Methionine metabolism is kind of complicated
as we study it coming from aspartic acid.
The breakdown of methionine
with cysteine metabolism
as we briefly seen.
The complicated synthesis
that comes from aspartate,
however, I'll show here
with a set of arrows.
Now, the other set of lectures where I've
talked about vitamin B12 metabolism,
I actually showed those
So I'm not going to
show them again here.
But suffice it to say that there are
seven chemical steps that occur:
phosphorylation, an oxidation that
another oxidation that occurs,
this process creates homoserine.
Homoserine we remember
was an intermediate in
the synthesis of the
cysteine from methionine.
Succinylation involves the
addition of a succinate molecule.
The cysteine is replacing the succinate
in the next step of the process.
There's then loss of a
pyruvate and an ammonion
ion to produce the
homocysteine and homocysteine
was the molecule we remember
is a problem in the
production of cysteine.
And finally, methylation of the homocysteine
by an N5-methylfolate creates methionine.
This was the reaction that I described in the
other lecture that requires vitamin B12.
There are other ways
of making methionine.
Homocysteine, as we've seen,
can be converted to methionine
by the alternate pathway that
I'm going to show you here.
Now, this pathway is another way of
getting a methyl group on to homocysteine.
The difference between homocysteine
and methionine is that methyl group.
So glycine betaine is the
source of methyl group here.
It combines with homocysteine to make
dimethylcysteine and methionine.
So we can see that a methyl
group has transferred
from the glycine betaine on to the
homocysteine to make methionine.
The enzyme catalyzing this reaction is
betaine, homocysteine, methyltransferase.
And the reaction here
occurs in the liver.
We see the transfer of the methyl
group as occurs right here.
Now, methionine is modified in bacteria
before it is put in to making proteins.
We remember in
that the very first amino acid
that makes it into the proteins
is not methionine but a
modified form of it,
and that's known as
So this set of reactions
I'm going to show you
shows how that
formylmethionine is made.
Like the synthesis
formylmethionine is made by modifying
a methionine tha's on a transfer RNA.
This is shown in the
set of reactions.
Methionine combines first with
its initiation tRNA that's used
to put it into the protein during
the process of translation.
This produces the methionine
joint to the tRNA.
And then the reaction that
we see that the formyl group actually
comes form 10 formyltetrahydrofolate
and produces the formulated
methionine on the transfer RNA.
The product to that reaction
And the enzyme catalyzing in this
The next amino acid metabolism we
will consider as that of threonine.
Threonine's metabolism overlaps a
bit with methionine metabolism.
The first three steps of the synthesis
of threonine from aspartate
are the same as first three steps
in the synthesis of methionine.
The aspartate is converted to
aspartyl-beta-phosphate by a phosphorylation.
is converted to
by a reduction.
And the homoserine is created from the
last intermediate by a reduction as well.
These three steps also happen
in the synthesis of methionine.
In the next step of the process,
homoserine is phosphorylated to make
phosphohomoserine using energy from ATP.
And then phosphohomoserine
and that dephosphorylation involves
a molecular rearrangement,
that molecular rearrangement
This whole process involves the use of 2 ATP and 2 NADPH molecules.
Lysine is made in also another set
of very complicated reactions.
The first two reactions are same as
threonine and methionine metabolism.
There are total of nine steps.
I'm not going to step you through all of
those here because they're not really
relevant or needed for us to understand
what's happening in the process.
Lysine is one of the most
post-translationally modified amino acids
and we'll talk about that at the
end of this set of lectures.
It's very important for that
the modifications that happen
to lysine allow for so many
things to happen in the cells.
The hydroxylation of lysine,
we've seen in another lecture,
is important for making
And a deficiency of one of the
enzymes in the lysine pathway,
an alpha-aminoadipic semialdehyde
synthase, leads to hyperlysinemia.
And hyperlysinemia leads to
accumulation of lysine in
the blood which has some
very severe consequences.