FAS complex, malonyl-CoA elongation, and NADPH consumption
Fatty acid synthesis occurs in the cytosol via the multifunctional Fatty Acid Synthase (FAS) complex in animals. Acetyl-CoA carboxylase (ACC) first carboxylates acetyl-CoA to malonyl-CoA — the committed, regulated step, stimulated by insulin and citrate. Each elongation cycle adds 2 carbons from malonyl-CoA via condensation, keto-reduction, dehydration, and enoyl-reduction, consuming 2 NADPH. After 7 cycles, palmitate (C16) is released by thioesterase. One palmitate requires 8 acetyl-CoA, 7 ATP (for ACC), and 14 NADPH, which is largely supplied by the pentose phosphate pathway.