So what you have seen so far is all the
reactions involved in the breakdown of glycogen.
It's a very simple pathway.
And as we will see
the synthesis of glycogen
is almost as simple.
In the synthesis of glycogen
we start with glucose-1-phosphate.
So, where would glucose-1-phosphate come from originally?
Well imagine that you had a cell that
had gotten a bunch of glucose into it.
The cell could use hexokinase
in the glycolysis pathway
to convert glucose into glucose-6-phosphate.
And the phosphoglucomutase which catalyzes
the reversible reaction, I showed earlier,
can convert glucose-6-phosphate back to
glucose-1-phosphate, as you see here.
Now that turns out to be
pretty important; because, cells,
liver cells in particular,
will release glucose.
They will also take up glucose after a meal, for example
when the glucose concentrations are high.
Dealing with that glucose is important
and storing it as a glycogen as we see
here is essential for them to do.
So that phosphoglucomutase plays a dual role both
in the breakdown and the synthesis of glycogen.
Glucose-1-phosphate in the first step of glycogen synthesis
combines with a molecule called UTP.
Now UTP is, of course, a ribonucleotide
that's used in the making of RNA.
In this case it's used to make a molecule
that can be used to build glycogen.
Now the enzyme that catalyzes this reaction is
UDP-glucose pyrophosphorylase, as you can see.
And what happens in
this reaction is that a
pyrophosphorylase, the PPi that you can see in
the lower part of the figure on the lower right
is released and the remaining part, the
UDP is attached to the glucose molecule.
Now that bond between the glucose molecule
and the UDP is a high energy bond,
is an example of what I'd like to call an activated
intermediate. Now activated intermediates
have a very important property. First
of all, they have high energy bonds.
And the high energy
bonds are used to transfer
a part of themselves to something else.
Now that's actually what we see
in this image on the screen.
UDP-glucose is getting ready to use it's
energy to transfer a part of itself
to glycogen and the part of itself that
its transferring of course is glucose.
So this is a reaction that is catalyzed
by glycogen synthase, as you can see,
and it's a fairly simple reaction.
What the glycogen synthase is
doing, is grabbing that glucose
and making a 1,4 bond so that the glycogen
chain is growing by 1 more glucose.
The remaining product UDP can then go back
and be a phosphorylated and used for making
RNA or making additional UDP-glucoses.
Now, of course, all the glycogen synthase is going to do
is make a glycogen with
α-1,4 bonds and you recall
that glycogen has many branches.
So, then question is how
do the branches get there?
And the branches get there by an enzyme
that has a name that is very long
that most people call branching enzyme and
so I have decided to call branching enzyme here.
You can see that what branching enzyme does, is
it grabs a section of a long α-1,4 polymer of glucose
on a glycogen chain and then clips
it and transfers part of it backwards
to where it can make an
α-1,6 bond with another sugar.
And it is in this way that the
branches actually get into glycogen.