- [Speaker] To save us some time, I've went ahead
and drawn out simplified version of the citric acid cycle
here and if you remember, it begins with acetyl-CoA
entering the cycle, and it combines with this molecule
oxaloacetate to form citrate.
And this citrate undergoes various conversions
and oxidations, which eventually cause the two carbons
that entered with acetyl-CoA,
because remember acetyl-CoA is a two-carbon molecule.
These two carbons exit as carbon dioxide.
And as it's oxidized and loses it carbon dioxide,
acetyl-CoA also allows us to produce the electron
carrier molecules FADH2 and NADH, which if you recall,
go to the electron transport chain
to allow us to produce ATP.
So to summarize, let's go ahead and write out the
kind of overall chemical reaction of the citric acid cycle.
So we have as our reactants, acetyl-CoA entering the cycle,
we also have some co-enzymes like NAD+ and FAD,
and we also have a freed GDP, remember GTP is formed
in this cycle as well.
And all of this eventually will produce carbon dioxide
from the oxidation of acetyl-CoA.
We reduce our electron carrier molecules.
So NADH and FADH2,
we also form a GTP.
Now the reason I wanted to go ahead and write out
this entire overall reaction of the citric acid cycle
is because when I was first learning the cycle,
I kind of often got stuck in the individual reactions
that were taking place in this kind of merry-go-round.
And getting confused by all these names like isocitrate
and succinyl-CoA, and I think that when we're trying to
understand in particular how this cycle is
regulated, that is trying to figure out when essentially,
this cycle is in full speed an when it's kind of
slowing down, it's actually nice to kind of
step back and look at the big picture.
Now the first point I want to make is that in contrast
to something like glycolysis, which we usually think about
as a metabolic pathway that we really consider to be
either on or off, the citric acid cycle is something
that we usually consider always to be on,
but to various degrees, depending on kind of the
energy needs of the cell.
And the reason it always has to be on, right is because
it needs to be delivering these high-energy
electron carriers to the electron transport chain,
to allow at least some constant flow of ATP production,
which is vital to a lot of the tissues in our body.
In contrast, you can probably think of a case when,
you know a theoretical case perhaps, where someone
might not have any glucose in their diet.
And of course glycolysis then could be off,
but the body might be able to use fatty acids
and other kind of sources of kind of fuel
to enter the citric acid cycle.
And indeed, kind of a clinical proof of this is that
there are kind of disease states that involve mutations
of enzymes in glycolysis, for example.
But you would be hard pressed to find a living
individual who has a mutation in their citric acid cycle
because this cycle is so vital to life.
So that brings me to my first point, which is that
there is no hormonal control in the citric acid cycle,
because it's on regardless of whether we're in the
fed state or the fast state.
Instead the major form of regulation of
the citric acid cycle is through allosteric regulation.
So I'll remind you that allosteric regulation is simply
the ability of a molecule floating around in the cell,
to bind to a specific enzyme that's part of this pathway,
and it binds through a part of the enzyme that's not
the active site, and essentially by doing this,
it causes the enzyme to undergo a conformational change,
and it can either make the enzyme work better,
in which case we call it an allosteric activator.
Or it can make the enzyme not work as good,
in which case we call it an allosteric inhibitor.
And the way that I remember that allosteric regulation
is the major kind of form of regulation in the
citric acid cycle, is I remind myself that since this cycle
is always on, it still kind of wants to be able to adjust
on a minute to minute, so rather fast way
to the kind of energy needs of the cell.
So I'm gonna go ahead and write that here, wants to respond
to the energy needs of the cell.
And they way that it can figure this out is by looking
at the molecules it has floating around.
And these molecules are often those that are involved
in regulating this pathway allosterically.
And finally, a third way that this cycle is regulated
is by looking substrate availability.
And this is exactly what it sounds like.
Essentially, if the body doesn't have a lot of acetyl-CoA
around for example, remember this is one of the major
kind of substrates for this pathway.
Then it makes sense of course that the speed by which
this NADH and FADH2 is produced is gonna slow down
because there's just not enough to enter the cycle.
Now one high-yield example of this is when citrate,
under conditions of high ATP generally shuttles
a lot of it's acetyl-CoA into the cytoplasm
for fatty acid synthesis.
And of course when this happens, it's taking the citrate
out of the cycle,
and so it will slow down the overall cycle.
On the flip side, let's say our body was in a very
starving state, we haven't had food for quite some while.
And in some cases amino acids can actually begin
to break down from our muscles and enter in various places
along the citric acid cycle.
And one place they enter is, they actually are converted
into alpha-ketoglutarate here.
