00:01
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.
00:16
So those have to be dealt with appropriately.
00:18
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.
00:25
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.
00:38
When that happens we create an intermediate
that looks here like this one that has
a double at positions 3 and 4.
00:45
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.
01:02
The cis 3,4 double bond is converted
to a trans 2,3 double bond.
01:05
Now that trans 2,3 double bond
is an intermediate normally in beta oxidation,
we can see that happening right here.
01:14
That bond is then metabolized and broken down and
two carbons are broken off just like
it has happened before in beta oxidation.
01:22
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.
01:42
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.
01:48
And that enzyme is shown here,
it's 2,4 Dienoyl-CoA reductase.
01:54
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.
02:04
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.
02:14
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.
02:24
At this point we have an intermediate
that has a trans bond in positions 2 and 3
and beta oxidation can continue as normal.
02:32
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.
02:45
The oxidations of these starts
not in the mitochondrion
but in another organelle called the peroxisome.
02:52
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.
03:05
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.
03:16
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.
03:28
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.
03:38
First there is an enzyme called
propionyl-CoA carboxylase
which combines with bicarbonate
to add a carboxyl group
to the propionyl-CoA creating methylmalonyl-CoA
that you can see right here.
03:52
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.
04:15
That enzyme that catalyzes this reaction is called
methylmalonyl-CoA mutase, mouth full of a name.
04:22
But this enzyme requires cobalt
and this enzyme requires vitamin B12.
04:28
So, it's one of the reasons that we have
to have B12 in our diet; because,
this reaction very much depends on it.
04:34
Succinyl-CoA is an intermediate in the citric
acid cycle and can be metabolized there.
04:42
Now each round of fatty acid oxidation
creates one molecule of FADH2
and one NADH, as we have seen.
04:47
Along with one acetyl-CoA and a fatty
acid is been shorten by two carbons.
04:52
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.