Myosin and actin | Circulatory system physiology | NCLEX-RN | Khan Academy
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Myosin and actin | Circulatory system physiology | NCLEX-RN | Khan Academy

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 conformation 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 conformation 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 conformation 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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