Insight into a natural Diels-Alder reaction from the structure of macrophomate synthase

The Diels-Alder reaction, which forms a six-membered ring from an alkene (dienophile) and a 1,3-diene, is synthetically very useful for construction of cyclic products with high regio- and stereoselectivity under mild conditions(1). It has been applied to the synthesis of complex pharmaceutical and biologically active compounds(2). Although evidence(3-7) on natural Diels-Alderases has been accumulated in the biosynthesis of secondary metabolites(8), there has been no report on the structural details of the natural Diels-Alderases. The function and catalytic mechanism of the natural Diels-Alderase are of great interest owing to the diversity of molecular skeletons in natural Diels-Alder adducts(8). Here we present the 1.70 Angstrom resolution crystal structure of the natural Diels-Alderase, fungal macrophomate synthase (MPS)(3), in complex with pyruvate. The active site of the enzyme is large and hydrophobic, contributing amino acid residues that can hydrogen-bond to the substrate 2-pyrone. These data provide information on the catalytic mechanism of MPS, and suggest that the reaction proceeds via a large-scale structural reorganization of the product.