00:01
That next amino acid we consider
in biosynthesis is that a proline.
00:06
Proline is made in a relatively simple fashion
in three steps starting from glutamate.
00:11
The first step involves
the synthesis of
glutamate-5-semialdehyde
which I don't show here.
00:17
The first step beyond that then is the
phosphorylation of the glumate-5-semialdehyde.
00:23
That oxidation and
dephosphorylation is necessary
to produce the cyclization
intermediate.
00:28
And that's what we see
on the screen below.
00:31
The cyclization that happens
of glutamate-5-semialdehyde
is spontaneous and does
not require an enzyme.
00:37
We can see that cyclization
actually happening
to make one
pyrroline-5-carboxylic acid.
00:43
Cyclization happens without any other
thing being present in the reaction.
00:47
The last step that one
pyrroline-5-carboxylic acid gets reduced
and the reduction of that
produces the amino acid proline.
00:56
Arginine is an amino acid that
can be made in multiple ways.
01:00
In arginine, as we will see
in the urea cycle, is a very
important sort of central enzyme
for doing a variety of things.
01:07
So there four ways to make
arginine and I'll describe them
right here and then show you
later how those reactions occur.
01:14
The first is a forward
reaction that takes two steps
coming from citrulline
and ATP and aspartate.
01:20
That process occurs in the urea cycle in
the normal direction of the urea cycle.
01:26
The second way of making
arginine is production
from asymmetric dimethyl
arginine or ADMA.
01:32
This is a modified form of
arginine that's found in proteins.
01:36
It differs from arginine
in having a metal group
and that metal group is put on to
arginine after the protein is made.
01:43
To get ADMA, one has to
break a protein down
by using proteolysis and
then ADMA is released.
01:50
So, ADMA is made into arginine after
a protein has been broken down.
01:55
The third reaction for making
arginine involves a reversal
of a reaction of the urea cycle
using the enzyme arginase.
02:03
This reaction involves the
molecule known as ornithine
and we'll see how that
actually is produced.
02:09
The last way of making arginine is a
reaction that involves nitric oxide.
02:13
This is reaction that is actually
used to make nitric oxide
in cells and it involves
citrulline, nitric acid, and NADP.
02:20
So arginine is a central
part of the urea cycle.
02:23
And remember that the function of
the urea cycle is to produce urea
for excretion because excess
amines are problematic.
02:30
The cell has to be able to have
arginine in order to do that.
02:34
The ADMA reaction that actually
goes backwards to produce
arginine interferes with the
production of nitric oxide.
02:40
Nitric oxide is a very important
signaling molecule that cells use.
02:45
So again, cells have to balance need
for one molecule versus need for
another in considering which pathway
is used to make a molecule.
02:54
Arginine may play role in very important
diseases involving cardiovascular disease,
diabetes mellitus, erectile
dysfunction and kidney disease.
03:02
So it's a very
important amino acid.
03:05
Deficiency of the enzyme arginase
is another important reaction to
consider because it leads to the
genetic disease known as argininemia.
03:15
And argininemia as we will see for
other amino acids involves the
accumulation of arginine and more
importantly ammonium ions in the blood.
03:23
Thos ammonium ions as we
can describe are toxic.
03:27
Well, in this set of slides, I want
to show now the reactions that
I've just described previously the
four ways of making arginine.
03:36
We see arginine in the lower part
of the screen and we see how
it connects to these molecules
that I've been describing.
03:42
We start with reaction number one which
involved the production of arginine
from citrulline reaction -- set of
reactions that occurs in the urea cycle.
03:51
There are two steps
as I've described.
03:53
The first step being a
very energy-intensive step
that involves the cleavage of ATP to make
AMP plus two inorganic phosphate ions.
04:02
This reaction is central
to making arginine in the
urea cycle but it's a very
energy-intensive reaction.
04:08
So cells again, are cautious in
how much they use to do this.
04:12
In each of the reaction
that I show here,
the green arrows point in the
direction of making arginine.
04:18
And we see that each reaction is
reversible so there's an arrow
pointing the other direction that
can go on different circumstances.
04:26
Argininosuccinate which
is a product of the
first reaction is converted
into arginine in
the next step, and we can see that was the
product of arginine for pathway number 1.
04:36
ADMA after approaching
has been broken down
can be demethylated to make
arginine as we have seen.
04:42
And we also see that by starting
with ornithine and urea and
going upwards with the lost of
water, we can make arginine.
04:50
Now, normally, this reaction
as it occurs in the urea cycle
involves the downward direction.
04:57
So, in order to make arginine, we have
to actually reverse that reaction.
05:00
Normally, that's not a very
common way of making arginine.
05:04
The last way of making arginine is also
a reversal of a reaction that occurs in
the cells and this involves the nitric
acid signaling that I've described before.
05:12
If we make arginine using this
pathway, we use nitric oxide.
05:18
And nitric oxide, as I said, is an
important molecule for signaling.
05:21
So, cells are careful again
in how much this reaction is
moved to the right versus how
much it's moved to the left.
05:28
This reaction is an oxidation
and we can see the oxidation
occurring as a result of the
NAPD being converted into NAPDH.