Fatty acid desaturase

A fatty acid desaturase is an enzyme that removes two hydrogen atoms from a fatty acid, creating a carbon/carbon double bond. These desaturases are classified as
 * delta - indicating that the double bond is created at a fixed position from the carboxyl group of a fatty acid (for example, Δ9desaturase creates a double bond at the 9th position from the carboxyl end).
 * omega (e.g. ω3desaturase) - indicating the double bond is created between the third and fourth carbon from the methyl end of the fatty acid.

In the biosynthesis of essential fatty acids, an elongase alternates with different desaturases (for example, Δ6desaturase) repeatedly inserting an ethyl group, then forming a double bond.

Role in human metabolism
Four desaturases occur in humans: Δ9desaturase, Δ6desaturase, Δ5 desaturase, and Δ4 desaturase.

Δ9desaturase, also known as Stearoyl-CoA desaturase-1, is used to synthesize oleic acid, a monounsaturated, ubiquitous component of all cells in the human body. Δ9desaturase produces oleic acid by desaturating stearic acid, a saturated fatty acid either synthesized in the body from palmitic acid or ingested directly.

Δ6 and Δ5 desaturases are required for the synthesis of highly unsaturated fatty acids such as eicosopentaenoic and docosahexaenoic acids (synthesized from a-linolenic acid), and arachidonic acid (synthesized from linoleic acid). This is a multi-stage process requiring successive actions by elongase and desaturase enzymes. The genes coding for Δ6 and Δ5 desaturase production have been located on human chromosome 11. DEGS1;    DEGS2;     FADS1;     FADS2;     FADS3;     FADS6;     SCD4; SCD5;
 * Human fatty acid desaturases:

Classification
Δ-desaturases are represented by two distinct families which do not seem to be evolutionary related.

Family 1 includes Stearoyl-CoA desaturase-1 (SCD).

Family 2 is composed of:
 * - Bacterial fatty acid desaturases.
 * - Plant stearoyl-acyl-carrier-protein desaturase, this enzyme catalyzes 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.
 * - Cyanobacterial DesA, 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.