Studies on the Mechanism of Fatty Acid Synthesis

Salts activate the fatty acid-synthesizing system of Escherichia coli and several of its component enzymes. The 3hydroxyacyl coenzyme A dehydrogenase from pig heart, one of the enzymes of fatty acid oxidation, is also stimulated by salts. Although all cations examined produce this stimulation, the activation patterns caused by monovalent cations are clearly distinct from those of divalent cations. When acyl carrier protein (ACP) and CoA thioesters are used as substrates, the activities of the P-ketoacyl-ACP reductase and the enoyl-ACP reductase are stimulated by salts. However, there is no stimulation when N-acetylcysteamine thioesters serve as substrates. With the /3-ketoacyl-ACP reductase an increase of the salt concentration causes an increase in V,,, for both acetoacetyl-ACP and acetoacetylCoA and a decrease in K,,, only for acetoacetyl-ACP. The chromatographic behavior of ACP on Sephadex G-100 is influenced by salts. ACP interacts with MgS04 to form complexes which elute in earlier fractions from Sephadex G-100 than salt-free ACP, although the molecular weight of the ACP is unchanged. When acetoacetyl-ACP, complexed with MgS04, is used as substrate for the /3-ketoacyl-ACP reductase, an increase of the salt concentration increases the V,,, for the reaction but has no effect on the K,,, for the acetoacetyl-ACP-MgS04 complex. It is suggested that cations complex with the protein moiety of ACP substrates, thereby facilitating binding to the enzymes, and that cations increase reaction rates possibly by reducing the repulsion between the negatively charged groups of the substrates and their respective enzymes.