Fatty Acid de novo Synthesis

Much of our need for fatty acids as constituents of phospholipids and other complex lipids is met by the diet. In addition, certain lipogenic tissues are capable of the de novo synthesis of fatty acids (Figure 6). These tissues include liver (hepatocytes), adipose tissue, and lactating mammary gland. Much of the fatty acids synthesized by all three tissues is incorporated into triacylglycerol. Hepatic synthesis is primarily for export to other tissues (in very low-density lipoproteins), while synthesis in adipocytes and mammary gland is for local storage.

The carbon used for fatty-acid synthesis typically derives from the products of glycolysis. The end product of glycolysis, pyruvate, enters the mitochondria and becomes the substrate for two separate

Figure 6 Fatty-acid biosynthesis. Cytoplasmic acetyl-CoA (AcCoA) is the primary substrate for de novo fatty-acid synthesis. This two-carbon compound most commonly derives from the glycolytic degradation of glucose, and its formation is dependent upon several reactions in the mitochondria. The mitochondrial enzyme pyruvate carboxylase is found primarily in tissues that can synthesize fatty acids. AcCoA is converted to malonyl-CoA (MalCoA) by acetyl-CoA carboxylase. Using AcCoA as a primer, the fatty-acid synthase multienzyme complex carries out a series of reactions that elongate the growing fatty acid by two carbon atoms. In this process MalCoA condenses with AcCoA, yielding an enzyme-bound four-carbon ,3-ketoacid that is reduced, dehydrated, and reduced again. The product is enzyme-bound 4:0. This process is repeated six more times, after which 16:0 is released from the complex. The reductive steps require NADPH, which is derived from enzyme reactions and pathways shown in grey. Enz refers to the fatty acid synthase multienzyme complex.

Glucose

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