>> Welcome to Metabolism and Bioenergetics Part II.
In this tutorial, we'll look closely
at the citric acid cycle.
Let's get started.
Alrighty, so the citric acid cycle it's pretty long,
so we'll abbreviate it CAC and Krebs was the gentleman
that first discovered it, and so it's often referred
to as the Krebs cycle.
This is the older name,
and citric acid cycle is replacing it.
And so this is a true cycle.
It is a circular pathway.
It is a series of biochemical reactions that are going
to break down acetyl groups
to create biochemical energy currency of reduced coenzymes
and GTP, so this whole concept of energy currency,
how does the body transport energy throughout us?
So we get it from where we have it to where we need it.
Alrighty, so we will look at it very simply now, and look at--
so we see that we are going to have CO2 production
and we are going to have reduced coenzymes.
So the energy currency you want to start thinking of it,
I think we all know about ATP, but what can be moved
to your awareness now is the reduced coenzymes.
These are all, we can all think of these as energy sources.
So, in the first two reduced coenzymes are NADH
and then we do produce some GTP, and so it's closely related
to ATP and so the body is able to convert that GTP into ATP.
Then we get some, an FADH2 reduced coenzyme
and then one more NADH.
So, the real essence of the citric acid cycle is
that we have 2 carbons in--
And we will get 2 carbons out as CO2,
and we get these reduced coenzymes and GTP
which are the energy currency.
So we'll see that the reduced coenzymes then will go
on to the electron transport chain.
So let's take this introductory review of the citric acid cycle
and connect it to the big picture, right?
So now we are down here for energy production,
so the acetyl CoA, right, so this is the 2 carbons in
and then we have our citric acid cycle so we have seen
that there's-- the CO2 is the carbons out,
and then in the diagram here, we described it as electrons
but how do those electrons move?
Well, they're transported by the reduced coenzymes,
FADH2 and NADH, so it is a little misleading to think
that the electrons are just floating
around freely in our body.
The reduced coenzymes are the transport molecules--
Right, so these are the transport molecules
for the electrons, OK?
So, and then ultimately that leads
to the electron transport chain and then we can see oxidative,
right, because oxygen is present and phosphorylation
because we see that we are combining the phosphate group
with ADP to produce the ATP.
So the names are all very descriptive and helpful
in explaining what is going on.
All right, so now that we have the big picture,
let's focus our attention and look at the details more closely
of the citric acid cycle.
So we'll look at this page centered.
Alrighty, so let's look at the changes
that are occurring, so here we have it.
Right? So here is our acetyl CoA entering the citric acid cycle
at stage I, right, so here is that acetyl group right there,
and we have oxaloacetate
and then this is citric acid right here,
so that's why it gets called the citric acid cycle.
So we have the enzyme citrate [inaudible], very well named.
Alrighty, so notice the acetyl CoA then we'll join with water
to the oxaloacetate and we will get coenzyme A released back
into our body to go produce more acetyl CoA,
and then we have our finished product here.
So now we want to look at the structure so we can,
there's quite a bit happening.
It's a pretty busy reaction but if we look here,
there we see the carboxylate and the CH2,
and then this carbon here we can see we have this carbonyl
changing and so here, here is our acetyl group--
Hooking on to the molecule, so now let's look
at the next [inaudible].
So the first step is basically,
we have this acyl group transfer reaction
and it's very complicated.
There are a lot more steps than what we show
in this simple schematic, so we'll just focus on the fact
that we have an acyl group transfer.
OK, now, let's look at the changes that occur
from the citrate to isocitrate.
So if we scan the molecules and you look for differences,
we go hmm, there really is no change in the chemical formula
but if we look here, we can see
that this alcohol group was a tertiary alcohol,
and so there's been some rearrangement of the molecule.
If we look at the alcohol here, now we can see
that it's a secondary alcohol, so in this second step,
we simply had a nice summarization
of citrate to isocitrate.
Let's see what happens in step 3.
So as we look at isocitrate, we look for changes that occur
as it produces alpha-ketoglutarate.
And as we look here, we notice
that this carboxylate group is gone.
It has been released as CO2, so now we see that, right,
we had a basically an oxidation.
We fully oxidized our carboxylate to CO2, so we notice
that we have an oxidation reaction.
And whenever we have an oxidation reaction we know
that we need to look for the accompanying reduction
and sure enough, we see the formation
of our reversed reduced coenzyme.
OK, now, from alpha-ketoglutarate what happens
to change it into succinyl CoA,
so let's compare these two molecules to look for changes.
And sure enough, if we'll notice that we have the loss
of another CO2 molecule, so we have another oxidation,
and that's combined with the reduction of another coenzyme,
so another oxidation reduction reaction.
OK, in step 5, we have succinyl CoA.
Notice that we're going
to produce coenzyme-A GTP and succinate.
So let's start looking for differences.
So as we start at the top, right, carboxylate, CH2,
CH2 ah-ha, right, so what do we see?
We see that there is something, this is breaking off
and so notice right there at we're basically,
we're taking a large molecule and we're breaking it apart
into two smaller molecules, and energy is produced,
so here's some energy production.
