Crystal Structures of Fsa2 and Phm7 Catalyzing [4 + 2] Cycloaddition Reactions with Reverse Stereoselectivities in Equisetin and Phomasetin Biosynthesis

[Image: see text] Fsa2 and Phm7 are a unique pair of pericyclases catalyzing [4 + 2] cycloaddition reactions with reverse stereoselectivities in the biosynthesis of equisetin and phomasetin, both of which are potent HIV-1 integrase inhibitors. We here solve the crystal structures of Fsa2 and Phm7, both of which possess unusual “two-β barrel” folds. Different residues are evident between the active sites of Fsa2 and Phm7, and modeling experiments provide key structural information determining the reverse stereoselectivities. These results provide a better understanding of how natural pericyclases control the catalytic stereoselectivities and benefit the protein engineering in future.