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In humans there are four distinct desaturases, each with a different specificity:
9,
6,
5,
4
(that is, they act at the 9-, 6-, 5- or 4-carbons.)
A minimum chain length of 16-18 carbons is required.
The specificity of distance from the carboxyl carbon, along
with the need for a 16-18 carbon chain means that n-6 and
n-3 fatty acids are not synthesized in humans. Because
some of these fatty acids are metabolically essential,
they must be supplied in the diet.
Desaturation and
2-carbon elongation often alternate, e.g.,
conversion of dietary linoleic acid (18:2 9,12)
to arachidonic acid (20:4
5,8,11,14),
which is important because it is a precursor of eicosanoids.
18:2
9,12
--> 18:3
6,9,12
-->
20:3
8,11,14
--> 20:4
5,8,11,14
-->
eicosanoids