Forum 09/25/2008

“Single-Particle Electron Microscopy Reveals Conformational Changes
of Metazoan Fatty Acid Synthase That Enable Catalysis”

Edward Brignole – Department of Cell Biology, The Scripps Research Institute, La Jolla, CA

Fatty acid synthase (FAS) is fundamental to cellular homeostasis, is a target for treatment of microbial infections, obesity, and certain cancers, and serves as a model for understanding homologous natural product synthases. In contrast to the FAS systems of prokaryotes and fungi, in metazoans all of the enzymes required for de novo fatty acid biosynthesis are covalently linked in a single polypeptide that dimerizes to form a giant (MW ~500 kDa) multifunctional enzyme. Recent crystallographic studies have revealed that the FAS active sites are arranged around two reaction chambers, however, the static X-ray structures cannot explain the transfer of pathway intermediates between distant catalytic domains. Using single-molecule electron microscopy (EM) we have identified a near continuum of FAS conformations that is influenced by the presence of substrates and altered by different catalytic mutations. A molecular interpretation of the 3D EM reconstructions of FAS was provided by docking high-resolution structures of individual domains. Dramatic swinging and swiveling motions give access to substrates within either reaction chamber. This functional redundancy in substrate loading and condensation may synchronize with rearrangements of the β-carbon processing domains, alternating acyl-chain reduction in one reaction chamber with acyl-chain elongation in the other. This series of structures illustrates how particular FAS conformations can be correlated with specific enzymatic activities and presents a unique portrait of a macromolecular machine in action.

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