When we think about other
fatty acid reactions,
fatty acids longer than 16 carbons
are made elsewhere in the cell.
They are made in the endoplasmic reticulum
and interestingly in the mitochondrion.
Unsaturated fatty acids are made by starting with the
saturated fatty acids and removing electrons from them
using an enzyme known asa desaturase,
and the desaturases are found in
the endoplasmic reticulum.
You can see here the numbering scheme
for fatty acids and on the top I have
indicated the omega numbering scheme that
is used to number carbons starting
at the right side of the methyl
group and moving inwards.
On the bottom, I have indicated the delta numbering scheme
which starts with the carboxyl and moves rightwards.
Now both systems are used
to number fatty acids,
and it's interesting to note
that in the delta numbering system
that the mammalian cells cannot
make an unsaturated fatty acid,
that is a chemical double bond
on any carbons past position 9.
Now for our practical purposes that
means that we as animals can make
oleic acid which has a double bond
at position 9. But we can't make linoleic
or linolenic acid which have
double bonds at positions 12 and 15.
So, linoleic acid is shown here
the linolenic acid is shown here,
and this limitation means that we must have
these unsaturated fatty acids in our diet,
linoleic and linolenic, because we can't synthesize
them. They are known as essential fatty acids.
Well, we have gone through now the synthesis
and the degradation of fatty acids
and with this we have seen that acetyl-CoA is produced
in one and used in the other to make them.
And as we will see in future lectures,
the acetyl-CoA is central
to all of lipid metabolism.