Algal Neurotoxin Biosynthesis Repurposes the Terpene Cyclase Structural Fold Into an N-prenyltransferase

Abstract

Prenylation is a common biological reaction in all domains of life whereupon prenyl diphosphate donors transfer prenyl groups onto small molecules as well as large proteins. The enzymes that catalyze these biotransformations are structurally distinct from ubiquitous terpene cyclases that instead assemble terpene molecules via intramolecular rearrangements. Herein we report the structure and molecular details of a new family of prenyltransferases from marine algae that repurposes the terpene cyclase structural fold for the N-prenylation of glutamic acid during the biosynthesis of the potent neurochemicals domoic acid and kainic acid. We solved the X-ray crystal structure of the prenyltransferase found in domoic acid biosynthesis, DabA, and show distinct active site binding modifications that remodel the canonical Mg2+-binding motif. We then applied our structural knowledge of DabA and a homologous enzyme from the kainic acid biosynthetic pathway, KabA, to alter their isoprene donor specificities (geranyl versus dimethylallyl diphosphate) by a single amino acid switch. While the diatom DabA and seaweed KabA enzymes share a common evolutionary lineage, they are distinct from all other terpene cyclases, suggesting a very distant ancestor. SignificanceDomoic acid is a neurotoxin produced by marine algae that readily bioaccumulates in shellfish and significantly impacts both human and animal life. The first committed step of the biosynthesis of domoic acid is the N-prenylation of L-glutamic acid by the enzyme DabA. By solving the crystal structure of DabA, we demonstrate that this enzyme has repurposed the common terpene cyclase fold to catalyze an extremely unusual reaction, N-prenylation of an unactivated primary amine. Application of these structural insights enabled rational engineering of two N-prenyltransferase enzymes to accept alternative prenyl donors. Ultimately, these results not only expand the scope of reactions catalyzed by a terpene cyclase family member, but will help inform future domoic acid environmental monitoring efforts.