Voiceover: So, today we're going to talk about
how allosteric regulation can affect enzyme kinetics.
But first, let's review the idea
that an enzyme's catalysis
can be divided into two steps.
First, the binding of enzymes to substrate,
and second the formation of products.
And using this information,
we can derive the Michaelis-Menten Equation,
which allows us to look at an enzyme's
rate of product formation
with respect to substrate concentration.
Also remember substrates will typically
bind to enzymes at the active site.
So what do we mean when we say allosteric regulation?
Well, we know that enzymes usually have
an active site where substrates combined,
but enzymes can also have what we call
an allosteric site.
And these allosteric sites are places on the enzyme
where any enzyme regulator can bind.
And I've put this star here
just to point out that allosteric sites
can be anywhere on a enzyme.
There can be any number of them as well.
So what do we mean when we say regulators?
Well, we generally say
there are two types of regulators.
There are allosteric activators,
which increase enzymatic activity and activate them,
and allosteric inhibitors,
which decrease ezymatic activity and inhibit the enzymes.
So let's take a look at what we mean
by increasing and decreasing ezymatic activity
from a kinetic perspective.
So, remember the Michaelis-Menten equation,
and if we're assuming substrate concentration
to be constant,
then there are two ways
to influence enzymatic activity, or VO.
In this first graph,
I've drawn three different curves.
The blue curve represents
the enzyme functioning without
an allosteric regulator at all.
The red curve represents the enzyme
with an allosteric inhibitor,
and the green curve represents the enzyme
with an allosteric activator.
And in this example, activators and inhibitors
affect VO by either increasing or decreasing KM
since the V max values seem to be
pretty close between the three curves.
So an activator here
might be decreasing KM.
Now, in this next example,
we have the same three colored curves,
but instead of KM changing significantly,
the regulators seem to be changing V max.
With the activator increasing the V max value.
So, now that we've talked about
activators and inhibitors,
let's introduce the idea of the feedback loop.
And, the basic idea is
that a feedback loop is when you have
downstream products regulating upstream reactions.
And I understand this can be a mouthful,
so let me show you this little reaction sequence,
where we have A forming B through reaction one,
and B forming C through reaction two,
and so on and so on.
Now let's say that molecule F
acted as an activator for the ezyme
powering reaction one.
So it had a positive effect on enzyme one's activity.
Now we would call this a positive feedback loop
since molecule F increases the rate
of reaction one,
which then causes even more F to be made,
since we've increased
the increase the rate of formation of molecule F.
Now, let's say that molecule F
had a negative effect on enzyme one,
we would call this a negative feedback loop
since molecule F decreases the rate of reaction one,
which leads to a decrease
in the rate of formation in molecule F.
So, let's look at an example of a feedback loop
just to really drive home the point
if you're still confused.
Now, phosphofructokianase is an enzyme
involved in glycolysis,
and it catalyzes the conversion of
fructose six phosphate and ATP
to form fructose one six bisphosphate and ADP.
Now, remember that glycolysis
is a metabolic process
that cells use to generate ATP.
So, here, our molecule F,
or downstream regulator from the last example,
is ATP.
and it turns out that ATP is an allosteric inhibitor
of phosphofructokianase.
And this makes sense
because if ATP is at a high level,
it's like the cell saying
"We have ATP and we don't really need any more.
"And we don't need phosphofructokianase
"to push glycolysis along."
So this would be a good example
of a negative feedback loop.
Since making ATP slows down glycolysis,
and thus slows down the rate of ATP production.
Now, because ATP is both an allosteric regulator
and a substrate for phosphofructokianase,
we can call it a homotropic inhibitor,
which is a new term,
and we call it a homotropic inhibitor
because the substrate and the regulator
are the same molecule.
Now AMP, which is used up ATP,
is an activator for phosphofructokianase,
and this also makes sense
because if AMP levels are high,
then ATP levels are probably low.
And it's like the cell saying
"We need ATP."
So we do need phosphofructokianase
to push glycolysis along.
Now, since AMP is a regulating molecule
but not an active site substrate
for phosphofructokianase
it would be considered
a heterotropic activator
since the substrate and regulator are different.
Now, the final point I want to make
is that specific reactions
make excellent control points
for long, multistep processes.
And remember that glycolysis
is a ten step sequence.
So why is there so much regulation
going on for this one step?
Well, this reaction in particular
has a very negative delta G,
and it's actually negative 4.5 kCal per mol.
And that means that it's not easily reversed
since there'll be a big release of energy
from the reaction,
and this makes THIS step of glycolysis
an excellent control point
for ALL ten steps together,
since it's more or less a one way reaction.
So, what did we learn?
Well, first, we learned about
the concept of allostery,
and how regulatory molecules bind
to allosteric sites instead of active sites.
Second, we learned
that these allosteric regulators
influence an enzyme's kinetics
by increasing KM or V max,
and third we learned about what
a feedback loop is,
and how in long, multi-step processes like glycolysis,
the best control points
are highly committing steps,
the ones with very negative delta G values.