Now the oxidation of unsaturated fatty acids posses
a little bit of a problem for
a cell; because, if you recall
the unsaturated fatty acids
have cis-double bonds
whereas the beta oxidation, I have just described to
you, creates intermediates that have trans double bonds.
So those have to be dealt with appropriately.
I show on the screen the way in which this is
handled and it's handled interesting by
only two enzymes that it requires to do it.
So I show the fatty acids that
has 2 double bonds on the top
and the oxidation of that fatty acid proceeds
by normal beta oxidation until
the oxidation process gets
close to that cis-double bond.
When that happens we create an intermediate
that looks here like this one that has
a double at positions 3 and 4.
Now you remember in the beta oxidation
that the trans double bond
was at positions 2 and 3 whereas we
have a cis at positions 3 and 4
in the structure. Now the cis 3,4 double bond in this is
handled very easily by an enzyme
called Enoyl-CoA isomerase.
The cis 3,4 double bond is converted
to a trans 2,3 double bond.
Now that trans 2,3 double bond
is an intermediate normally in beta oxidation,
we can see that happening right here.
That bond is then metabolized and broken down and
two carbons are broken off just like
it has happened before in beta oxidation.
After that two carbon pieces being released
we start the process again
of oxidizing what remains
and we get to an intermediate
that has a trans 2,3 bond
next to a cis 4,5. Now this
is getting a little tacky
but the important thing is that
the cell can't handle those.
So rather than deal, we
tried to deal with those two
use another enzyme that comes into play
that's start to simplify that situation.
And that enzyme is shown here,
it's 2,4 Dienoyl-CoA reductase.
And it uses electrons, because, it
has to do a reduction at this point
in order to convert two double bonds
into one double bond and
that's what this enzyme is doing.
So it's converting the two
double bonds that you see here
into one double bond and the one
double bond is between carbons 3 and 4.
Well, the one double bond between
carbons 3 and 4 is converted into
a double bond between carbons 2 and 3
by the same enzyme we saw
before, Enoyl-CoA isomerase.
At this point we have an intermediate
that has a trans bond in positions 2 and 3
and beta oxidation can continue as normal.
Now long chain fatty acids are metabolized
slightly differently than shorter chain fatty acids and
by long chain, I typically mean fatty acids that are
20-22 carbons or greater.
The oxidations of these starts
not in the mitochondrion
but in another organelle called the peroxisome.
And fatty acids with odd numbers of carbons
which the cell occasionally encounters
are oxidized in a little bit different
process; because, most fatty acids have
even numbers of carbons and even
numbers of carbons work really well.
Because if you are chopping off two carbon
pieces every time when you get to the last
4 carbon pieces you split it
in half and you have two
units of 2 carbons each acetyl-CoA.
Now if you have an odd number of
carbons in your fatty acid chain
then that means when you get down to the end,
and instead of having a 4 carbon piece
you have a 3 carbon piece and that
3 carbon piece is called propionyl-CoA.
Propionyl-CoA has to be altered in some
way for the cell to be able to handle it
and it's handled in this way
by a certain unusual scheme.
First there is an enzyme called
which combines with bicarbonate
to add a carboxyl group
to the propionyl-CoA creating methylmalonyl-CoA
that you can see right here.
Now you will think move forward from there but the
cell doesn't move from there. It actually converts the
forms and it converts the forms
from the D to the L configuration
before going forwards. As it goes forwards
then it takes that carboxyl group
that it added to the end and rearranges it so that
it creates a new molecule called auccinyl-CoA.
That enzyme that catalyzes this reaction is called
methylmalonyl-CoA mutase, mouth full of a name.
But this enzyme requires cobalt
and this enzyme requires vitamin B12.
So, it's one of the reasons that we have
to have B12 in our diet; because,
this reaction very much depends on it.
Succinyl-CoA is an intermediate in the citric
acid cycle and can be metabolized there.
Now each round of fatty acid oxidation
creates one molecule of FADH2
and one NADH, as we have seen.
Along with one acetyl-CoA and a fatty
acid is been shorten by two carbons.
Each acetyl-CoA that's released in the matrix of
the mitochondrion and this is important; because,
this is where the citric acid cycle actually uses
those. So it doesn't have to go anywhere else.