yeast FAS is incredibly unique due to its structural complexity, which contains 48 functional centers for one α6β6 complex and can efficiently performs 6 fatty acid syntheses separately at one time. There are seven, total enzymatic reactions in fatty acid synthesis. These reactions include: activation, priming, four reactions in elongation, and termination. Five these reactions are performed in the beta subunit and two reactions are performed in the alpha subunit. The 3D protein structure of the enzyme can be found here:PDB. The crystal structure of yeast fatty acid synthase has also been derived, showing both alpha and beta subunits. Mechanism Activation The activation of yeast FAS occurs in the alpha subunit. The reaction is performed by the holo-(acyl-carrier-protein) synthase (ACPS) domain. ACPS attaches the 4′-phosphopantetheine prosthetic group of CoA to the acyl carrier protein (ACP) domain, which is found in the N terminus of the α subunit. ACP is the only “mobile” domain of the enzyme complex, in which it moves intermediate substrates along all of the catalytic centers the enzyme, most notably the alpha and beta subunits. Priming The next step is priming, or the initiation of fatty acid synthesis. Priming is performed in the β subunit, and is catalyzed by the acetyltransferase (AT, equivalent to bacterial (acyl-carrier-protein) S-acetyltransferase) domain, which initiates the process of fatty acid synthesis. Here, acetyltransferase transfers the acetate group from acetyl-CoA onto the SH group of the 4′-phosphopantetheine prosthetic group of ACP, which had been attached during activation. Elongation Elongation involves four main reactions: The acetyl unit on ACP is condensed with malonyl-ACP to form β-ketobutyryl-ACP Ketobutyryl-ACP is then reduced by ketoacyl-ACP reductase to afford β-hydroxyacyl-ACP β-hydroxyacyl-ACP is then dehydrated to produce enoyl-ACP Enoyl-ACP is then reduced by Enoyl-ACP reductase (ER) to form a saturated acyl-ACP, which can be elongated again in a new cycle of elongation Elongation itself occurs in mainly in the α subunit, though the entire process required for elongation is a coordinated system which involves the α and β subunits. ACP first delivers the acetate group, which had been attached during priming, to the ketoacyl synthase (KS) domain in the α subunit. ACP then moves back to the β subunit to the malonyl/palmitoyl-transacylase (MPT, equivalent to bacterial malonyl transacylase) domain and binds to a malonyl of malonyl-CoA, which will be used for elongation. The newly bound malonyl-ACP then swings back to the KS domain and transfers the malonate group for