Aerobic desaturation is the most widespread pathway for the synthesis of unsaturated fatty acids. It is utilized in all eukaryotes and some prokaryotes. This pathway utilizes desaturases to synthesize unsaturated fatty acids from full-length saturated fatty acid substrates. All desaturases require oxygen and ultimately consume NADH even though desaturation is an oxidative process. Desaturases are specific for the double bond they induce in the substrate. In Bacillus subtilis, the desaturase, Δ-Des, is specific for inducing a cis-double bond at the Δ position. Saccharomyces cerevisiae contains one desaturase, Ole1p, which induces the cis-double bond at Δ.
In mammals the aerobic desaturation is catalyzed by a complex of three membrane-bound enzymes (NADH-cytochrome b reductase, cytochrome b, and a desaturase). These enzymes allow molecular oxygen, , to interact with the saturated fatty acyl-CoA chain, forming a double bond and two molecules of water, . Two electrons come from NADH +  and two from the single bond in the fatty acid chain. These mammalian enzymes are, however, incapable of introducing double bonds at carbon atoms beyond C-9 in the fatty acid chain..) Hence mammals cannot synthesize linoleate or linolenate (which have double bonds at the C-12 (= Δ), or the C-12 and C-15 (= Δ and Δ) positions, respectively, as well as at the Δ position), nor the polyunsaturated, 20-carbon arachidonic acid that is derived from linoleate. These are all termed essential fatty acids, meaning that they are required by the organism, but can only be supplied via the diet. (Arachidonic acid is the precursor the prostaglandins which fulfill a wide variety of functions as local hormones.)