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The synthesis of long-chain fatty acids from acetyl CoA and malonyl CoA by soluble extracts from E. coli has been studied by Vagelos and co-workers1' 2 and by Bloch and his group.3 These workers found that the responsible enzyme system was separable into a heat-stable and a heat-labile fraction, both of which were required for the synthesis of palmitic and cis-vaccenic acids. Recently, Majerus et al.4 and Wakil et al., independently, found that the activity of the heat-stable fraction resided in a protein with a molecular weight of about 9,000. During operation of the system, the heat-stable protein accepts the acyl groups from acetyl or malonyl CoA so as to form covalently linked acetyl and malonyl derivatives which have been found to be the immediate substrates for the fatty acid synthesizing enzymes. Thus, the heat-stable protein serves as a coenzyme rather than as an enzyme. The free form acts as an acyl acceptor while the acylated form can serve as acyl donor; all of the reactions in which the fatty acyl chain is elongated, reduced, and dehydrated appear to occur while the chain is in an acyl linkage to this protein. For this reason the heat-stable protein has been designated an acyl carrier protein, abbreviated herein as "ACP."Vagelos and his group4 as well as we5 have presented evidence which suggests that fatty acid biosynthesis proceeds according to the following sequential steps:CH3COSCoA + ACPSH = CH3COSACP + CoASH (1) HOOCCH2COSCoA + ACPSH = HOOCCH2COSACP + CoASH (2) CH3COSACP + HOOCCH2COSACP o CH3COCH2COSACP + ACPSH + CO2 (3) Downloaded by guest on July 5, 2020