Extended Data Fig. 10: Proposed mechanisms of PdxI- and EpiI-catalysed reactions.

a, The catalytic cycle of PdxI-catalysed reaction is initiated by the deprotonation of the 4-hydroxy group by K337 followed by the syn-dehydration to 7 assisted by the extend water hydrogen bonding network mediated by H336. Subsequently, protonated K337 serves as the general acid catalyst and forms the strong hydrogen bonding with 4-carbonyl oxygen of 7 to set the stage for the periselective Alder-ene reaction. Note that the steric effect of T232 inhibits the formation of the O2-hetero-Diels–Alder product 11 to further control regioselectivity. The alkyl chain folds to a reactive conformation and readily undergoes an Alder-ene reaction. After this, the tautomerization is facilitated by K337 and possibly water mediated by H336 to form and release 8. Then, the next catalytic cycle initiates. b, The catalytic cycle of EpiI-catalysed reaction, in contrast to PdxI, is initiated by the deprotonation of the hydroxy group by an alternative general base, possibly water followed by the syn-dehydration to 7. Since the key lysine residue does not form hydrogen bonding with 4-carbonyl oxygen of 7 due to the bulkier side chain of M411 (corresponding to V413 in PdxI), the favoured hetero-Diels–Alder reaction takes place to form and release 9. As same as PdxI, the steric effect of T231 inhibits the formation of the O2-hetero-Diels–Alder product 11 to further control regioselectivity. Then, the next catalytic cycle initiates.