In vertebrates, including fish, de novo synthesis of 20:5n−3 and
22:6n−3 from 18:3n−3 proceeds via a series of reactions catalysed
by fatty acyl desaturase (Fads) and elongation of very long chain fatty
acids (Elovl) enzymes (Fig. 1). The most commonly observed pathway
is as follows. In the first step, 18:3n−3 is desaturated to 18:4n−3 by
a Δ6 Fads. In the second step, 18:4n−3 is elongated to 20:4n−3 by
an Elovl5. This is followed by desaturation of 20:4n−3 to 20:5n−3
catalysed by a Δ5 Fads and elongation of 20:5n−3 to 22:5n−3
catalysed by the same Elovl5 as mentioned above. Subsequently,
22:5n−3 is elongated to 24:5n−3 by an Elovl2 and 24:5n−3 is
desaturated to 24:6n−3 by the same Δ6 Fads as mentioned above. Finally,
partial β-oxidation of 24:6n−3 to yield 22:6n−3 occurs in the
peroxisomes. It is important to note that there is competition between
different intermediates within this pathway for the same enzymes.
There is also competition from the corresponding omega-6 (n−6)
fatty acids that are abundant in certain vegetable oils such as linoleic
acid (18:2n−6) in soybean oil.