Enzymatic synthesis of fatty acids

Abstract

Available evidence indicates that there are two distinct systems for the synthesis of fatty acids. 1—The mitochondrial system which may involve some enzymes of the β-oxidation system plus TPNH-α-β-unsaturated acyl reductase and perhaps a new condensing enzyme. Both TPNH and DPNH are required for the synthesis. This system is for the elongation of fatty acids and is responsible for the formation of stearate from palmitate and arachidonate from lionoleate, etc. 2—The non-mitochondrial system which is located in the cytoplasm and catalyzes the conversion of acetyl CoA to palmitate in the presence of ATP, Mn 2+, HCO 3 − and TPNH. Acetyl CoA is condensed with HCO 3 − to form malonyl CoA, a reaction which is catalyzed by acetyl CoA carboxylase (a biotin-containing enzyme) in the presence of ATP and Mn 2+. Malonyl CoA condenses with acetyl CoA or propionyl CoA to yield a reduction by TPNH saturated fatty acids, CO 2 and CoA. Short-chain fatty acyl CoA's (C 4, C 6, C 8, etc.) did not accumulate suggesting they may not be intermediates (or in equilibrium with such intermediates) in the overall synthesis. Instead a compound (presumably not acetoacetate) was isolated from the condensation of malonyl CoA and acetyl CoA which could be converted to palmitate by the addition of TPNH to the reaction mixture.