Elementary plant Biochemistry and Biotechnology (2+1)

Oxidation of monounsaturated fatty acids Oxidation of monounsaturated fatty acids follows many of the reactions of saturated fatty acids except the requirement of two additional enzymes, an isomerase and a novel reductase. Reactions of monounsaturated fatty acid are explained by considering the oxidation of a C-16 unsaturated fatty acid, palmitoleic acid, having a single double bond between C-9 and C-10.Palmitoleic acid is activated and transported across the inner mitochondrial membrane in the same way as saturated fatty acids. Palmitoleoyl CoA undergoes three cycles of degradation as in ß oxidation. But the cis Δ3 decenoyl CoA formed after the third cycle does not serve as a substrate for acyl CoA dehydrogenase. The presence of a double bond between C-3 and C-4 prevents the formation of another double bond between C-2 and C-3. An isomerase converts the cis double bond into a trans double bond and shifts the position of double bond between C-2 and C-3. The subsequent or follow up reactions are those of the ß-oxidation pathway in which the trans Δ2 decenoyl CoA is a regular substrate Oxidation of polyunsaturated fatty acids The oxidation of a polyunsaturated fatty acid, linoleic acid, with cis Δ9 and cis-Δ12 double bonds, is considered. The cis-Δ3 double bond formed after three rounds of ß -oxidation is converted into a trans double bond by the isomerase. This permits one more round of ß- oxidation. The acyl CoA produced by four rounds of ß-oxidation of linoleic acid contains a cis-Δ4 double bond, which undergoes dehydrogenation by acyl CoA dehydrogenase yielding trans Δ2, cis-Δ4 dienoyl intermediate. This intermediate is not a substrate for the next enzyme in the ß -oxidation pathway. This intermediate is converted into a trans Δ3 enoyl CoA to the trans Δ2 form, an intermediate generally found in ß-oxidation pathway and results in complete oxidation of the fatty acid.