On the Mechanism and Control of the Malonyl‐CoA‐Dependent Chain Elongation of Fatty Acids. The Malonyl‐Transfer Reaction

The enoyl CoA reductase activity of the purified microsomal chain elongation system of rat liver is inhibited non competitively by long chain acyl CoA and competitively by malonyl CoA. The multienzyme complex catalyzes the transfer of the malonyl residue from malonyl CoA to pantetheine and CoASH with high affinities for the physiological acceptor and donator CoASH (Km = 20 μM) and malonyl CoA (Km = 22 μM), respectively. The malonyl transfer is competitively inhibited by octanoyl CoA, 2,3 transoctenoyl CoA and 3 oxo octanoyl CoA. From the above data a common transferase catalyzing the exchange of the acyl moieties of malonyl enzyme and of the various enzyme bound intermediates of chain elongation with free coenzyme A is deduced. Former observations by other laboratories, suggesting a microsomal chain elongation at the level of the CoA derivatives are explained by a rapid exchange of enzyme bound intermediates of the chain elongation process with free CoASH.