Now we see a similar thing happening
in glycogen metabolism and
in another presentation I have talked about glycogen
metabolism but let me go through it briefly here.
We are gonna see in these
various hormone pathways
similarities; because, many of the same cascade
systems are actually used to control them.
In this pathway, let's start again
with epinephrine being released.
This will be conditions of low glucose in the body
and the liver cells are being
stimulated to produce glucose.
Either epinephrine or glucagon
can cause this to happen.
Well, of course, the G protein
gets activated, as seen before.
The G protein activation causes production
of cyclic AMP by the adenylate cyclase.
The activation of protein kinase A happens;
because, the cyclic AMP is being produced.
And then protein kinase A
starts phosphorylating proteins.
There are two sets of proteins
that get phosphorylated.
First, there is a protein
called phosphorylase kinase
which catalyzes the addition of phosphate to
another enzyme that I will talk about in a second.
The other protein that gets phosphorylated by
protein kinase A is the enzyme that makes glycogen.
Now the phosphorylation of this
enzyme called glycogen synthase B,
converts the B into the A form and
the A form of the enzyme is inactive.
So what happened is protein kinase,
as part of this cascade system,
has turned off the synthesis of glycogen.
Well since glucose is used to
make glycogen that make sense.
This body doesn't have enough glucose
so there is no reason to be storing it.
In fact what the body is
trying to do is break it down.
Now we come back to our phosphorylated
protein known as phosphorylase kinase.
What it does, is catalyze the phosphorylation
of the enzyme that breaks down glycogen.
That's known as glycogen phosphorylase
and it converts glycogen phosphorylase
from the B form which doesn't have phosphate
to the A form that does have phosphate.
Now this enzyme with the addition
of phosphate becomes active.
That means that glycogen is broken
down and glucose is being released.
So again we see a sort of
a reciprocal regulation here.
Addition of phosphate is turning off the enzyme
involved in synthesizing glycogen
and addition of a phosphate to the
enzyme that breaks it down, turns it on.
The breakdown of glycogen by glycogen phosphorylase
And glucose-1-phosphate is readily converted to
glucose-6-phosphate by phosphoglucomutase.
Now the advantage of this is that glucose-6-phosphate
can be converted to glucose in the liver
if the liver is exporting it, or
if the glucose-6-phosphate makes it to a cell
the cell can take that glucose-6-phosphate and use it
directly into pathway of glycolysis.
In either event the goal of
increasing the blood glucose level
has been accomplished as a result of the
binding of epinephrine to this liver cell.
So binding of epinephrine or glucagon
to the receptor on the liver cell
cause the breakdown of glycogen to
favor the increase in blood sugar.
If we have a high blood sugar
concentration to begin with
then, of course, insulin is gonna be produced
by the body to deal with that.
And here at the cellular level of
the liver is how it deals with it.
Insulin binds to the receptor as before
and the phosphoprotein phosphatase that
we talked about on the last set
was activated by the binding of the
insulin to the cell surface receptor.
Now the G protein that we talked about being
inactivated before, is inactivated which means that
adenylate cyclase is not making cyclic AMP.
And so the protein kinase A which relies
on cyclic AMP is no longer active as well.
All of the other reactions in this pathway, now
involve removal of phosphates and
reversal of what happened
in the epinephrine pathway. And so we are going to see
phosphoprotein phosphatase, which is the protein
that has been activated here by insulin,
is going to turn everything
in a different direction.
We see first of all that the phosphorylase kinase
which was activated by phosphorylation
in the epinephrine pathway is inactivated here.
We see that the glycogen phosphorylase which was
activated by the phosphorylation is inactivated by the
removal of phosphate and it's converted back to B form.
By contrast, the glycogen synthase
which was in the A form
and was inactivated by phosphorylation is now activated
by the removal of the phosphate by phosphoprotein phosphatase.
The upshot of this now is that
glycogen synthesis is activated.
This turns out to be important, of course; because,
there is all this glucose coming in the cells
since insulin stimulates the uptake of glucose
and now the cell has a way of dealing with that
incoming glucose by making it into glycogen.
Glucose, as I have noted, is a molecule
that can act like a poison.
So dealing with that poison in this way
makes a lot of sense for the cell.