Fatty Acid synthesis (Chain desaturation)

Enzymes (fatty acyl CoA desaturases) also present in the SER are responsible for desaturating LCFA (that is, adding cis double bonds). The desaturation reactions require oxygen (O2), NADH, cytochrome b5, and its flavin adenine dinucleotide (FAD)-linked reductase. The fatty acid and the NADH get oxidized as the O2 gets reduced to H2O. The first double bond is typically inserted between carbons 9 and 10, producing primarily oleic acid, 18:1(9), and small amounts of palmitoleic acid, 16:1(9). A variety of
polyunsaturated fatty acids can be made through additional desaturation combined with elongation. Humans have carbon 9, 6, 5, and 4 desaturases but lack the ability to introduce double bonds from carbon 10 to the ω end of the chain. This is the basis for the nutritional essentiality of the polyunsaturated ω-6 linoleic acid and ω-3 linolenic acid.