Fig. 22. Synthesis of palmitic acid

The overall synthesis of palmitic acid: The fatty acyl chain grows by two-carbon units donated by activated malonate, with loss of CO₂ at each step. The initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO₂ is shaded green. After each two-carbon addition, reductions convert the growing chain to a saturated fatty acid of four, then six, then eight carbons, and so on. The final product is palmitate (16:0).

Stages of Fatty acid synthesis

• •  Overall goal is to attach a two-carbon acetate unit from malonyl-CoA to a growing chain and then reduce it.

•  Reaction involves cycles of four enzyme-catalyzed steps

−  Condensation of the growing chain with activated acetate.

−  Reduction of carbonyl to hydroxyl.

−  Dehydration of alcohol to trans-alkene.

−  Reduction of alkene to alkane.

• •  The growing chain is initially attached to the enzyme via a thioester linkage

•  During condensation, the growing chain is transferred to the acyl carrier protein

•  After the second reduction step, the elongated chain is transferred back to fatty acid synthase