So this is the opposite of what we normally think
of a ligase enzyme.
So we think of ligase enzymes as bringing together
and using energy to bring together.
Here, we've broken the molecule apart and produced energy.
So here it's, well, it's catabolic still.
OK. Now we have the succinate.
Let's look and see what happens here as it is transformed
into fumarate, so we're comparing contrast,
we see the carboxylate and oh, look, there's the 2 carbons.
This one's very obvious.
We noticed that we have, right, saturated to unsaturated,
so what do we have here?
We had a loss of hydrogen, right,
so a loss of hydrogen, right, is oxidation.
Spelling error.
Oxidation.
OK. There we are.
Alrighty, so our substrate hasn't been oxidized and lo
and behold, we have produced another reduced coenzyme, FADH2.
Alrighty, so we have produced another reduced coenzyme,
so another oxidation reduction reaction.
And now as we look at step 7, we will look at fumarate
to malonate and we have the double bond.
You may notice that we're bringing in the water
and we can see that water is adding across the double bond,
and so the water has added to the molecule.
So this would be an addition reaction catalyzed
by a ligase enzyme.
And then in our final step, we have the malonate and we notice
that here this secondary alcohol, right,
is oxidized to a ketone.
So we have another oxidation and producing our last coenzyme
through the reduction reaction.
And then this is considered why we call it the citric acid cycle
because notice that the product of our 8th step,
[inaudible] acetate is the substrate
of the first step as the cycle begins.
And so the acetyl CoA keeps feeding into carbon chunks
and then we go through a series of reactions.
I think it's also interesting to note that the two carbons
that are introduced by acetyl CoA are not the same carbons
that are released as CO2.
And then to keep in mind the big picture, right, these coenzymes
that have been produced, which we can think
of as our energy currency all the way up, all the way, right,
so every place you see are reduced coenzymes,
think of an energy source, right,
so all of these reduced coenzymes then,
we will see in the next tutorial go
to the electron transport chain with oxidative phosphorylation.
But before we move on, let's take some time
to answer a few questions to make sure
that we're really understanding the finer details
of the citric acid cycle.
All right, so you'll want to be looking
at the citric acid cycle while you answer these questions.
So this would be a great time to pause the video
and answer these yourself, and then we can compare answers.
Alrighty, so looking at the citric acid cycle,
what is the substrate of step 4?
Well, that's alpha-ketoglutarate
and what was the product of step 3?
Alpha-ketoglutarate, right.
That's why it's called a cycle, right.
OK. Now, in each turn of the citric acid cycle,
2 molecules of CO2 are released.
In which step are they released?
I think this is a pretty straightforward one.
Steps 3 and 4.
OK. In which step of the citric acid cycle do two carbons enter?
I think that one is pretty straightforward, too,
but you can double check your cycle
if you're not convinced and that's a one.
I kind of got pretty European there on that one.
Alrighty, which steps require the oxidoreductase enzyme,
right, and those are easy to spot, right?
Look for the reduced coenzymes, so look for NADH
and FADH2 formation, right?
So if we look back we'll see steps 3, 4, 6 and 8.
OK. Which of the steps require a ligase enzyme?
Right, so remember ligase enzymes is water, right,
plus a carbon-carbon double bond, right,
or a carbon-oxygen double bond, so for citric acid cycle,
we're here and you'll notice that is step 7.
Which steps require a transferase enzyme?
So notice that's step 1, right,
that's how we make the two carbons into--
Oops. Right, into the citric acid cycle, right,
by the acyl group transfer?
OK, which of the steps require a ligase enzyme.
This one can feel a little tricky because none
of the steps show that we need ATP.
So this one you have to remember to think
about the reverse nature of enzymes, right,
so the enzymes catalyze both the forward and reverse reaction,
so if we look at step 5, right, we see that it's
like a reverse ligase.
We took one large, broke it into two small compounds
and GTP was released as the energy source.
So it was like a reverse ligase, so that would be step 5.
And then, lastly, what's--
if we look at step 2, why is it needed?
And what subsequent step would not be possible without step 2?
So, if we look back at our cycle, we notice that step 2,
right, is an isomerase reaction.
Right, we go from a tertiary alcohol to a secondary alcohol,
and the reason for this is remember that tertiary alcohols,
right, cannot be easily oxidized.
So [inaudible] class we can just say they're not oxidized,
but the secondary alcohol can be oxidized.
So if you focus on the carbon
that becomes the secondary carbon and you look
through the pathways, what you'll notice is in step 8.
Right, step 8 would not be possible, right.
Step 8 needs step 2 because in step 8, we oxidize that, oops,
we oxidize that secondary alcohol.
So step 2 is well in advance of step 8, but it was setting it
up to create the cyclic nature of the citric acid cycle.
Alrighty, so that concludes our tutorial
on the citric acid cycle.
We've done a bit of review here and practiced
to reinforce our understanding but it'd be a good idea to work
through a few more homework problems so you're feeling solid
on this important biochemical pathway.