>>Welcome to this next segment of the chemistry of beer. Our topic for this segment is the
tricarboxylic acid cycle, or mercifully abbreviated the TCA cycle. It's also called the Krebs
cycle or the citric acid cycle. And please excuse me if I accidentally interchange those
names, but I'll try to stay consistent and use the TCA cycle. We will begin our discussion
by considering the big picture. What does the tricarboxylic acid cycle do for an organism?
Next we'll discuss the gateway reaction, how pyruvate is converted to acetyl-CoA because
it's actually acetyl-CoA that enters the citric acid cycle. Then we'll discuss the entire
pathway, and we'll make connections with the electron transport chain and oxidative phosphorylation.
Finally, we'll try to tie up some loose ends and summarize our discussion.
Our first reaction to consider is sometimes called the gateway reaction. Glycolysis, which
we discussed previously, occurs in the cytosol. It produces two pyruvates. The two pyruvates
are able to pass through both the relatively porous outer mitochondrial membrane and through
the inner mitochondrial membrane via pyruvate transporters. Once inside the matrix of the
mitochondrion, the pyruvates are oxidatively decarboxylated to form acetyl-CoAs. This sequence
of reactions is catalyzed by the pyruvate dehydrogenase, or PDH complex, which as the
name implies is made up of several different subunits.
The PDH reactions can be summarized as follows. Pyruvate plus our old friend NAD+ plus Coenzyme
A form acetyl-CoA plus carbon dioxide plus NADH. We can take another view of this reaction
with the help of this diagram. This explicitly shows the structure of pyruvate, which you
all hopefully know by now, and it shows the loss of CO2. Once that happens we have a two-carbon
fragment. This two-carbon fragment is transferred to Coenzyme A to make acetyl-CoA. And in the
process our cofactor NAD+ is reduced to NADH. Now the diagram says that this NADH can go
to the electron transport chain, and so keep that in mind because that's a topic we're
definitely going to encounter.
So now we are ready to discuss the TCA cycle. This diagram here shows the complete TCA cycle.
However, it has, for our purposes, a bit too much detail. Instead we're going to use the
help of another diagram. This diagram allows us to focus on some of the more salient features
of this cycle. If you go on and take a full biochemistry course, you will go in depth
in your study of the TCA cycle. But again, for our purposes, this diagram is sufficient.
Remember the gateway reaction, the pyruvate dehydrogenase reaction, formed acetyl-CoA.
Acetyl-CoA, which we're mainly interested in this two-carbon fragment of acetyl-CoA,
it condenses with the four-carbon compound oxaloacetate. This forms the six-carbon compound
citrate. Citrate then undergoes an isomerization reaction to make isocitrate. Isocitrate is
then oxidatively decarboxylated to make alpha-ketoglutarate, a five carbon containing compound. This reaction
forms NADH. Again, I want to draw your attention to this. Note all the points at which we form
NADH. The other product, since we're decarboxylating isocitrate, is CO2. The alpha-ketoglutarate
is also oxidatively decarboxylated, so we go from a five-carbon alpha-ketoglutarate
to a four-carbon molecule called succinyl-CoA. And during this reaction sequence, NAD+ is
reduced to NADH, we release carbon dioxide, and we bring in Coenzyme A to attach to the
succinyl moiety to make the succinyl-CoA. So we started with a four-carbon compound,
oxaloacetate. It condensed with the two-carbon fragment from acetyl-CoA, the acetyl fragment.
We made a six-carbon molecule, citrate. It isomerized to isocitrate, and then we had
two steps where we lost CO2. So we're back to our four-carbon compound, or our four-carbon
fragment since it's attached to Coenzyme A. Our succinyl-CoA then undergoes a reaction
whereby we form succinate, and in the process form GTP, guanosine triphosphate. Guanosine
triphosphate we can think of as being energetically equivalent to adenosine triphosphate, or ATP.
The succinate then undergoes a dehydration reaction to form fumarate, so we start with
a four-carbon compound and we still have a four-carbon compound, but it's undergone a
dehydration reaction and those electrons have been used to reduce the cofactor FAD to FADH2.
Now the FADH2 will further hand off the electrons to Q, which stands for Coenzyme Q or ubiquinone,
and that forms QH2, and that's going to be part of our discussion for the electron transport
chain. So moving back to fumarate, our next reaction catalyzes the hydration of the double
bond of fumarate to make malate, and specifically—not shown on this diagram, but it's L-malate.
