The next stem we'll be considering our
reactions is that of the aspartate family.
The aspartate family, as its name
suggests, all involve aspartate.
And aspartate can actually trace
its roots back one further
molecule and that's oxaloacetate
as you can see here.
All family members in this metabolic
pathway come from aspartate.
Aspartate can actually also
be made from one of them.
So there's reversed reaction that can
produce aspartate as we will see.
There are numerous ways
that lead to aspartate.
So I'm only going to talk
about a couple of them.
One of the ways and one of the more
common ways in which aspartate can be
made is by the process of transamination
that we've already talked about.
Here's one sample transamination
and there are many
that can produce aspartate.
One starts with glutamate
Glutamate being the amino acid
that is the source of the amine.
Oxaloacetate being the alpha keto acid
which is the source of the oxygen.
The product of that transamination, the
glutamate becomes alpha-ketoglutarate,
and the oxaleacetate
Amine has come from the glutamate,
onto the aspartate, the oxygen
has come from the oxaloacetating,
becomes the alpha-ketoglutarate.
Another way of making
aspartate is by hydrolysis.
This is a reversal of the
synthesis of asparagine in a way.
Now, we'll see later, it's not the exact
reversal of the synthesis of asparagine.
Asparagine can be cleaved using water in a
hydrolysis reaction to produce aspartate
and there's that
ammonium ion again.
Ammonia is a toxic compound.
The cell meets the aspartate,
they're going to do this reaction.
If the cell was breaking
it's got too much asparagine,
he'll do this reaction.
But that ammonium ion has to be gobbled up
somewhere or it's going to cost a problem.
The third way of making aspartate
involves the urea cycle.
The Urea cycle involves the
cleavage of argininosuccinate
with AMP to make aspartate
and citrullyl AMP.
Now this is kind of a complicated
reaction that I won't go into here,
but will be discussed later in another
one of these lectures in the urea cycle.
As I note, it's a reversal of a reaction
that occurs normally in the urea cycle.
So normally the reaction, the forward
direction of the urea cycle goes upwards.
The synthesis of asparagine occurs from
aspartic acid in a fairly simple fashion.
We can see here that if we start with
aspartate and glutamate and we add ATP,
we can create asparagine and
alpha-ketoglutarate plus AMP.
Now this is a transamination but this
is a transamination that's a little bit
different in the sense that it's
requiring additional energy to do that.
Why does that happen?
Well, the reason that
that happens is we're not
putting the amine onto
the alpha keto group.
We're actually putting the amine onto the
carboxamide group at the end of asparagine.
That requires energy.
And because that requires so much
energy, we see ATP becoming AMP.
When we see that happen, we know
there's a lot of energy involved.
When that happens, it means that this
reaction, even though it's drawn
in a reversible fashion, isn't very
practical in the reversed direction.
The enzyme catalyzing this
reaction is asparagine synthetase.
It's energetically costly and
essentially not reversible.
So the other way of course of
making aspartate and the much more
common way of making aspartate from
asparagine was by the breakdown
of asparagine that involves the
use of water, production of
aspartate and the enzyme catalyzing
that is known as asparaginase.
This is the most common way
that asparagine is broken down
and the most common way that aspartate
is produced from asparagine.
We produce the ammonium
ion in this process.
We remember that's toxic and it will have
to be gobbled up in another reaction.