And suffice to say that the general idea here is that if
you have of this substrate around, it's gonna push
the cycle to go faster which makes sense in this case
right, because the body is alerted to the fact that
it's starving, and so it wants to be able to produce
more NADH and FADH2 to produce more ATP.
Alright, so that's kind of a general overview
of substrate availability, but now I wanna talk
more in depth about this allosteric regulation.
Now you wouldn't notice if I had told you, but it turns out
that there are three reactions in the citric acid cycle
that have a very large negative delta G.
And remember large negative delta G means that these
reactions are largely irreversible, which means that
they are good targets for regulation because once these
reactions essentially occur, it's usually like
a ball rolling down a hill and will allow everything
else to occur.
And these three reactions are the conversion of
oxaloacetate and acetyl-CoA into citrate,
as well as the conversion from isocitrate
to alpha-ketoglutarate, and alpha-ketoglutarate
to succinyl-CoA.
Now to go ahead and keep this kind of diagram as
clear as possible, I'm gonna go ahead and kind of
abbreviate the names of these enzymes, but of course
you can always go to Wikipedia or a textbook
and remind yourself what these enzymes are called.
But of course this enzyme from octyl acetate
and acetyl-CoA to citrate, is citrate synthase,
and isocitrate to alpha-ketoglutarate is
isocitrate dehydrogenase, so I'm gonna say ID.
And then alpha-ketoglutarate to succinyl-CoA
is alpha-ketoglutarate dehydrogenase.
Now the allosteric regulators that these three enzymes
can end up being kind of a long list,
but my hope in this video is to just be a resource
for you to come back to and to kind of justify
why certain things are allosterically inhibiting
or activating these enzymes.
So why don't we start off with the allosteric inhibitors,
and one kind of easy allosteric inhibitor to remember
is NADH because it allosterically inhibits
all three of these enzymes.
And the reason it does this, and it should be apparent
to you why this is so, is that, notice that NADH is a
product of the overall citric acid cycle, right?
So it's a product right here.
And so if we're building up NADH, it's essentially
a sign to the body that the citric acid cycle is going
faster than the, essentially the electron transport chain,
which is using up these NADH, can consume those NADHs.
And so it's time for the citric acid cycle to slow down.
Now another allosteric inhibitor that should make some
sense to you is ATP.
So if we have a lot of ATP in the body, it makes sense that
this would be an allosteric inhibitor
of processes that produce energy right,
Because we want to conserve energy and not kind of
make more energy than our body needs.
So in this case, it turns out experimentally for
some reason, only two of these enzymes have been
shown to be inhibited by ATP and those are citrate synthase
and isocitrate dehydrogenase.
Now the final allosteric inhibitors to talk about
are actually products that form in the citric acid cycle
and these products, if they accumulate in excess amounts,
can actually negatively feed back by allosterically
inhibiting some of these enzymes.
And the two notable products that do that are first,
citrate, which can allosterically negative feed back
on it's enzyme citrate synthase, as well as succinyl-CoA,
which not only negatively allosterically feed backs
onto the alpha-ketoglutarate dehydrogenase,
but it also can actually negatively feed back on the
citrate synthase enzyme as well.
And one way that I kind of remember why succinyl-CoA
might wanna kind of feed back all the way back
to the citrate synthase is to recognize that the
citrate synthase is the kind of first kind of point of entry
into the citric acid cycle.
And so, if it can stop the citric acid cycle sooner,
it will essentially waste less energy so to say.
Alright so that kind of sums up the allosteric inhibitors,
but what about the allosteric activators?
The first allosteric activator that always comes to
my mind is ADP.
Remember that ATP is hydrolyzed by a water molecule
into ADP, and a free phosphate group.
So if ADP levels accumulate in excess of ATP,
then it's basically a sign that the cell is
running out of it's power, it's running out of it's ATP,
and it will therefore need to produce more ATP.
And so, if it needs to produce more ATP, it makes sense
that it would want to activate the enzymes
in the citric acid cycle.
And it's easy to remember because it activates the same
enzymes that ATP inhibits.
So those are citrate synthase and isocitrate dehydrogenase.
Alright, so we're hitting the home stretch here
and there's actually only one more allosteric activator
that we need to talk about, and that is calcium.
So why might calcium be an allosteric activator?
Well remember that our muscle cells require an influx
of calcium to contract.
So presumably you know, if we're exercising really hard,
of course our energy needs go up, but our calcium levels
inside of our cell are also going up,
because of all that muscle contraction.
So, this is essentially a way for the body,
especially in skeletal muscles to essentially
couple muscle contraction with producing more ATP
to meet the needs of those contracting muscles.
And again, this is experimental evidence,
but calcium has been shown to allosterically activate
isocitrate dehydrogenase, as well as
alpha-ketoglutarate dehydrogenase.