The next step in the reaction involves
the enzyme argininosuccinate synthetase.
And in this reaction, you remember
there's a two-step process.
In the first step of the process,
ATP cleaves off a pyrophosphate.
We see that happening in that
very first step in the top.
We see a little
molecule in blue, PPi.
That is released from the ATP.
That allows the remaining part
of the ATP in the form of AMP
to combine with the citrulline to make
the L-citrulline adenylate complex.
In the second part
of the process,
aspartate displaces the AMP and
that’s where we see the AMP
leading also shown in blue to
create the L-argininosuccinate.
The chemical mechanism of the
reaction is shown below.
Now this two-step process, as I
said, involves AMP attachment.
And we see the AMP
attachment going here,
followed by displacement of
the AMP by the aspartate.
And the displacement
is shown here.
The product to the
is the substrate for the
next step of the reaction.
This reaction is the rate
limiting step of the cycle.
Meaning that it is the
slowest and it's the step
that other reactions await
for it to be completed.
The gene expression of the
enzyme is reduced by arginine.
Now arginine will see seized
further ahead in the pathway.
And as arginine accumulates, there's no
reason to continue making this enzyme.
So this regulation by
arginine, of this enzyme
that feeds and makes
arginine is important.
On the other hand, the expression of
the enzyme is increased by citrulline.
And citrulline is further
behind that in the cycle
and it's literally pushing
the thing forward.
So this kind of regulation
ensures that the cell has the
proper amount of the
argininosuccinate synthetase enzyme.
Defects in this enzyme
lead to citrullinemia
which involves an
accumulation of ammonia.
And in fact, most of the enzymes in the
urea cycle when their deficient result in
accumulation of ammonia and all the
consequences that arise as a result of that.
Like other enzyme deficiencies
at the urea cycle,
it is treated with a low protein
diet and in some cases,
with supplementation of arginine which
is needed in the next reaction.
The next step in the process is catalyzed
by the enzyme, argininosuccinate lyase.
We see the structure of the enzyme
in the far left of the slide.
In this reaction, argininosuccinic acid
or argininosuccinate, as we describe it,
is converted into arginine
and fumaric acid.
This involves a cleaving of the
bond that shown as on the screen.
In this reaction,
arginine is produced.
Now arginine really has two
or even three ultimate fates.
Most commonly, it can go to proteins or it
can remain and continue in the urea cycle.
The fumaric acid or fumarate,
as we also call it,
can be released and oxidized
in the citric acid cycle.
Now, remember what's happen as
we added an aspartic acid in the
previous reaction and we've
lost fumarate in this reaction.
The net result was an
additional amine got built into
our molecule in this case
in the form of arginine.
Now this reaction is very important
for the production of arginine.
Cells need a lot of arginine,
not just for the urea cycle
but also for making proteins.
It's also a source of fumarate.
And the citric acid cycle is a reaction
cycle where a lot of energy is produced.
So having more fumarate available
allows the cell to have more energy.
A deficiency of this enzyme,
like that of other urea
cycle enzymes, results in
excess ammonia accumulation.
In the next step of the reaction,
arginine is cleaved to
make urea and ornithine in a
reaction catalyzed by arginase.
Now the ornithine can
go ahead and go back to
the mitochondria and
complete the urea cycle.
The urea cycle can be
excreted into the urine.
This reaction occurs by cutting the bond
shown in arginine here to make the urea.
Now this enzyme is co-expressed with
nitrogen oxide synthase in smooth muscle.
These two compete for
the use of arginine.
The more arginase available, the more
arginase that's used in the urea cycle,
the less arginine is available
to make nitric oxide.
So again, we have to balance
how much of each of
these is use for the cell
to properly function.
Nitric oxide functions and
signaling to relax smooth
muscle and facilitates the
erection of the penis.
A deficiency of arginase is the
rarest of the urea cycle enzymes.
And the reason for that
is because there were two
forms of arginase that
are present in cells.
They provide some backup
when one is deficient.