Modular polyketide synthases (PKS) make novel natural products through a series of preprogrammed chemical steps catalyzed by an assembly line of multidomain modules. Each assembly-line step involves unique extension and modification reactions, resulting in tremendous diversity of polyketide products. Dehydratase domains catalyze formation of an α,β-double bond in the nascent polyketide intermediate. We present crystal structures of the four dehydratase domains from the curacin A PKS. The catalytic residues and substrate binding site reside in a tunnel within a single monomer. The positions of the catalytic residues and shape of the substrate tunnel explain how chirality of the substrate hydroxyl group may determine the configuration of the product double bond. Access to the active site may require opening the substrate tunnel, forming an open trench. The arrangement of monomers within the dimer is consistent among PKS dehydratases and differs from that seen in the related mammalian fatty acid synthases.
Highlights
► Active site location at sharp bend within a tunnel explains how substrate stereochemistry can determine product conformation ► Access to active site is likely mediated by movement of specific structural elements, opening the substrate tunnel to accommodate bulky and/or inflexible substrates ► Re-annotation of Curacin A biosynthetic pathway suggests dehydration reactions may occur across module boundaries ► Orientation of dehydratase monomers in PKS modules differs from that seen for FAS modules but is nonetheless consistent with overall shared architecture of PKS and FAS modules.
