Professor Dave again, let's talk about the citric acid cycle.
We learned about glycolysis, which is an anaerobic process, meaning it
does not require oxygen to occur. Since oxygen is not necessary,
the first simple organisms on earth were able to generate energy through
glycolysis for millions of years. But two ATPs per glucose just isn't that much.
In order for higher organisms like animals to evolve that can run, and jump, and swim,
they had to evolve additional metabolic pathways that generate far more energy
than glycolysis. This became possible once plants covered the earth, thereby
filling the atmosphere with oxygen, which is a product of photosynthesis,
thus the possibility for large-scale oxygen dependent aerobic respiration was born.
The location in the cell where this process occurs is the mitochondria, which
are eukaryotic cell organelles. According to endosymbiotic theory, mitochondria
seem to have been entirely separate organisms that were incorporated into
eukaryotes specifically for the respiratory abilities they possessed.
This activity begins with the pyruvate molecules that were generated in the
cytoplasm during glycolysis. These pyruvates will enter the mitochondrial
matrix to find Coenzyme A. In the presence of NAD+ pyruvate will undergo
decarboxylation, oxidation by NAD+, and then attachment to Coenzyme A, generating
acetyl CoA. Acetyl CoA will then enter the citric acid cycle, also known as the
Krebs cycle, or the tricarboxylic acid cycle. This is an eight-step pathway
requiring eight separate enzymes. In the first step, the enzyme citrate synthase
removes the acetyl group and tacks it on to oxaloacetate to form citrate.
Next, with help from aconitase, a water molecule is removed
and another one is added to generate a structural isomer of citrate called
isocitrate. Then, catalyzed by isocitrate dehydrogenase, isocitrate is oxidized by
NAD+ and then decarboxylates to form alpha-ketoglutarate. Next, another CO2
is lost and further oxidation by NAD+ takes place, with the help of
ketoglutarate dehydrogenase. The resulting molecule will join with Coenzyme A once
again to form succinyl-CoA. CoA is then displaced by a phosphate group to
form succinate, which is catalyzed by succinyl-CoA synthetase. This will
make one molecule of guanosine triphosphate, or GTP, in the process, which
can be used to make one ATP. Then with the help of succinate dehydrogenase
succinate is oxidized by a different molecule, FAD, which will result in
fumarate and FADH2. Next, fumarase will catalyze hydration which results in
malate, and lastly, one more oxidation by NAD+ takes place with the help of malate
dehydrogenase to give oxaloacetate, which will restart the cycle, reacting with a
new acetyl CoA. Overall, for every acetyl CoA that enters, this cycle
will produce three NADHs, one FADH2, and one ATP. Since one glucose will produce
two pyruvates in glycolysis and therefore two acetyl CoAs, we can double
these numbers to get the amounts per glucose molecule. Once again, for the ease
of memorization, here is a list of each step of the citric acid cycle with the
names of the respective enzymes. We can see that there still hasn't been a huge
payoff in terms of energy, but the products of the citric acid cycle will
then move on to oxidative phosphorylation, which will generate the
majority of the ATP produced in aerobic respiration.
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