Mechanism of the Stereoselective Catalysis of Diels–Alderase PyrE3 Involved in Pyrroindomycin Biosynthesis
Bo Li, Xingyi Guan, Song Yang, Yike Zou, Wen Liu, K. N. Houk
Abstract
The biosynthesis of pyrroindomycins A and B features a complexity-building [4 + 2] cycloaddition cascade, which generates the spirotetramate core under the catalytic effects of monofunctional Diels–Alderases PyrE3 and PyrI4. We recently showed that the main functions of PyrI4 include acid catalysis and induced-fit/conformational selection. We now present quantum mechanical and molecular dynamics studies implicating a different mode of action by PyrE3, which prearranges an anionic polyene substrate into a high-energy reactive conformation at which an inverse-electron-demand Diels–Alder reaction can occur with a low barrier. Stereoselection is realized by strong binding interactions at the endo stereochemical relationship and a local steric constraint on the endo-1,3-diene unit. These findings, illustrating distinct mechanisms for PyrE3 and PyrI4, highlight how nature has evolved multiple ways to catalyze Diels–Alder reactions.