That is the stereochemistry. L-malate is then oxidized back to our starting compound oxaloacetate,
and in the process NAD+ is reduced to NADH.
So let's summarize. We start with oxaloacetate, a four-carbon containing compound, and it
condenses with the acetyl group, the two-carbon fragment of acetyl-CoA, and forms the six-carbon
citrate. We have two decarboxylations, during which we form NADHs. We form one GTP—that's
referred to as a substrate-level phosphorylation reaction. We form one FADH2, and the remaining
reactions get us back to oxaloacetate, and in the process form another NADH. So we want
to keep track. We want to keep that in our minds, what happens to the NADHs and the FADH2.
So now we're ready to think about the fate of NADH and FADH2. Because the citric acid
cycle occurs in the matrix of the mitochondrion, that is where we're forming the NADH and the
FADH2. The NADH hands its electrons off to a complex, which is simply referred to quite
frequently as Complex I. So as NADH is reoxidized back to NAD+, electrons are handed off through
Complex I. While this occurs, hydrogen ions, or protons, from the matrix are moved from
the matrix to the intermembrane space. Recall that mitochondria have an outer membrane and
an inner membrane, and so the protons are moved from the matrix to the intermembrane
space. As the electrons move through Complex I, they're transferred to Q, which I mentioned
earlier. This is ubiquinone. It's, it's basically an electron carrier that moves within the
bilayer of the inner mitochondrial membrane. The electrons move from reduced Q through
Complex III, where more protons are translocated from the matrix to the intermembrane space.
Then the electrons go one at a time to cytochrome c. We actually need two cytochrome c's to
accomplish this. The electrons pass from cytochrome c to Complex IV where more protons are then
pumped out from the matrix to the intermembrane space, where finally oxygen is our final electron
acceptor, and it accepts the electrons and is reduced to water. What we've accomplished
by handing the electrons off down the electron transport chain is to capture some of that,
that free energy in the form of an electrochemical potential. We have a higher hydrogen ion concentration
in the intermembrane space with respect to the matrix. Those hydrogen ions can then move
in through ATP synthase. And as that occurs, this helps bring about the formation of ATP.
Actually, it helps bring about the release of tightly bound ATP.
As we noted on the previous slide, succinate is one of the intermediates of the citric
acid cycle. As succinate undergoes the dehydration reaction to form fumarate, those electrons
are handed off to FAD to make FADH2. This occurs in Complex II, shown on the bottom
of this scheme. Those electrons move through Complex II to Coenzyme Q, or ubiquinone, which
is simply symbolized by the capital Q. The Q moves to Complex III, and basically we continue
our discussion. Complex III pumps protons, hands the electrons off to cytochrome c. The
electrons move from cytochrome c to Complex IV, where oxygen is the final electron acceptor,
and more protons are translocated, and those protons can flow through ATP synthase and
bring about, effectively, the synthesis of more ATP.
So now we are in a position to summarize our discussion of cellular respiration. During
our discussion of glycolysis, we saw that we started with glucose, and we ended up with
two pyruvate molecules. In the process we formed some ATP, specifically we formed two
net ATPs. The pyruvate is then transformed into acetyl-CoA through the pyruvate dehydrogenase
complex reactions. This formed an NADH. The acetyl-CoA then entered the citric acid cycle,
and we formed one GTP, which I said was equivalent to an ATP in terms of energy, but importantly
we formed three NADHs and one FADH2. Those electrons were then handed through the electron
transport chain where oxygen was the final electron acceptor, and we formed water. One
thing I didn't mention is that we form a lot more ATP by undergoing oxidative phosphorylation
than we do by simply undergoing glycolysis. So if a cell is aerobic, it is to its advantage
to undergo oxidative phosphorylation. It's much more efficient. You get a lot more ATP
formed as a result of oxidative phosphorylation.
So now we're ready to summarize our discussion. We found that the pyruvate, which was formed
in glycolysis, enters the mitochondrial matrix and is oxidatively decarboxylated to acetyl-CoA
and carbon dioxide. The acetyl-CoA enters the citric acid cycle by condensing with oxaloacetate
and forming citrate. One turn of the TCA cycle converts citrate, a six-carbon compound, back
into oxaloacetate, a four-carbon compound. Along the way, two carbon dioxides were released,
one GTP was formed, three NADHs were formed, and one FADH2 was formed. When we follow the
electrons, we see that the NADHs and the FADH2 hand off their electrons to the electron transport
chain, which in turn leads to the production of ATP. This aerobic process is much more
efficient than the anaerobic process of glycolysis.