Now if we look at what’s happening with this system, and by
the way this system is known as the β-adrenergic receptor.
What’s happening in this system?
We can actually see
the overall process.
There’s the hormone that binds to the receptor, there is the
activation of the G-protein with the α subunit getting a GTP.
The interaction of the α subunit
with the adenylate cyclase.
The formation of cyclic AMP.
The activation of protein kinase A, the
activation of phosphorylase kinase.
The inactivation of glycogen synthase.
The activation of
And the breakdown of glycogen.
This process happens very rapidly.
Everything that we see here
is an enzymatic reaction.
And enzymes work
really, really quickly.
You notice that in this pathway,
there’s no movement into the nucleus.
This is important because movement into the nucleus and
activation of gene expression is a very slow process.
In our cells it takes about
24 hours for that to happen.
Well it’s important if you’re in a dark alley for
example because imagine that someone’s chasing you.
You need to have energy and you
need to have that energy right now.
You can’t wait 24 hours for gene expression to occur so
that you can make something that can make you some glucose.
This pathway which regulates the activity
of enzymes and doesn’t have anything
to do with gene expression is an example
of one that works very, very quickly.
Now, when we think about turning on a system which
I’ve described here, that’s pretty straightforward.
But we think, cells can’t continually have
the system turned on because if they left the
system turned on all the time, they would be
continually breaking down their glycogen.
They wouldn’t have any glycogen.
And glycogen’s a pretty important
storage molecule for energy.
So just like they’ve got to be able to turn on
glycogen breakdown quickly, they’ve also got to
be able to turn it off quickly when the need for
glucose from the glycogen breakdown diminishes.
So there are several things that have
to be deactivated in this process.
So I’m going to step you through the process
of deactivation of the β-adrenergic receptor.
The receptor itself
has to be deactivated.
The G-protein that I described with
the α subunit has to be deactivated.
The cyclic AMP, the small molecule has to either be destroyed
or hidden in some way so that it can’t exert its effects.
Protein kinase A needs to
regain the regulatory subunits.
Phosphorylase kinase needs
to lose its phosphate.
Glycogen phosphorylase A
needs to be inactivated.
And when this all happens, glycogen
synthase, the synthesis of-- the
enzyme involved in the synthesis
of glycogen has to be activated.
So all these things have to happen,
and the reversal of that process.
Well let’s look to
see how that happens.
We’ll start with the receptor.
The receptor is shown
here as we saw before.
The receptor is depicted with the
exterior part of the cell being up and the
interior part being below which is
where the G-protein interacts with it.
One of the things that happens in the inactivation process
of the receptor, is the receptor gets phosphorylated.
It gets a phosphate attached to it.
And this happens as a result of action of a
protein known as G-protein receptor kinase.
You can see the addition of the
phosphate to the bottom of the receptor.
That phosphate is target for binding
of a protein known as arrestin.
Now arrestin sees that phosphate,
grabs a hold and covers it up.
And the effect of arrestin being there prevents
the G-protein from interacting with its receptor.
This therefore blocks the receptor and prevents it
from communicating further signals into the cell.
It also favors the process of endocytosis,
that is the pulling in of the receptor
into the cell, so that cell can either
break it down or do something with it.
So we’ve inactivated the receptor as a result
of this action I’ve just described here.
Next we think about-- and by the way,
I’m describing these in an order.
There is not an order associated
with them for the most part.
But I will talk about them in the way that they
happen during the overall signaling process.
The next thing is that the G-protein
has to itself be inactivated.
And that turns out to be a
really interesting phenomenon.
We see here first of all, the G-protein
can’t interact with the receptor.
But we remember that cells have hundreds of
copies of a receptor, and we’re looking at one.
So just because we block one receptor, doesn’t
mean that the other receptors aren’t available.
This means that the G-protein
has to itself be deactivated.
And that deactivation process is something
that it does to itself which is kind of cool.
Well how does that happen?
Well it turns out that the α subunit
of the G-protein is a very bad enzyme.
Yeah, I said that right.
It’s a very bad enzyme.
What does that mean?
It means that it catalyzes a reaction but it
doesn’t catalyze a reaction very well or very fast.
The reaction that it catalyzes
is it breaks down GTP.
And in breaking down GTP,
the product is GDP.
Well why is that important?
Well of course, remember when the GDP was bound to
the α subunit, it was no longer in the active state.
It will no longer interact
with adenylate cyclase.
So the α subunit in this
reaction has turned itself off.
And it being a very inefficient or a very bad enzyme, means
that it didn’t turn itself off as soon as it got the GTP.
If it was a good enzyme, it would break down the
GTP immediately and no signal will be communicated.
This process of breaking down the GTP can take
from a few seconds to a few minutes which allows
enough time to communicate that signal but not
allow the signal to continue to propogate.
Once the GDP is back in the α subunit as we see here,
the β and γ subunits can then interact with it again.
And this will then ultimately go to another β-adrenergic
receptor or GPCR and wait for the next signal.
The next molecule that I’ll describe, the
elimination of, is that of cyclic AMP.
Cyclic AMP you recall was necessary
to activate protein kinase A.
Cyclic AMP is broken down by an
enzyme known as phosphodiesterase.
Now the cell is full
Phosphodiesterase is usually present and
it’s usually present in an active form.
So cyclic AMP, once it’s been made,
usually also doesn’t hang around for
very long because it gets found by
phosphodiesterase and cleaved to make AMP.
Well as AMP it can’t do anything, so this phosphodiesterase
being present allows cyclic AMP to rapidly be degraded.
Well cyclic AMP was necessary for of
course activating protein kinase A.
And you’ll see the green form there.
So without the cyclic AMP being present, the regulatory
subunits can now come back and replace what little cyclic AMP
was associated with the protein kinase A, and recreate the
inactive protein kinase A bound to the regulatory subunit.
The protein kinase A has
therefore been turned off.
Alright, well the next process in
the scheme, is removing phosphates.
And it turns out this is actually
the simplest step that happens.
There’s an enzyme known as
And phosphoprotein phosphatase is an enzyme that’s stimulated
by the addition of insulin to the outside of a cell.
Insulin causes this protein to become active and what this
protein does is it removes phosphates from other proteins.
Well we can imagine what’s
going to happen here.
The removal of phosphate from phosphorylase kinase causes the
phosphorylase kinase to move from
the active to the inactive form.
We see the removal of the phosphate from
the glycogen phosphorylase-a causes
it to flip into the glycogen phosphorylase-b
form which is also inactive.
And the removal of the phosphate
from glycogen synthase causes it to
flip into the green form which is the
active form of glycogen synthase.
So with this last step, now all of
the processes that got activated
during the binding of the epinephrine have been reversed.
There’s one other thing I
want to say something about.
And that one thing I want to say something about
is to take you back to this slide right here.
This slide you remember still showed
active cyclic AMP being present.
And when cyclic AMP was present, we saw all these other
proteins being active and glycogen synthase being inactive.
But why am I coming
back to this slide?
I’m coming back to this slide because remember
the phosphodiesterase breaks down cyclic AMP.
A really cool fact is that phosphodiesterase breaking
down cyclic AMP to make AMP is inhibited by caffeine.
So that morning cup of coffee that you had, you said
it gave you a little buzz, well the little buzz that it
gave you came from the fact that with cyclic AMP levels
present in your cells, you’re putting out more glucose.
So that little buzz that you had was a little bit of sugar
that came from the inhibition of phosphodiesterase by caffeine.