Mammalian fatty acid synthase (mFAS) is a homodimeric multienzyme complex and its polypeptide chain carries all the functional domains required for fatty acid synthesis. During the FAS reaction cycle, the growing fatty acid chain remains attached to the arm of acyl carrier protein (ACP), which moves it sequentially from domain to domain before being released by thioesterase (TE).

Maier et al presented the crystal form of mammalian FAS at 4.5 Å resolution. Purified crystals of porcine FAS, highly homologous to the human multienzyme system, were grown by vapor-diffusion method, and its structure determined by x-ray crystallography. Mammalian FAS is X-shaped with two lateral clefts or reaction chambers on either side, with each chamber folding into seven catalytic domains. The homology between prokaryotic and eukaryotic analogs of FAS was used to deduce the function of each domain from the crystallographic data. Based on their secondary-structure elements, five domains of the fatty acid elongation cycle were observed in the electron density map. These were sequentially as follows: β-ketoacyl synthase (KS), malonyl/acetyl-CoA transacylase (MAT), dehydratase (DH), β-enoyl reductase (ER) and lastly β-ketoacylreductase (KR). The ACP and TE interdomain linking regions could not be accurately resolved because of their inherent flexibility.At the center, there is a catalytically inactive “core” or “interdomain”, about 450 residues in length. The lower end of each cleft contains the condensing domains of MAT and KS, and the upper part contains the β-carbon processing KR, DH and ER domains. Such a conformation makes it difficult for ACP to reach the active sites on the opposite cleft; firstly, because of the long distances between active sites and secondly, blockage of the other cleft by KS and DH domains.

The left reaction chamber is considerably narrower than the right with a difference of about 15 Å in distance between KR and MAT active sites. The observed asymmetry at both sides of the clefts is caused by three hinge regions and suggests that only a single substrates can bind to the KS domain at any one time. Maier et al speculated that asymmetry induced opening and closing (clamping) of the reaction chambers around the hinges at opposite clefts, thus aiding mobility of ACP. Hence, a combination of inherent and linkage flexibility as well as asymmetrical motions is required to transfer ACP-attached reaction intermediates among the several deep-set active sites in each half of the mFAS complex.

The work of Maier et al enabled better understanding of the organization of five catalytic domains within the FAS complex. However, their crystal resolution was not discerning enough to trace interdomain connecting regions and the exact reaction mechanism followed by ACP. Future studies should focus on linkages/interactions of domains within FAS  and tracking the ACP arm from higher resolution crystals. The effect of protein conformational changes on substrate binding should also be examined.  This would clarify whether asymmetry is an innate property of the FAS enzyme that allows ACP mobility- and in turn substrate binding- or whether such an effect is only found in the specific conformation state shown in the paper.

Maier et al carried out a follow-up study in 2008, where they resolved mFAS at a higher resolution of 3.2 Å. This structure revealed additional alternating linkers and a possible substrate shuffling mechanism facilitated by the flexible tethering of ACP in the enzyme.

Attaining a complete knowledge of the crystal structure of mammalian FAS would lead to the realization of the atomic model of the enzyme. This would aid the development of drug inhibitors targeting FAS for prevention of obesity, obesity-related diseases such as diabetes, and even some forms of cancer. FAS is also a paradigm for megasynthases. For example, FAS shows a close evolutionary relationship to PKS for each catalytic domain despite low sequence conservation. Hence, the structural mode of action of megasynthase enzymes could be understood as well, from resolving mFAS.