Uncoupled activation and cyclization in catmint reductive terpenoid biosynthesis

Terpene synthases typically form complex molecular scaffolds by concerted activation and cyclization of linear starting materials in a single enzyme active site. Here we show that iridoid synthase, an atypical reductive terpene synthase, catalyzes the activation of its substrate 8-oxogeranial into a reactive enol intermediate, but does not catalyze the subsequent cyclization into nepetalactol. This discovery led us to identify a class of nepetalactol-related short-chain dehydrogenase enzymes (NEPS) from catmint (Nepeta mussinii) that capture this reactive intermediate and catalyze the stereoselective cyclisation into distinct nepetalactol stereoisomers. Subsequent oxidation of nepetalactols by NEPS1 provides nepetalactones, metabolites that are well known for both insect-repellent activity and euphoric effect in cats. Structural characterization of the NEPS3 cyclase reveals that it binds to NAD+ yet does not utilize it chemically for a non-oxidoreductive formal [4 + 2] cyclization. These discoveries will complement metabolic reconstructions of iridoid and monoterpene indole alkaloid biosynthesis

Structure and Function of Enzymes Involved in the Biosynthesis of Tropane Alkaloids

Tropane alkaloids are found in a scattered distribution among the angiosperm families including members within the Solanaceae, Erythroxylaceae, Convolvulaceae, and Brassicaceae. Recent studies regarding the origins of tropane production provide strong evidence for a polyphyletic origin, suggesting that novel enzymes from different gene families have been recruited during the course of flowering plant evolution. Tropane alkaloid biosynthesis is best documented on the molecular genetic and biochemical level from solanaceous species. Regardless of the system chosen, there are currently gaps in the knowledge of enzyme structure-function relationships and how they influence tropane alkaloid biosynthesis. Obtaining insights on structure-function relationships of tropane biosynthetic enzymes is critical to understanding regulation, turnover, and flux of metabolites through the pathway. In this review, we discuss the current state of knowledge regarding structure-function relationships of the known steps involved in tropane biosynthesis.This is a post-peer-review, pre-copyedit version of a chapter published as Kim, Neill, Benjamin Chavez, Charles Stewart, and John C. D’Auria. "Structure and Function of Enzymes Involved in the Biosynthesis of Tropane Alkaloids." In Tropane Alkaloids (Srivastava V., Mehrotra S., Mishra S., eds.) 2021: 21-50. The final authenticated version is available online at DOI: 10.1007/978-981-33-4535-5_2. Posted with permission.</p