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VI. Modification of Dietary and Endogenous Fatty Acids

              

Desaturation: Specificity

In humans there are four distinct desaturases, each with a different specificity:

⁹, ⁶, ⁵, ⁴ (that is, they act at the 9-, 6-, 5- or 4-carbons.) A minimum chain length of 16-18 carbons is required.

The specificity of distance from the carboxyl carbon, along with the need for a 16-18 carbon chain means that n-6 and n-3 fatty acids are not synthesized in humans. Because some of these fatty acids are metabolically essential, they must be supplied in the diet. Desaturation and 2-carbon elongation often alternate, e.g., conversion of dietary linoleic acid (18:2 ^9,12) to arachidonic acid (20:4 ^5,8,11,14), which is important because it is a precursor of eicosanoids.

18:2 ^9,12 --> 18:3 ^6,9,12 --> 20:3 ^8,11,14 --> 20:4 ^5,8,11,14 --> eicosanoids