Unsaturated Acids and Desaturase Enzymes

There are two types of desaturase the first are soluble enz mies, found only in plants, and located in the plastids. Their substrate is a fatty acid attached to acyl carrier protein (acyl ACP). The second type is integrally bound to the membrane of the endoplasmic reticulum. These enzymes are found in animals generally, including insects, as well as in plants and fungi. Their substrate is the fatty acid bound to coenzyme A (acyl Co A). 

Soluble enzymes are much easier to study, so more is known of the first type, but from many studies with a variety of spectroscopic. X-ray and molecular biological techniques, it seems the mechanism of reaction is the same in both types. Although the full story of the enzymes is not yet known, the description here summarizes our present knowledge of membrane-bound fatty acid desaturases found in insects. The description is of a A9-desaturase, the most common type, which converts stearic acid to oleic acid. The location of the double bond is measured from the carboxylate end of the molecule. Palmitic acid with the same enzyme gives palmitoleic acid. If an unnatural Cjy or C19 acid is supplied to the 

Two further enzymes, both bound in the same membrane, and two coenzymes are required to complete the cycle. Cytochrome Z j restores the desaturase iron to its reduced state, and the cytochrome in turn is reconverted to its reduced state by cytochrome Z j reductase which uses FAD as coenzyme. The FAD is converted to FADH2 (see Chapter 2) by NADPH. 

Small variations in the enzyme structure give other desaturases. 

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