Now one of the considerations that we have with
metabolic pathway is that metabolic pathways
that breakdown and make things, if
they are occurring at the same time,
it can cause problems for cells and we
refer those problems as a futile cycle.
We see on the screen the schematic pathway
for glycolysis and gluconeogenesis.
Now these two pathways overlap
a lot and that one pathway is
partly the reverse of the other pathway.
We see the glycolysis direction
moving down, as you can see here,
and the gluconeogenesis pathway
starts at the bottom and moves up.
And with the exception of the oxaloacetate, shown
in the bottom right of the gluconeogenesis pathway,
there is really no difference in the molecules
that are present in either pathway.
The pathway on the left, glycolysis generates ATP energy.
It also, as a result of oxidation, generates
2 reduced electron carriers known as NADH.
Gluconeogenesis by contrast to start with
pyruvate and move upwards, requires
4 ATPs, 2 GTPs and the 2 NADHs;
because, a lot of the reactions of glycolysis and
gluconeogenesis occurring at the same place,
there is concerned that if they are both
happening at the same time. If that happens
then the products of gluconeogenesis
are made and then burned in glycolysis in the
pathway where the glycolysis intermediates are made
are burned and feed into gluconeogenesis.
When this happens we have what's
referred to as a futile cycle.
The cell is making things and
breaking things down at the same time
without any productive output of energy.
In fact, this is energy consuming.
So the futile cycle is something for a cell to avoid.
The consequences of running a futile
cycle are shown in this slide.
The output of glycolysis is 2 pyruvates, 2 ATPs,
and 2 NADHs plus some other molecules.
The outputs of gluconeogenesis are glucose plus
2 NADs plus 4 ADPs and 2 GDPs and 6 phosphates.
You notice that the output of glycolysis
becomes the input of gluconeogenesis.
But similarly the output of gluconeogenesis
can become the input of glycolysis.
Well this would be fine and dandy except for
the fact that when these two are running together
the gluconeogenesis pathway is
taking more triphosphate energy
than the glycolysis pathway is generating.
So this futile cycle if it runs goes in the circle
and the output of that circle is simply heat
and loss of triphosphate energy.
This is something for a cell to avoid.