Now fructose-6-phosphate is converted
in the next reaction, and
this is probably the most
interesting reaction of glycolysis.
In this reaction, we are seeing fructose-6-phosphate
gaining an additional phosphate
and becoming the 1,6-bisphosphate on
the side. Why is that interesting?
It's interesting because of the enzyme
that catalyzes that reaction.
Phosphofructokinase also known as PFK-1
catalyze a reaction very
much like what hexokinase did
uses the energy of ATP to put a phosphate
onto one of the hydroxyls
In this case the phosphate is going
onto hydroxyl on position number 1.
Now this, as we can see, is going through the same
mechanism or the same steps that we are going through
in the hexokinase reaction so
the phosphate ends up on here.
Now this enzyme is interesting and important because
it's a regulatory enzyme. When I describe
regulatory enzyme, what I am talking about
are enzymes that can be turned on
or turned off by various things.
In the case of hexokinase, for example, it was a
regulatory enzyme because it was turned off
by it's product, which was glucose-6-phosphate.
Now this enzyme is turned off or on
by several products and these products
are interesting for us to consider.
The delta G zero prime for this
reaction is -14kJ/mol.
So very favorable in the forward direction
and the reason it's favorable in the forward
direction is for the same reason for hexokinase
reaction that's favorable and that is
coupled with the hydrolysis of ATP.
Now discussing phosphofructokinase and it's
regulation gives us some interesting perspectives.
First of all, there are two
materials or two compounds
that favor the activation of
phosphofructokinase. One of these is AMP.
Now AMP is related to ADP and ATP.
The ATP being the triphosphate.
The ADP being the diphosphate
and the AMP being the monophosphate.
Well as we talk about the nucleotides, the more
phosphates they have, the more energy they have.
And AMP is a form of adenosine
nucleotide that has very low energy.
Cells usually don't have much AMP in them
and the only time they have AMP in them is when
they are running out of the energy.
So when they are running out of energy, they really need to have
a boost in that energy, and one of the boost in that energy
is to activate a process that will make
energy such as the breakdown of sugars.
So a low energy indicator like AMP
encourages the process of glycolysis to occurs
so that the cell can generate some energy.
Now fructose-2,6-bisphosphate doesn't fit into
that scheme. Fructose-2,6-bisphosphate
regulation is a little bit different and it will
be covered in another one of the these lectures.
ATP inhibits phosphofructokinase.
Now that make sense because ATP in abundance
indicates the cell has got a plenty of energy.
You don't wanna be breaking your sugar
down if you have more than enough
energy just like you don't wanna be lighting
your fire place in the middle of summer.
It doesn't make sense to waste energy that way.
Now, how does this happen? If we think about PFK,
it has a substrate for ATP.
And one of the things that we learned about enzymes
is that the greater the substrate concentration,
the faster the reaction goes. So how is it that one
of the substrates of PFK is inhibiting the enzyme?
Well it turns out that the enzyme actually
has two different binding sites for ATP.
One of the binding sites
is the binding site for the
allosteric affects that
is turning the enzyme off.
And the other binding site on the enzyme is for
the active site where the reaction is catalyzed.
Now you are probably getting ahead of me at this point.
But what happens is the these two binding sites
have different affinities for ATP.
That's important because if we have low
energy we want this reaction to go.
That means that the active
site which binds the ATP
has a greater affinity for
ATP than the allosteric site.
And only at high concentrations of ATP where
the allosteric site start to bind ATP.
That's a remarkable process and there are
very few enzyme that exhibit this property.