Now the pathway I’ve drawn here
is a little unusual.
This isn’t the way the pathway
is commonly drawn.
But in fact, it shows all the reactions of the
pathway and some of the crosses that happen.
This is sometimes a pathway that’s
difficult for students to learn
because in the multiple directions
that it can take.
We’re going to go through the
individual reactions in just a bit.
But I want to show you first of all,
the connections of the Pentose Phosphate pathway
to the other pathways that are
important in the cell.
I said for example, there are several intermediates
important in glycolysis and gluconeogenesis.
Gluconeogenesis of course being
the reverse of glycolysis.
We see glucose 6-phosphate,
we see glyceraldehyde 3-phosphate
and we see fructose 6-phosphate
as well coming from glycolysis.
We see an intermediate and nucleotide synthesis
as I’ve noted ribose 5-phosphate shown here.
And we also have erythrose 4-phosphate
which is important for
amino aromatic amino acid synthesis.
Now, this provide both entrance points
and exit points into the pathway.
Now, the first reaction of the
Pentose Phosphate pathway is labelled here.
This reaction is catalyzed by the enzyme
In this process, glucose-6-phosphate
is converted into
Now, full the name,
which I abbreviated to 6-PG delta L.
This is the first of two oxidations that
happens in the Pentose Phosphate pathway.
and is one of the sources of
NADPH produced by the pathway.
So, this reaction that you see,
I often describe this as rate limiting step
in the process of the
Pentose Phosphate pathway.
And it’s true that it is rate limiting
for the entry of glucose 6-phosphate.
But since we have multiple points
of entry into the pathway,
it’s not as important
as it might otherwise seem.
The reaction is inhibited by the molecules
acetyl-CoA and NADPH.
Acetyl-CoA and NADPH of course are indicators
the fact the cell has plenty of things,
plenty of intermediates and doesn’t need
to be making other things like NADPH.
And so, the enzyme gets turned off
when they bind to it.
Deficiency of the enzyme that catalyzes
this reaction can lead to acute hemolytic anemia
due to reactive oxygen species damages that happens
from lack of reduced glutathione where
the NADPH is needed to make glutathione.
So, if we ran out of it of NADPH
then we will have not enough glutathione
to help protect cell from reactive oxygen species.
A deficiency of this enzyme on the other hand,
may give some protection to people
who get infected by malarial parasites
because malarial parasites require
more NADPH for them to function.
The second process of the second reaction cell
is depicted in the rectangle here.
We’ll look at a little bit closely up here.
We see in this reaction that
is hydrolyzed using water
to open the ring structure that we see in
the top molecule to a linear at the bottom.
Now, this turns out to be important
because in the next reaction
that end of the molecules
kind of get broken off
and it wouldn’t get broken off
if it wasn’t the ring structure.
This enzyme that catalyzes the reaction is
6-phosphoglucono-lactase as you can see here.
And this linearization as I noted
is important to allow the molecule
in the next reaction to be decarboxylated.
In the next reaction
6-phosphogluconate is oxidized
and the only decarboxylation reaction
of the Pentose Phosphate pathway.
We see 6-phophogluconate at
the top molecule here being oxidized
and the decarboxylation occurring
to make the five carbon molecule
ribulose diphosphate in the reaction below.
6-phosphogluconate of course has six carbons.
This reaction produces NADPH
and this second and the last reactions
of the Pentose Phosphate pathway
that make NADPH.
The enzyme catalyzing this reaction
is 6-phosphogluconate dehydrogenase
and its essentially irreversible
release carbon dioxide.
and the carbon dioxide goes away.
So that the process
can’t be reversed very well.
A deficiency of this enzyme
is harmful to red blood cells.
And so, this is very important
reaction to be functioning
if we want to have regular
healthy supply of red blood cells.
There are two possible fates
of this molecule,
one is at this point the pathway
gets a little bit more complicated.
In this reaction we see ribulose-5-phosphate
being converted into ribose-5-phosphate.
Now, this reaction is a fairly simpler.
and we see the ribulose-5-phosphate
on the left is a ketose
meaning it has a ketone
of bonded carbon number two.
The reaction converts the ketose
on the left into an aldose on the right.
The aldose being ribose-5-phosphate with
this aldehyde group on carbon number one.
The reaction is fairly simple
and the enzyme involve that
catalyzes the reaction is known as
Now, this reaction is very important
for nucleotide metabolism
as I’ve said I think three times now
because of the importance of having the sugar
Is the only source of the sugar for
making nucleotides inside of the cell.