Acyl-bound Intermediates in Fatty Acid Synthesis.

This over-all equation is thought to consist of the repetition of a series of reactions, each series being initiated by the condensation of malonyl coenzyme A with an unsubstituted straight chain acyl compound to form ,&ketoacyl-S-enzyme (5, 8). A series is then presumably concluded by reactions in which the P-ketoacyl compound is reduced and dehydrated to form an acyl compound 2 carbon atoms longer than the one initiating the series and ready to participate in a condensation with another molecule of malonyl coenzyme A. Evidence now available seems to eliminate acyl-CoA compounds as intermediates in these reactions. This is indicated by the finding that crotonyl-CoA and P-hydroxybutyryl-CoA are not reduced in a fatty acid-synthesizing system from pigeon liver (9) and by the observation that butyryl-CoA is not an intermediate in the synthesis of palmitate from acetyl-CoA and malonyl-CoA in an enzyme system from adipose tissue (10). Presumptive evidence exists which suggests that the 4 carbon intermediates in the further metabolism of the initial condensation product, acetoacetyl-s-enzyme, are enzyme-bound D( -)-phydroxybutyrate, crotonate, and butyrate (5,11,12). However, this concept is based on studies with model compounds and the enzyme-bound intermediates themselves, with the exception of enzyme-bound acetoacetate and butyrate in Escherichia coli (8,13), have not been isolated and characterized. The bacterial fatty acid-synthesizing system offers a unique advantage in the study of intermediates in the over-all reaction because the system can be fractionated into a number of protein fractions (8, 14, 15). Of considerable interest in the fractionated bacterial system has been Enzyme II, the heat-stable, acid-stable protein that binds the /3-ketoacyl intermediate formed from the condensation of acetyl-CoA and malonyl-CoA (8). This role of Enzyme II as an acyl-binding protein in fatty acid synthesis has now been extended by the demonstration that butyrate formed by the enzymatic reduction of acetoacetyl-SEnzyme II is also in acyl linkage with Enzyme II (13), presumably in the form butyryl-S-Enzyme II. With the isolation of these two enzyme-bound intermediates of fatty acid synthesis, the series of reactions that catalyzes a 2 carbon elongation in fatty acid synthesis has been separated into two parts, the condensation and the reduction. It is the purpose of this paper to present evidence for Scheme 1 in the enzymatic synthesis of long chain fatty acids. EXPERIMENTAL PROCEDURE