SCIENCE & TECHNOLOGY

SUPERSIZE ENZYMES COME INTO FOCUS Architectures of fungal and mammalian fatty acid synthases are determined at 5-Â resolution CELIA HENRY ARNAUD, C&EN WASHINGTON

FATTY ACID SYNTHASES, T H E CEL-

lular machines that produce fatty acids, are coming in to sharper focus. The best structural informa­t ion tha t has been available on

these large, complex enzymes has been based on low-resolution electron micrographs. N o w , a t eam led by structural biologist Nenad Ban at the Swiss Federal Institute of Technology, Zurich, has solved X-ray crystal structures of mammalian and fungal fatty acid synthases at 5-Â resolution {Science 2006 ,3111258 and 1263).

Fat ty acid synthases are mul t idomain proteins tha t add two carbon atoms at a time to a fatty acid chain until it has 16 or 18 carbon atoms. Besides being curious about how these enzymes do their iterative work, scientists are intr igued by t h e m because they are potential targets for antiobesity, anticancer, and antimicrobial drugs.

T h e structures obtained by Ban's team, which includes postdocs T i m m Maier and Marc Leibundgut and grad student Simon Jenn i , "required overcoming significant technical challenges," Ban says. "Mamma­lian fatty acid synthase yielded only very small crystals, which, combined wi th the size and complexity of the molecule, made data collection almost impossible. T h i s crystal s tructure was solved wi th crystals tha t only several years ago I would have considered unusable."

T h e fungal fatty acid synthase is a 2.6-megadal ton complex, m u c h larger t h a n most individual enzymes, and the mamma­lian enzyme is a complex of two identical 270-kilodalton polypeptide chains. Both enzymes have multiple active sites.

T h e two prote ins have markedly dif­ferent architectures. T h e fungal synthase forms a barrel-shaped dodecamer with six

X FILES The mammalian version of a fatty acid synthase is formed from two identical polypeptides that assemble into an X-shaped structure, a central body with arms and legs. The different colors represent the domains where different steps in fatty acid synthesis occur.

each of two different polypeptides. A cen­tral wheel-and-spoke structure divides the barrel into two reaction chambers.

I n contrast, the mammal i an synthase has an X-shaped structure with two flexible "arms" and "legs" extending from its body. It also has two reaction chambers, found in the semicircular regions on either side of the body. Although the two peptides are identical, the structure is asymmetric, with substantially different size openings for the two reaction chambers.

T h e structures revealed some surprises. "The active sites are relatively far apart from each other, and the products of one reac­t ion must be channeled very far in order to be used as a substrate for the next reac­tion," Ban says.

Craig A. Townsend, a chemistry profes­sor at J o h n s H o p k i n s University, po in ts out that the structures will force people to change their view of how fatty acid synthe­sis is carried out. Because the enzyme cycles

ENZYME PROBERS Ban (standing) and coworkers Leibundgut (sitting, f rom left), Maier, and Jenni solved the structures of two fatty acid synthases.

through its reactions without releasing an intermediate, the only way the synthesis is possible is because the arms are flexible enough to bring the growing fatty acid to the different active sites, he says.

These enzymes are potential targets for antiobesity, anticancer, and antimicrobial drugs. 3 4 C & E N / M A R C H 1 3, 2 0 0 6 W W W . C E N - O N L I N E . O R G

As the first reported structures of very large so-called megasynthases, these struc­tures could be helpful in understanding oth­er enzymes that catalyze iterative syntheses, such as the modular polyketide synthases and the nonribosomal peptide synthetases, bo th of which take a closely related assem­bly-line approach to synthesis. (In enzyme

F A T B A R R E L The fungal version of a fatty acid synthase is a barre l -shaped dodecamer with six each of two different polypeptides, shown here f rom the side. A central whee l -and-spoke structure divides the bar re l into two reaction chambers. The different colors indicate the domains where different reactions occur.

nomencla ture , synthetases or synthases catalyze the synthesis of other molecules with or without the direct participation of a nucleoside triphosphate.) "Structural in­formation about the spatial organization of these multidomain systems has been sorely lacking," Townsend says. "These papers will doubtless provide a guide bo th to under­standing the mechanisms of these enzymes and to engineering experiments."

THE CURRENT STRUCTURES are a good start, according to Salih Waki l , a profes­sor at Baylor College of Medicine who has studied fatty acid synthesis for more than 50 years, but they "are no t at the resolu­tion that everybody is dreaming of." A lot of questions remain, he says. For example, key parts of the enzymes still can't be de­finitively located.

Ban hopes to obtain higher resolution s t ruc tures in the future. " W e are deter­mined to solve the structures at high resolu­tion and to characterize the enzymes with respect to their interaction wi th reaction substrates and inhibitors," he says. •

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