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complex, fatty-acid synthase. After binding of one molecule each of acetyl-CoA and malonyl-CoA to unique binding sites within the complex, a condensation reaction occurs in which carbon dioxide is released and an enzyme-bound 4-carbon 3-ketoacid is formed. Subsequent reactions include a reduction step, a dehydration step, and a second reduction step. The intermediates produced in these reactions are similar to those seen in f-oxidation (Figure 2), in reverse order. The product (enzyme bound) is the saturated fatty acid 4:0, which can then condense with another molecule of malonyl-CoA to start the process anew. After seven such cycles, the ultimate product is 16:0, which is released from the complex.

The reductive steps in fatty-acid synthesis require reduced nicotinamide adenine dinucleotide phosphate (NADPH). Some NADPH is produced during recycling of the oxaloacetate formed during the cytoplasmic hydrolysis of citrate, described above. Oxaloacetate is first converted to malate (via cyto-plasmic malate dehydrogenase). Malate is then dec-arboxylated to pyruvate in an NADP+-dependent reaction catalyzed by malic enzyme; NADPH is produced in this reaction. NADPH for fatty-acid biosynthesis also comes from reactions in the pentose phosphate pathway (hexose monophosphate shunt).

In several respects, the enzymatic reactions of fatty-acid synthesis are the converse of those in fatty-acid oxidation. However, there are key differences, which are summarized in Table 1.

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