Submitted to: ChemBioChem
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/1/2008
Publication Date: 5/15/2008
Citation: Crawford, J.M., Vagstad, A.L., Ehrlich, K., Udwary, D.W., Townsend, C.A. 2008. Acyl-carrier protein - Phosphopantetheinyltransferase partnerships in fungal fatty acid synthases. ChemBioChem. 9(10):1559-1563. Interpretive Summary: Production of the agricultural toxins called aflatoxins in foods and feeds begins with the chemical synthesis of a fatty acid. This fatty acid is then incorporated into a larger molecule by the action of two enzymes. The first, a fatty acid synthase, makes the fatty acid and binds to the second enzyme, called a polyketide synthase. When the two enzymes bind to each other, the fatty acid is transferred to the polyketide. The transfer requires a special molecular “arm” to help carry the fatty acid to the polyketide synthase. This arm is made by two specialized proteins in the fungal cell. One is necessary for all processes that require such an arm, including aflatoxin biosynthesis. The second is part of the aflatoxin specialized fatty acid synthase. This only is able to transfer the fatty acid to the polyketide synthase and cannot carry out any other function. This knowledge is not only important in understanding the beginning steps of aflatoxin synthesis, but also may be a potential target for preventing aflatoxin biosynthesis. A second important finding in the paper is a correction of the previously reported sequence for one of the subunits of the fatty acid synthase.
Technical Abstract: The synthesis of fatty acids is an essential primary metabolic process for energy storage and cellular structural integrity. Assembly of saturated fatty acids is achieved by fatty acid synthases (FASs) that combine acetyl- and malonyl-CoAs by repetitive decarboxylative Claisen condensations with subsequent reduction and dehydration steps. In mammals, seven catalytic domains are encoded by a single gene giving rise to an A2-homodimeric protein. The acyl-carrier protein (ACP) tethers the growing fatty acid during the iterative catalytic cycle. Such attachment fosters high effective substrate concentrations during the synthesis. In fungi, however, eight catalytic domains are divided between two subunits, and an architecturally distinct A6B6 canonical complex releases the final product as a CoA ester rather than as a free-acid, as occurswith animal FASs. Several examples are known in fungi where dedicated FASs have evolved to interact with polyketide synthases (PKSs) in secondary metabolic pathways. For example, norsolorinic acid synthase (NorS) is comprised of a pair of fatty acid subunits, HexA and HexB, that synthesize a C6-fatty acid starter unit to prime the associated PKS, PksA, in the formation of the aflatoxin precursor, norsolorinic acid. (These subunits associate into an approximately 1.4 MDa species as determined by size exclusion chromatography, and they are thought to form an A2B2Y2 complex that is quite distinct from the FAS of primary fungal metabolism. Hexanoyl-CoA was not detected as a free intermediate in in vitro assays, suggesting, but not proving, that a direct transfer could take place between the FAS and PKS subunits. A starter unit: ACP transacylase (SAT) domain in the accompanying PKS was identified that exhibited C6- chain length specificity and catalyzed transfer to the PksA ACP to bridge fatty acid and polyketide synthesis. Such drastic differences in the protein organization of primary and secondary metabolic FASs reflect different evolutionary histories.