Now we have covered pretty much there fatty acid oxidation
including the longer chains and the unsaturated
fatty acids as well as the saturated ones.
Now let's turn our attention
to how we make fatty acids.
Fatty acid synthesis doesn't
occur in the mitochondrion
it occurs in the cytoplasm. It is sequestered
from the fatty acid oxidation and
this simplifies considerably
the regulation needed to control the two pathways.
Fatty acid synthesis is similar
chemically to the reverse of oxidation.
For example looking at the bottom we see that in
synthesis we are doing the joining of two things
instead of the splitting of two things.
The next step moving upwards involves
the reduction whereas in fatty acid
oxidation it involved oxidation.
The next step above that involves the loss
of water whereas in fatty acid
oxidation that was a gain of water.
And finally the last step involves a
reduction whereas in fatty acid oxidation
it was an oxidation. So in many ways it's very
much the reverse of the oxidation process.
Now there are some differences and one of the
differences is on the end of the fatty acid.
Fatty acid synthesis occurs
with the fatty acid group being
carried by a molecule called
acyl-carrier protein or ACP
as people most commonly call it.
That's different from the CoA that was on the
fatty acid during the oxidation process.
Notably also the electron source
in fatty acid synthesis is NADPH.
The electron carriers in fatty acid
oxidation were, of course, FAD and NAD+.
Now let's step through the process to
learn a little bit about how this happens.
The very first step is little
unusual in fatty acid synthesis.
It's the only regulated reaction
in the entire synthesis of fats
and this reaction occurs, as
I noted, in the cytoplasm.
ACP will get involved as a carrier.
But the very firt step involves the synthesis of a
three carbon intermediate. That three carbon intermediate
is known as malonyl-CoA that you can see right here.
Now malonyl-CoA is made from acetyl-CoA
by the addition of a carboxyl group
by the enzyme acetyl-CoA carboxylase.
And you can see that this reaction
is also a very
energy requiring reaction; because,
ATP is being converted to AMP.
That's the equivalent of burning two ATP molecules.
The enzyme acetyl-CoA carboxylase
is the only enzyme that is regulated
in fatty acid synthesis, as I noted.
You will also see that this enzyme
is using biotin as a co-enzyme.
Biotin is a co-enzyme that
is involved in grabbing
carbons to add them to things
as we have seen with the
previous reactions in some
of these presentations.
Acetyl-CoA carboxylase is regulated
by two different mechanisms,
a covalent modification and the allosteric
binding of small molecules.
Now we can see this depicted on the screen here.
The carboxylase that is
the acetyl-CoA carboxylase
can be phosphorylated by a protein known
as AMP activated protein kinase.
That enzyme converts the activate carboxylase into
an inactive form by putting a phosphate onto it.
That also has the effect of changing the
organization of the acetyl-CoA carboxylase.
The active carboxylase is actually a polymer
that is a bunch of linear chains of individual units
that are attached to each other and that polymer
is the most active form of the enzyme.
Adding phosphates to individual units as
happens with the AMP activated protein kinase
converts that polymer into individual monomers.
And the individual monomers are significantly
less active than the polymer.
You can see that phosphates are taken off by
an enzyme known as protein phosphatase 2.
Now one of the things that can be happen
to the inactive carboxylase, however,
is it can be activated a little bit at least
by binding of an allosteric factor known as citrate.
Citrate is an intermediate
in the citric acid cycle
and citrate accumulates when
cells have a lot of energy.
So if cells have a lot of energy what they
do is they store that energy as something
and one of the things they stored
as is fatty acid. So this
citrate is indicating that
their energy level is high
that energy level being high favors
the synthesis of fatty acids.
Now this enzyme, as I said,
is located in the cytoplasm
and it is also feedback inhibited
by the end product of the pathway.
So this is catalyzing the first step in
the pathway that leads to fatty acids
and as long chain fatty acids start to accumulate
they will come back as allosteric
factors and inhibit the enzyme.
So the enzyme can be inhibited by phosphorylation
and binding of long chain fatty acids
and it can be stimulated by dephosphorylation
and the binding of citrate.
Polymerization, as I noted, favors activation
and depolymerization favors inactivation.
Now the fatty acid synthesis
proceeds with an intermediate
carrier called ACP, as I noted.
That carrier is added in this
reaction as you can see here
and you also see that there
is two starting molecules.
The two starting molecules are the
malonyl-CoA that we started with
that is converted into
malonyl-ACP. And an acetyl-CoA
that is converted into acetyl-ACP.
Now those two are the starting points
for the synthesis of the fatty acid.
The reaction proceeds, as you can see here,
by the joining of these two components.
And the joining of these two components
creates a molecule that has four carbons even though
we started with two components that had 5 carbons.
Well that means, of course, in the process that
one of those carbons had to get lost.
The enzyme catalyzing, as you
can see on the screen there,
and one of the ACPs gets lost as well.
So that carbon is added and then the carbon
is lost in the process so that's
seems inefficient process. But that's
the way cell have evolved to do this.
By the way all the reactions that
occur in fatty acid synthesis
also occur between carbons 2 and 3 or the
α and β as we have described before.
We see that the ketone on carbon
number 3 of this acyl-ACP group
is reduced in the next reaction
by 3-ketoacyl-ACP reductase.
That enzyme uses electrons from NADPH
creating an ADP and produces the
intermediate shown on the right.
Now the intermediate in the right is in the opposite
configuration to the same intermediate we saw in oxidation.
In oxidation, the hydroxyl on position 3 was in the
L form and you can see here it's in the D form.
In this reaction we can see that there is a dehydration
that happens, a removal of water to create a double bond.
This reaction which is catalyzed
by 3-ketoacyl-ACP dehydrase
creates a trans double bond an intermediate very
much like what we saw in fatty acid oxidation.
In the last step of this round of
the process of making fatty acids
the double bond is reduced
to a single bond by addition of
electrons that come from NADPH
catalyzed by the enzyme enoyl-ACP reductase.
Now at this point we have created a molecule that is
grown by two carbons from the first round of the cycle
and the continuing synthesis of the fatty acid
only requires repeating the cycle over and over,
until we get to a molecule that has 16 carbons.
At 16 carbons we have made a fatty acid called palmitic
acid and it's released from this complex that occurs.
The release of palmitic acid
from the palmitoyl-ACP is
procuded by the catalysis of the enzyme, thioesterase
which simply hydrolysis the bond between
ACP and the palmitic acid to create the
intermediate that you see on the right.
Now I have shown you a series of reactions that
goes through the cycle of adding two carbons
to a growing acyl-CoA. But what
I didn't tell you was that
all of those enzymatic properties
are contained within one giant enzyme.
And that giant enzyme is known
as fatty acid synthase.
Now fatty acid synthase is a big structure like
what you have seen on the screen in blue.
Now I am gonna step you through the reactions
to show you how they are performed
on individual parts of this bigger enzyme.
But each of the individual activity is part
of a big complex and this is true
for fatty acid synthesis in most cells from
bacteria all the way up to humans.
So if we look at the reactions that we have just gone
through, I have highlighted in green on the screen
the place in this fatty
acid synthase complex
where each activity is located.
You can see the green here
in the first reaction, the joining of 2 carbons occurring in
the second reaction and the location of the activity is moving.
Some have compared the movement of the catalysis
around the enzyme to the moving the hands of the clock.
Although as you will see the clock actually has to
run backwards in some cases as it has does here.
Last we get to the step where the palmitoyl ACP
is cleaved into palmitic acid and released.
And is because of the limitations of
this enzymes in terms of how big
of a fatty acid that can handle. That fatty acids
up to 16 carbons are only made here.