Now as we consider
amino acid catabolism,
I’ve talked about some of
the individual reactions,
but I want to now describe
them in the entirety
of the picture of all
the amino acids.
Amino acid catabolism is broken
down to three categories.
The first category being amino
acids that are glucogenic.
These are amino acids
that are broken down into
glycolysis or gluconeogenesis
And the glucogenic amino acids are shown in
the figure on the right labeled in green.
The ketogenic amino acids are those that
are broken down to form acetyl-CoA.
And they’re shown in the figure
on the right on the red.
There are some amino acids
that appear as intermediates in
both pathways and they’re known
as ketogenic and gluconeogenic.
And those amino acids are
shown in the pathway in blue.
Now the pathways that are shown on the
right schematically depict on the left,
the glycolysis pathway starting with
glucose and ending with pyruvate.
And the circle on the right
depicts the citric acid cycle
and the intermediates of
the citric acid cycle.
So we can see how the breakdown
of these amino acids
feed into these
The glucogenic amino acids, you can see
here, alanine, I won’t describe them all.
You can see the list of amino
acids that are present.
The ketogenic amino acids are not
nearly as a bond, but there’s
only two that primarily produce the
acetyl-CoA, lysine and leucine.
And here are the amino acids
that are involved in both.
Yeah, it’s complicated.
It looks complicated.
But there’s actually some
simplification to it.
And that is that there
are only six molecules
that are involved in the
catabolism of amino acids
They’re shown here.
succinyl-CoA and fumarate.
All of the animo acids can be broken
down to produce those six intermediates.
It’s because of this,
and you see the four of
these individual molecules
up here in citric
acid cycle, it’s because
of this that we
describe the citric acid
cycle is anaploratic.
It can use intermediates from
elsewhere to break down.
It can also be a source of intermediates
for making some of these amino acids.
So amino acid catabolism is
pretty important as we can see.
It has to be able to
deal with the means.
The cell has to be
able to shuffle,
make the proper amount of amino acids
and not produce too much ammonia
if it’s going to
Most of the pathways ultimately shuffle
amines through transamination.
But the addition in the movement of ammonia
is also an important consideration.
That’s why glutamate transamination
is important because it
can transfer or accept amines
and it can also handle ammonia.
Glutamate is therefore essential for
the transport of all of these nitrogen
containing compounds away from sensitive
tissue and delivering it to the liver.
And we’ve seen how the glucose alanine
cycle helps the brain, for example,
to take away that excess amine
without losing glutamate.
In the liver, this is where the ammonia is
used for urea synthesis and for excretion.
The liver handles a lot
of things in our body.
The brain as I’ve said is
sensitive to ammonia levels
and so they have to be very careful to
balance things appropriately there.
Glutamate is a neurotransmitter
as I said and that’s
why the glucose alanine
pathway is important
because alanine provides a
way of getting the amine
out of the brain or the
ammonia out of the brain.
Now as you might imagine, we’ve got
some complicated pathway that we’ve
talked about and I’ve talked about some
of the diseases that are involved.
I want to review some of those
and bring up some of the
others that were involved
in amino acid catabolism.
Alcaptonuria, we’ve seen, is involved in the
catabolism of phenylalanine and tyrosine.
which I haven’t describe,
arises from defects in the
catabolism of methionine,
Mapel syrup disease arose
from the breakdown problems
associated with the
branched-chain amino acids.
Homocystinuria arises from the
problems with breakdown of methionine.
Tryrosinemia, of course, from the
breakdown products of tyrosine.
And argininemia arises from problems
associated with the breakdown of arginine.
And I’ll say more about that in
the lecture of the urea cycle.
and its name is suggests
from the deficiencies in
breakdown of methionine.
Hyperlysinemia from deficiencies
in the breakdown of lysine.
Glycine encephalopathy from the
deficiencies in breakdown of glycine.
Propionic acidemia from the deficiencies
to the breakdown of these four enzymes.
And finally, hyperprolinemia from
deficiencies in the breakdown of proline.
Now one of the things happens to amino acids
that we also have to consider chemically
is that after they get built into proteins,
many of them are chemically modified.
That chemical modification actually enhances
the ability of a protein to function
or may have important things
relating to the signaling
involved in proteins or
So the most commonly chemically modified
amino acids are shown on the screen here.
I’ve got two examples
in terms of structure.
And you can imagine
using your knowledge of
chemistry what the others
actually look like.
Hydroxylysine is one.
It’s very commonly
modified that we’ve seen.
And phosphotryrosine is important
in the signaling process.
Serine is a target in the cell for
glycosylation and phosphorylation.
This arises from the side chain of
serine that has a hydroxyl group,
and that’s where a phosphate
can be attached by a kinase
or a sugar can be attached in
the process of glycosylation.
Threonine has the same side chain
as serine does, a hydroxyl group.
And like serine, it’s a target for
phosphorylation and glycosylation.
Now lysine is really a complicated
amino acid in terms of modification.
The most common modifications
of it involved
methylation and acetylation
I’ve listed below a variety of other kinds
of modifications that can happen to serine.
I won’t list all of them here.
But suffice it to say that
lysine is the most commonly
enzyme found in proteins.
Methionine is modified in prokaryotes
by the addition of a formal group
as I’ve described earlier in the
lectures on amino acid metabolism.
And that happens only in
prokaryotes and only for the
first amino acid going into
proteins and prokaryotes.
Arginine can be acetylated and the
acetylation of amino acids like
arginine and also lysine is important
because they have positive charges.
And the addition of an acetyl group changes
that positive charge to a neutral charge.
If we’re talking about histones which
is where this modification occurs,
histones are very positively-
charged proteins that
interact with DNA, which
So if we take that
arginine or lysine
and convert it from a positive
charge to a zero charge,
we change the attraction that occurs
between the histone and the DNA.
And we’ll see more about that in our discussion
of the control gene expression of DNA.
Proline is a target
Of course, that happens in collagen as
I’ve described in the collagen lecture.
Cysteine as I’ve mentioned earlier
is a very important amino acid,
because within proteins, it can
react with a second cysteine.
So two cysteines can get together
and make a disulfide bond.
I’ve mentioned that earlier in the
lectures on amino acid metabolism.
And that disulfide bond that
forms between two cysteines is a
very important structural feature
to help stabilize proteins.
Histidine is not
very much modified.
But occasionally, histidine
That’s something we’ve learned
only in recent years.
The significance of that phosphorylation
is not completely clear at this time.
Glycine is a target
That involves the attachment of
a myristic acid to a glycine.
Glutamic acid is a target for carboxylation,
and I’ve talked about that earlier
as related to the importance of
carboxylation, the blood clotting process.
And finally, asparagine is a target for
glycosylation just like the serine and
threonine, meaning that this amino acid
side chain is linked to sugar residues.
In this set of lectures, I’ve described
the metabolic pathways involved
of the 20 amino acids, plus the one
rare amino acid, selenocysteine.
There’s a lot of diversity here.
There’s a lot of different
reactions that are
occurring in a lot of
This is necessary as we’ve described
to balance the individual amino acids.
We’ve also seen how those amino
acids are chemically modified
and how the breakdown of those amino acids
can lead to numerous genetic diseases.