Figure 1. Biosynthesis of n-3 and n-6 polyunsaturated fatty acids

22:6n-3 and 22:5n-6 are synthesized from 18:3n-3 and 18:2n-6, respectively, by a series of elongation and desaturation reactions. All reactions occur in the endoplasmic reticulum (microsomes) unless otherwise stated (P, peroxisomes). An interesting feature is that the final step in 22:6n-3 and 22:5n-6 synthesis is the peroxisomal retroconversion (beta-oxidation) of their respective C-24 precursors.