Fatty Acid Elongation

The primary product synthesized by the de novo pathway is 16:0. While 16:0 is an important fatty acid, there is a need to synthesize longer-chain acids. Enzymes for elongation of fatty-acids have been found in membranes of the endoplasmic reticulum and mitochondria. However, these pathways are less well-characterized than that of fatty-acid synthesis. In the endoplasmic reticulum, the reactions involved in fatty-acid elongation are very similar to those of cytoplasmic fatty-acid synthesis. The donor of the added carbon atoms is also malonyl-CoA, indicating that an active acetyl-CoA carboxylase is required for elongation. Whereas the primary reactions of fatty-acid synthesis are found within the fatty-acid synthase multienzyme complex, individual proteins catalyze the four elongation reactions (condensation, reduction, dehydration, and reduction). Like synthesis, elongation in the endoplasmic reticulum requires reducing equivalents in the form of NADPH.

Fatty-acid elongation in mitochondrial membranes is thought to be slightly different from the process in the endoplasmic reticulum. The primary difference is that the donor of elongation units is thought to be acetyl-CoA, not malonyl-CoA. The four elongation reactions are similar, but may require NADH rather than NADPH as source of reducing equivalents. Little is known about how fatty-acid elongation in either the mitochondria or the endoplasmic reticulum is regulated.

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