 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 and as soon as that
- happens, that causes the myosin to release actin.
- So that's step one.
- So I start it off with this guy just touching the actin,
- the ATP comes, and it gets released.
- So in the next step-- so after that step, it's going to look
- something like this-- and I want to draw
- it in the same place.
- After the next step, it's going to look
- something like this.
- It will have released.
- So now it looks something like that and you have the ATP
- attached to it still.
- I know 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 hydrolizes, the phosphate gets
- pulled off of it.
- This is an ATPase enzyme.
- That's what it does.
- Let me write that down.
- And what that does, that releases the energy to cock
- this myosin protein into kind of a high energy state.
- So let me do step two.
- This thing-- it gets hydrolized.
- It releases energy.
- We know that ATP is the energy currency of biological
- systems. So it releases energy.
- I'm drawing it as a little spark or explosion, but you
- can really imagine it's changing the confirmation of--
- it kind of spring-loads this protein right here to go into
- a state so it's ready to crawl along the myosin.
- So in step two-- plus energy, energy and then this-- you can
- say it cocks the myosin protein or
- enzyme to high energy.
- You can imagine it winds the spring, or loads the spring.
- And confirmation for proteins just mean shape.
- So step two-- what happens is the phosphate group gets--
- they're still attached, but it gets detached from
- the rest of the ATP.
- So that becomes ADP and that energy changes the
- confirmation so that this protein now goes into a
- position that looks like this.
- So this is where we end up at the end of step two.
- Let me make sure I do it right.
- So at the end of step two, it might look
- something like this.
- So the end of step two, the protein looks
- something like this.
- This is in its cocked position.
- It has a lot of energy right now.
- It's wound up in this position.
- You still have your ADP.
- You still have your-- that's your adenosine and 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 let me write
- this as step three.
- Remember, when we started, we were just sitting here.
- The ATP binds on step one-- actually, it does definitely
- bind, at the end of step one, that causes the myosin protein
- to get released.
- Then after step one, we naturally have step two.
- 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 think of it, to the next rung
- of our actin filament.
- Now we're in a high energy state.
- In step three, the phosphate releases.
- The phosphate is released from myosin in 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 this myosin protein
- to push on the actin.
- This is the power stroke, if you imagine in an engine.
- This is what's causing the mechanical movement.
- So when the phosphate group is actually released-- remember,
- the original release is when you take
- ATP to ADP in a phosphate.
- That put it in this spring-loaded position.
- When the phosphate releases it, this releases the spring.
- And what that does is it pushes on the actin filament.
- So you could view this as the power stroke.
- We're actually creating mechanical energy.
- So depending on which one you want to view as fixed-- if you
- view the actin as fixed, whatever myosin is attached to
- it would move to the left.
- If you imagine the myosin being fixed, 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-- you have the ADP released.
- And then we're exactly where we were before we did step
- one, except we're just one rung further to the left on
- the actin molecule.
- So to me, this is pretty amazing.
- We actually are seeing how ATP energy can be used to-- we're
- going from chemical energy or bond energy in ATP to
- mechanical energy.
- For me, that's amazing because when I first learned about
- ATP-- people say, you 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 what we see in
- our everyday world as 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 this thing change
- shape and all that?
- And you have to remember, these proteins, based on
- what's bonded to it and what's not bonded to
- it, they change shape.
- And some of those shapes take more energy to attain, and
- then if you 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.
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