Sarpagan bridge enzyme has substrate-controlled cyclization and aromatization modes

Cyclization reactions that create complex polycyclic scaffolds are hallmarks of alkaloid biosynthetic pathways. We present the discovery of three homologous cytochromes P450 from three monoterpene indole alkaloid-producing plants (Rauwolfia serpentina, Gelsemium sempervirens and Catharanthus roseus) that provide entry into two distinct alkaloid classes, the sarpagans and the β-carbolines. Our results highlight how a common enzymatic mechanism, guided by related but structurally distinct substrates, leads to either cyclization or aromatization.