Alpha-linoloyl-CoA is converted to Stearidonoyl-CoA (SDA, 6,9,12,15-18:3(n-3)) by delta-6-destaurase (fatty acid desaturase-2, FADS2). FADS2 uses the cytochrome b5 system to add a double bond at position 6. This is the rate limiting step in alpha-linolenic acid (ALA) metabolism (Horrobin 1993).
Tetracosapentaenoyl-CoA (9,12,15,18,21-24:5(n-3)) is further converted by delta 6-desaturase (FADS2) to tetracosahexaenoyl-CoA (6,9,12,15,18,21-24:6(n-3)).
The first major step in the metabolism of linoleic acid (LA) is desaturation of delta 9,12-octadecadienoyl CoA/linoleoyl CoA (LA-CoA) to delta 6,9,12-octodecatrienoyl CoA/gamma-lenoleoyl CoA (GLA-CoA). This step is catalyzed by delta-6-destaurase (fatty acid desaturase-2, FADS2) which introduces a cis-double bond between carbons 6 and 7. This is the rate limiting step in LA metabolism (Horrobin 1993).