Discovery of Semi-Pinacolases from the Epoxide Hydrolase Family during Efficient Assembly of a Fungal Polyketide

The semipinacol rearrangement (SPR) is highly useful in the asymmetric synthesis of complex compounds. In biological systems, only a few semi-pinacolases belonging to a few families have been identified to catalyze the SPR on alkaloids. Here, based on the biosynthesis of a fungal mycotoxin asteltoxin (1), two semi-pinacolases AstD/MrvD were identified from the epoxide hydrolase family to catalyze type III SPR on the polyketide backbone. They were proposed to catalyze efficient regio-selective hydrolysis on the bis-epoxide and 2,3-migration on the epoxide alcohol for the rearrangement. Based on the comprehensive mutations and chemical calculations, a critical Asp residue was identified as an acid for the coupled catalysis of selective epoxide collapse and subsequent SPR, while other critical residues facilitated efficient hydrolysis and protected carbocation for SPR. Thus, this study expanded the SPR biocatalyst family and provided an understanding of the catalytic mechanisms of these bifunctional semi-pinacolases.