Catalytic mechanism and endo-to-exo selectivity reversion of an octalin-forming natural Diels–Alderase

We have previously reported the identification of CghA, a proposed Diels–Alderase responsible for the formation of the bicyclic octalin core of the fungal secondary metabolite Sch210972. Here we show the crystal structure of the CghA–product complex at a resolution of 2.0 A. Our result provides the second structural determination of eukaryotic Diels–Alderases and adds yet another fold to the family of proteins reported to catalyse [4 + 2] cycloaddition reactions. Site-directed mutagenesis-coupled kinetic characterization and computational analyses allowed us to identify key catalytic residues and propose a possible catalytic mechanism. Most interestingly, we were able to rationally engineer CghA such that the mutant was able to catalyse preferentially the formation of the energetically disfavoured exo adduct. This work expands our knowledge and understanding of the emerging and potentially widespread class of natural enzymes capable of catalysing stereoselective Diels–Alder reactions and paves the way towards developing enzymes potentially useful in various bio/synthetic applications. Enzymatic Diels–Alder reactions are of high synthetic interest, but mechanistic insights remain scarce. Now, a structure of the Diels–Alderase CghA in complex with its product is reported, a catalytic mechanism proposed and the enzyme is engineered to form the energetically disfavoured exo adduct.