Now, homocystinuria, as I said,
is pretty severe disease.
It's a genetic disease caused
by deficiency of this enzyme
known as cystathionine
There are many problems that
can arise in homocystinuria.
There are musculoskeletal
anomalies that occur,
And one of the reasons we have
these intellectual disabilities
is because of the problems with
the nerve tissue in the brain.
Seizures can result.
There are numerous eye
anomalies that can arise.
Vascular disease because again, we have
a lot of homocysteine that's occurring
because it's not being converted
into cysteine in the process.
We see cystinuria which is
an unrelated genetic disease
that has many symptoms
that are similar.
We have the inability of the
kidneys to reabsorb cysteine.
So when this happens, then we see
problems associated with the kidneys
including high levels of
cysteine, high levels of lysine,
ornithine and arginine
that appear in the urine.
And because we have these
we develop kidney
stones that also occur.
This is a not disease that
you would wish on anyone.
There are at least three other
ways of making cysteine
and I illustrate these here.
One of these comes
from serine directly.
In this process, acetylcholine gets --
It donates an acetyl group to
serine to make O-acetyl L-serine.
That is shown here.
And the next step of the process,
the sulfur that ends up being cysteine
comes with the addition
of a hydrogen sulfite.
This splits out the acetate in the process
leaving behind cysteine
for making proteins.
A second way of making cysteine
actually comes from proteins itself.
One of the things that cysteine
can do on a protein is combine
with another cysteine in a covalent
bond to make this dicysteine
as it were called L-cysteine.
This involves the joining of
the two cysteines together
by a disulfide bond as shown in
the middle of the molecule.
When proteins are cleaved and they've
had this bond occur in them,
L-cysteine is the product of
the breakdown of the protein.
Making cysteine from L-cysteine
is a trivial matter.
It simply involves breaking that bond.
And breaking that bond
involves a reduction.
The electrons in the process come
from NADH as you can see here,
the product of the reaction
producing two cysteins.
The last way of making cysteine
starts with an oxidized form
of cysteine known
as cysteic acid.
Cysteine is fairly, readily oxidized
and so it's not uncommon that cells
will have L-cysteic acid within them.
To make cysteine from that,
simply involves a reduction
and that reduction involves again,
hydrogen sulfite as we can see here.
In the process, the sulfur is
donated to the cysteic acid
The sulfite which was the oxidized sulfur
is released and cysteine is produced.
Another member of
the cysteine family
is that of one of the rare amino
acids I talked about earlier.
This amino acid is known as selenocysteine.
And it does not normally occur in proteins.
It's very rarely appearing.
And it's way of appearing
in proteins is unusual
and its synthesis is also a little unusual.
Selenocysteine is sometimes
called the 21st amino acid
because it's not coded
for genetic code.
It uses a stop codon that's in the
translational process to get into a protein,
and I won't talk
about that here.
The way in which selenocysteine is made
is actually made on a transfer RNA.
Now, the other amino acid who has a
modification that occurs on a transfer RNA
is that of methionine, and I'll talk about
that later in this set of lectures.
When we look at this process, we see what
the way in which selenocysteine is made.
starts with serine,
which is why it's
in the serine family.
But serine itself is not directly modified
until after the serine has been joined
to a transfer RNA.
You'll notice that this is
a non-serine transfer RNA.
So this is a special transfer RNA
that has joined the serine in the
process of making selenocysteine.
So, here we have the
product of that reaction.
The serine has been linked
to the special transfer RNA.
There are two chemical reactions
in which the serine is modified
and the modification of the serine involves
the alteration of the hydroxyl group
to put a seleno group into that.
Two enzymes are involved
known as cell A and cell B.
The product of that reaction is this
selenocysteine linked to the transfer RNA.
And the selenocysteine
linked to the transfer RNA
is then incorporated into proteins
using this odd stop codon
mechanism of getting it in.
It's because of this that selenocysteine
is so rarely found in proteins.