myosin

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    As you already learned, each myosin molecule in skeletal muscle has two globular heads and one tail.  These myosin molecules gather together to form a myosin filament-- the "thick" filament.

    Now what you will need to understand is that both the heads and the tail have some dynamic properties.  Let's discuss these:

Binding sites

    Each myosin head has two binding sites (myosin tails do not have special binding sites):

1.  a binding site for actin

2.  a binding site for ATP

    Why does it have both of these sites and what do they do?  Well, first of all, let's discuss the binding site for actin.   In order for the thick filaments to move along the thin filaments, as we discussed in class, they have to be able to interact with them.  This interaction is provided by the binding site on myosin for actin, called the actin-binding site(You will see that actin also has a myosin-binding site)

    The binding site for ATP is a little bit different.  It should be easy to understand why the ATP-binding site is found on each myosin head-- in order for the thick filament to pull along the thin filament, it needs energy.  ATP is cellular energy, so myosin will have to be able to grab some ATP.

    The part about the ATP-binding site that makes it different from the actin-binding site is that once ATP binds, it gets used, whereas, when actin binds to the actin-binding site, it remains unaltered.  You see, it is not enough to bind to ATP, you have to use it, too.  To understand this, let's consider a more familiar source of energy--

    gasoline for your car.  In order for your car to run, you need to put gas into it.  There is one place in your car for gas, the gas tank, so you have to put it there.  Once inside your car, the gas allows the car to run.  But quickly, the gas is used up and your tank begins to empty.  It was not enough for your car to have the gas inside it, but your car had to also use the gas to run.

    Myosin heads have to use the ATP that they bind in order to move.  ATP gets "used" when it is broken down through a hydrolysis reaction into ADP + inorganic phosphate (typically symbolized as Pi).  In order to cause a hydrolysis reaction to occur, one needs an enzyme.  Luckily, myosin is made out of protein, and some proteins can be enzymes.  So, the myosin head will actually act like an enzyme, and break down (hydrolyze) ATP.  Since all enzymes end in the suffix "-ase" and are usually named for their particular substrate, the myosin head has also been called an "ATPase."  We can write this reaction out, like any other chemical reaction, as:

                             myosin
            ATP --------------------> ADP + Pi

The above reaction would be spoken as soundicon.gif (538 bytes).  The only thing not included in this formula is the fact that as ATP is broken down, it releases its energy for use.  We could then re-write this formula as:

                             myosin
            ATP --------------------> ADP + Pi + energy

It is this energy that powers muscular contraction.   So now, you can think of myosin as both a structural protein, and binding protein, and an enzyme.  Wow!  Some protein!

    To see an image of the myosin head that indicates these two binding sites (as well as the light chains and other stuff), take a look at this image!

Movements:

    Myosin heads also move.  And so do myosin tails.  We will consider the movements of both of these regions on myosin.

Myosin Head Movement:

    The myosin heads bend, in a ratchet-like fashion.  This bending of the myosin head (powered by ATP hydrolysis) provides the power-stroke of muscle contraction.  Therefore, when myosin has to move past actin, it is the movement of the myosin heads, fueled by ATP that the heads hydrolyzed, that causes the movement.  You will see more about this on the sliding filament theory page.

Myosin Tail Movement:

    The myosin tail also moves.  It bends.  Why?  Because in order for myosin to move past actin by pulling on actin, myosin heads have to be able to get close enough to the actin filaments to reach them.  myosintailbend.gif (11492 bytes) When the myosin tail bends, the heads are in a position for contact with actin; but when the tails are not bent, the myosin heads cannot reach the actin.

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.