SCIENCE & TECHNOLOGY X FILES The mammalian version of a fatty acid synthase is formed f r o m 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.
SUPERSIZE ENZYMES COME INTO FOCUS Architectures of fungal and mammalian fatty acid synthases are determined at 5-Â resolution
each of two different polypeptides. A central wheel-and-spoke structure divides the barrel into t w o reaction chambers. I n contrast, t h e m a m m a l i a n synthase has an X-shaped structure with two flexible "arms" and "legs" extending from its body. It also has t w o reaction chambers, found in t h e semicircular regions o n either side of the body. Although the two peptides are identical, the structure is asymmetric, with substantially different size openings for the t w o reaction chambers. T h e structures revealed some surprises. "The active sites are relatively far apart from each other, and t h e products of one reaction m u s t b e channeled very far in order to be used as a substrate for t h e next reaction," Ban says. Craig A. Townsend, a chemistry professor at J o h n s H o p k i n s University, p o i n t s out that the structures will force people t o change their view of how fatty acid synthesis is carried out. Because the enzyme cycles
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lular machines that produce fatty acids, are coming i n t o sharper focus. T h e best structural information t h a t has b e e n available o n these large, complex enzymes has been based o n low-resolution electron micrographs. N o w , a t e a m led b y structural biologist N e n a d Ban at t h e Swiss Federal Institute of Technology, Zurich, has solved X-ray crystal structures of mammalian and fungal fatty acid synthases at 5-Â resolution {Science 2 0 0 6 , 3 1 1 1 2 5 8 and 1263). F a t t y acid synthases are m u l t i d o m a i n proteins t h a t add t w o c a r b o n 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 intrigued b y 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 M a r c Leibundgut and grad student Simon J e n n i , "required overcoming significant technical challenges," Ban says. " M a m m a lian fatty acid synthase yielded only very small crystals, which, combined w i t h t h e size and complexity of the molecule, made d a t a collection almost impossible. T h i s crystal structure was solved w i t h crystals t h a t only several years ago I would have considered unusable." T h e fungal fatty acid synthase is a 2.6m e g a d a l t o n complex, m u c h larger t h a n most individual enzymes, and the mammalian enzyme is a complex of t w o identical 270-kilodalton polypeptide chains. B o t h enzymes have multiple active sites. T h e t w o p r o t e i n s have m a r k e d l y different architectures. T h e fungal synthase forms a barrel-shaped dodecamer with six
ENZYME PROBERS Ban (standing) and coworkers Leibundgut (sitting, f r o m left), Maier, and Jenni solved the structures of two fatty acid synthases. through its reactions without releasing an intermediate, t h e only way t h e synthesis is possible is because the arms are flexible enough t o bring the growing fatty acid t o the different active sites, he says.
These enzymes are potential targets for antiobesity, anticancer, and antimicrobial drugs. 34
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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 t h e modular polyketide synthases and the nonribosomal peptide synthetases, b o t h of which take a closely related assembly-line approach to synthesis. (In enzyme
Full dose of business
F A T B A R R E L The f u n g a l version of a fatty acid synthase is a b a r r e l - s h a p e d d o d e c a m e r w i t h six each of two different polypeptides, shown h e r e f r o m the side. A c e n t r a l w h e e l - a n d spoke s t r u c t u r e divides the b a r r e l into two reaction c h a m b e r s . The different colors indicate the domains w h e r e different reactions occur.
n o m e n c l a t u r e , 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 b o t h to understanding the mechanisms of these enzymes and to engineering experiments." THE CURRENT STRUCTURES are a good start, according to Salih W a k i l , a professor at Baylor College of Medicine w h o has studied fatty acid synthesis for more t h a n 50 years, b u t t h e y "are n o t at t h e 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 r u c t u r e s in t h e future. " W e are determined to solve the structures at high resolution and to characterize the enzymes with respect to their interaction w i t h reaction substrates and inhibitors," he says. •
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