Studies on the Mechanism of Fatty Acid Synthesis

Acetyl coenzyme A-acyl carrier protein transacylase (acetyl transacylase) and malonyl coenzyme A-acyl carrier protein transacylase (malonyl transacylase) have been purified 90- and 255-fold, respectively, from Escherichia coli extracts and exhibited the following properties. Acetyl transacylase is heat-labile whereas malonyl transacylase is comparatively heat-stable, 80% of its activity surviving for 20 min at 80°. Both enzymes have pH optima in the region of 6.5. The reactions are readily reversible, and equilibrium constants are 2.09 for acetyl transacylase and 2.33 for malonyl transacylase. Inhibition of the enzymes by N-ethylmaleimide and iodoacetamide demonstrates that both are sulfhydryl enzymes. Acetyl transacylase is relatively specific for acetyl-CoA but can transacylate coenzyme A esters of propionic, butyric, and hexanoic acids at a rate of 23.4, 9.8, and 4.5% that of acetyl-CoA, respectively. Evidence is presented in support of a mechanism of action of acetyl transacylase involving the intermediary formation of an acetyl—S—enzyme which can then transfer the acetyl group to acyl carrier protein or coenzyme A.