The Specificity of Yeast Fatty‐Acid Synthetase with Respect to the “Priming” Substrate

Abstract

Yeast fatty acid synthetase catalyzes the synthesis of palmityl‐CoA and stearyl‐CoA from acetyl‐CoA and malonyl‐CoA. Acetyl‐CoA serves as “primer” of the reaction cycle. The specificity of the fatty acid synthetase with respect to the “priming” substrate was studied on the purified enzyme. In its function as primer acetyl‐CoA was replaced by homologous saturated acyl‐CoA derivatives (butyryl‐CoA, capronyl‐CoA, etc. ). In these experiments nonanoyl‐CoA and decanoyl‐CoA were good primers in low concentration range. This kind of fatty acid synthesis leads to normal end products. At higher concentrations of priming substrate an inhibitory effect became apparent with all long‐chain acyl‐CoA derivatives. With decanoyl‐CoA the degree of inhibition was dependent on the concentration of malonyl‐CoA. This was interpreted as competition between decanoyl‐CoA and malonyl‐CoA. The rate of synthesis with dl ‐3‐hydroxydecanoyl‐CoA and Δ 2 ‐decenoyl‐CoA as primer was 25–50 times slower than with decanoyl‐CoA. The rate‐limiting step was the transfer of the acyl residue from CoA to the enzyme. The saturated acids C 12 –C 18 were isolated as end products of this synthesis. Derivatives of decanoyl‐CoA with substitutions at positions more distant from the carboxyl group were not so strongly depressed in their priming activity, e.g. Δ 3 ‐decenoyl‐CoA or 4‐oxo‐decanoyl‐CoA. In these cases the functional groups were retained during chain elongation. As reaction products unsaturated acids or oxo acids respectively were isolated. The enzymatic reduction of the double bond (Δ 3 ‐decenoyl‐CoA) or the oxo group (4‐oxo‐decanoyl‐CoA) could be demonstrated only as a slow side reaction.