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What I want to do in this video
is try to understand how

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two proteins can interact with
each other in conjunction with

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ATP to actually produce
mechanical motion.

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And the reason why I want to
do this-- one, it occurs

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outside of muscle cells as well,
but this is really going

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to be the first video on really
how muscles work.

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And then we'll talk about how
nerves actually stimulate

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muscles to work.

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So it'll all build up
from this video.

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So what I've done here is I've
copy and pasted two images of

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proteins from Wikipedia.

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This is myosin.

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It's actually myosin II because
you actually have two

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strands of the myosin protein.

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They're interwound around each
other so you can see it's this

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very complex looking protein or
enzyme, however you want to

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talk about it.

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I'll tell you why it's called
an enzyme-- because it

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actually helps react ATP into
ADP and phosphate groups.

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So that's why it's
called an ATPase.

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It's a subclass of the
ATPase enzymes.

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This right here is actin.

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What we're going to see in
this video is how myosin

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essentially uses the ATP to
essentially crawl along.

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You can almost view it as an
actin rope and that's what

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creates mechanical energy.

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So let me draw it.

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I'll actually draw it on
this actin right here.

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So let's say we have one
of these myosin heads.

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So when I say a myosin head,
this is one of the myosin

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heads right here and then it's
connected, it's interwound,

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it's woven around.

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This is the other one and it
winds around that way.

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Now let's just say we're
just dealing with one

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of the myosin heads.

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Let's say it's in
this position.

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Let me see how well
I can draw it.

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Let's say it starts off in a
position that looks like that

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and then this is kind of the
tail part that connects to

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some other structural and we'll
talk about that in more

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detail, but this is my myosin
head right there in its

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starting position, not
doing anything.

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Now, ATP can come along and bond
to this myosin head, this

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enzyme, this protein,
this ATPase enzyme.

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So let me draw some ATP.

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So ATP comes along and bonds
to this guy right here.

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Let's say that's the-- and it's
not going to be this big

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relative to the protein,
but this is just to

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give you the idea.

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So soon as the ATP binds to its
appropriate site on this

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enzyme or protein, the enzyme,
it detaches from the actin.

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So let me write this down.

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So one, ATP binds to myosin
head and as soon as that

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happens, that causes the myosin
to release actin.

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So that's step one.

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So I start it off with this guy
just touching the actin,

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the ATP comes, and
it gets released.

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So in the next step-- so after
that step, it's going to look

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something like this--
and I want to draw

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it in the same place.

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After the next step,
it's going to look

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something like this.

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It will have released.

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So now it looks something like
that and you have the ATP

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attached to it still.

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I know it might be a little
bit convoluted when I keep

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writing over the same thing,
but you have the

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ATP attached to it.

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Now the next step-- the ATP
hydrolizes, the phosphate gets

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pulled off of it.

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This is an ATPase enzyme.

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That's what it does.

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Let me write that down.

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And what that does, that
releases the energy to cock

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this myosin protein into kind
of a high energy state.

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So let me do step two.

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This thing-- it gets
hydrolized.

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It releases energy.

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We know that ATP is the energy
currency of biological

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systems. So it releases
energy.

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I'm drawing it as a little spark
or explosion, but you

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can really imagine it's changing
the confirmation of--

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it kind of spring-loads this
protein right here to go into

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a state so it's ready to
crawl along the myosin.

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So in step two-- plus energy,
energy and then this-- you can

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say it cocks the myosin
protein or

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enzyme to high energy.

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You can imagine it winds the
spring, or loads the spring.

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And confirmation for proteins
just mean shape.

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So step two-- what happens is
the phosphate group gets--

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they're still attached, but
it gets detached from

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the rest of the ATP.

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So that becomes ADP and that
energy changes the

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confirmation so that this
protein now goes into a

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position that looks like this.

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So this is where we end up
at the end of step two.

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Let me make sure
I do it right.

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So at the end of step
two, it might look

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something like this.

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So the end of step two,
the protein looks

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something like this.

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This is in its cocked
position.

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It has a lot of energy
right now.

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It's wound up in
this position.

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You still have your ADP.

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You still have your-- that's
your adenosine and let's say

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you have your two phosphate
groups on the ADP and you

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still have one phosphate
group right there.

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Now, when that phosphate group
releases-- so let me write

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this as step three.

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Remember, when we started, we
were just sitting here.

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The ATP binds on step one--
actually, it does definitely

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bind, at the end of step one,
that causes the myosin protein

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to get released.

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Then after step one, we
naturally have step two.

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The ATP hydrolyzes into
ADP phosphate.

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That releases energy and that
allows the myosin protein to

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get cocked into this high energy
position and kind of

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attach, you can think of
it, to the next rung

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of our actin filament.

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Now we're in a high
energy state.

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In step three, the phosphate
releases.

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The phosphate is released from
myosin in step three.

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That's step three right there.

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That's a phosphate group
being released.

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And what this does is, this
releases that energy of that

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cocked position and it causes
this myosin protein

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to push on the actin.

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This is the power stroke, if
you imagine in an engine.

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This is what's causing the
mechanical movement.

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So when the phosphate group is
actually released-- remember,

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the original release
is when you take

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ATP to ADP in a phosphate.

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That put it in this
spring-loaded position.

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When the phosphate releases it,
this releases the spring.

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And what that does is it pushes
on the actin filament.

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So you could view this
as the power stroke.

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We're actually creating
mechanical energy.

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So depending on which one you
want to view as fixed-- if you

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view the actin as fixed,
whatever myosin is attached to

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it would move to the left.

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If you imagine the myosin being
fixed, the actin and

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whatever it's attached to
would move to the right,

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either way.

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But this is where
we fundamentally

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get the muscle action.

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And then step four-- you
have the ADP released.

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And then we're exactly where
we were before we did step

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one, except we're just one rung
further to the left on

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the actin molecule.

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So to me, this is
pretty amazing.

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We actually are seeing how ATP
energy can be used to-- we're

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going from chemical energy
or bond energy in ATP to

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mechanical energy.

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For me, that's amazing because
when I first learned about

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ATP-- people say, you use ATP to
do everything in your cells

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and contract muscles.

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Well, gee, how do you go from
bond energy to actually

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contracting things, to actually
doing what we see in

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our everyday world as
mechanical energy?

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And this is really where
it all occurs.

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This is really the core issue
that's going on here.

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And you have to say, well, gee,
how this thing change

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shape and all that?

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And you have to remember,
these proteins, based on

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what's bonded to it and
what's not bonded to

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it, they change shape.

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And some of those shapes take
more energy to attain, and

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then if you do the right things,
that energy can be

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released and then it can
push another protein.

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But I find this just
fascinating.

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And now we can build up from
this actin and myosin

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interactions to understand how
muscles actually work.