Fatty acid desaturases (EC 1.14.19.-) are enzymes that catalyze the insertion of a double bond at the delta position of fatty acids.

There seems to be two distinct families of fatty acid desaturases which do not seem to be evolutionary related.

Family 1 is composed of:

• Stearoyl-CoA desaturase (SCD) (EC 1.14.19.1) [1]. SCD is a key regulatory enzyme of unsaturated fatty acid biosynthesis. SCD introduces a cis double bond at the delta(9) position of fatty acyl-CoA's such as palmitoleoyl- and oleoyl-CoA. SCD is a membrane-bound enzyme that is thought to function as a part of a multienzyme complex in the endoplasmic reticulum of vertebrates and fungi.

As a signature pattern for this family we selected a conserved region in the C-terminal part of these enzymes, this region is rich in histidine residues and in aromatic residues.

Family 2 is composed of:

• Plants stearoyl-acyl-carrier-protein desaturase (EC 1.14.19.2) [2], these enzymes catalyze the introduction of a double bond at the delta(9) position of steraoyl-ACP to produce oleoyl-ACP. This enzyme is responsible for the conversion of saturated fatty acids to unsaturated fatty acids in the synthesis of vegetable oils.
• Cyanobacteria desA [3] an enzyme that can introduce a second cis double bond at the delta(12) position of fatty acid bound to membranes glycerolipids. DesA is involved in chilling tolerance; the phase transition temperature of lipids of cellular membranes being dependent on the degree of unsaturation of fatty acids of the membrane lipids.

As a signature pattern for this family we selected a conserved region in the C-terminal part of these enzymes.