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Fatty acid desaturases (also called unsaturases) are a family of enzymes that convert saturated fatty acids into unsaturated fatty acids and polyunsaturated fatty acids. For the common fatty acids of the C18 variety, desaturases convert stearic acid into oleic acid. Other desaturases convert oleic acid into linolenic acid, which is the precursor to alpha linolenic acid, gamma linolenic acid, and eicosatrienoic acid. Two subgroups of desaturases are recognized:
Delta indicating that the double bond is created at a fixed position from the carboxyl end of a fatty acid chain. For example, Δ9 desaturase creates a double bond between the ninth and tenth carbon atom from the carboxyl end. Omega indicating the double bond is created at a fixed position from the methyl end of a fatty acid chain. For instance, ω3 desaturase creates a double bond between the third and fourth carbon atom from the methyl end. In other words, it creates an omega 3 fatty acid. For example, Δ6 desaturation introduces a double bond between carbons 6 and 7 of linoleic acid (LA C18H32O2; 18:2 n6) and α linolenic acid (ALA: C18H30O2; 18:3 n3), creating γ linolenic acid (GLA: C18H30O2,18:3 n6) and stearidonic acid (SDA: C18H28O2; 18:4 n3) respectively. In the biosynthesis of essential fatty acids, an elongase alternates with various desaturases (for example, Δ6 desaturase) repeatedly inserts an ethyl group, then forms a double bond.

Desaturases have diiron active sites reminiscent of methane monooxygenase. These enzymes are O2 dependent, consistent with their function as either hydroxylation or oxidative dehydrogenation. Desaturases produce unsaturated fatty acids. Unsaturated fatty acids help maintain structure and function of membranes. Highly unsaturated fatty acids (HUFAs) are incorporated into phospholipids and participate in cell signaling. Unsaturated fatty acids and their derived fats increase the fluidity of membranes.

Acyl CoA dehydrogenases are enzymes that catalyze formation of a double bond between C2 (α) and C3 (β) of the acyl CoA thioester substrates. Flavin adenine dinucleotide (FAD) is a required co factor.