Cells use a control mechanism called reciprocal
regulation to make sure that futile pathways don't happen.
And in reciprocal regulation
what the cell does, is it uses
small molecules or alterations to one pathway
that have opposite effects on the other pathway.
So in the case of glycolysis and gluconeogenesis
we will see what happens with this.
We can see for example here glycolysis shown in blue,
going down and gluconeogenesis in yellow, going up.
We see that the control points
on these pathways are labelled
with the PKF and the pyruvate kinase
on the right side. The pyruvate carboxylase, the PEPCK
and the F1,6BPase on the right side.
We notice that they are opposite each other
and these enzymes that are opposite each other
are regulated by opposite things.
So here is how it works.
If we look at the activators and an inhibitors
of these enzymes, they are as follows.
In the case of PFK which controls that essential
reaction of going from fructose-6-phosphate
to F1,6BP, we see that F2,6BP, the
activates PFK and the same
molecule inhibits the F1,6BPase.
This is a perfect example of reciprocal regulation.
We see similar things happening with citrate
which is inactivating PFK but it's
Again, AMP has opposite effects, as you can see.
Now not everything parallels
in the reciprocal regulation.
But when we look at what's happening this
simple organization that happens with the
molecules that you see here allows the cell
to use a mechanism to control
different pathways with 1 molecule.
At the bottom we see that
pyruvate kinase is regulated
by for example ATP and ATP is
an indicator of high energy.
So ATP turns off pyruvate kinase
which also helps to stop the pathway.
We saw ATP for example inhibited the PFK.
On the gluconeogenesis side we don't
see ATP but we do see ADP,
and ADP is an indicator of low energy.
Now this makes very good sense; because, the cell
doesn't wanna be trying to synthesize glucose under
conditions of low energy and the reason for that
is, it doesn't have the energy it takes to make the glucose.
So this balance in terms of energy
or the balance in terms of molecules
allows reciprocal regulation to operate and
control both pathways in a simple mechanism.