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Abstract
Iridoids are plant-derived terpenoids with a rich array of bioactivities. The key step in iridoid skeleton formation is the reduction of 8-oxogeranial by certain members of the progesterone 5β-reductase/iridoid synthase (PRISE) family of short-chain alcohol dehydrogenases. Other members of the PRISE family have previously been implicated in the biosynthesis of the triterpenoid class of cardenolides, which requires the reduction of progesterone. Here, we explore the occurrence and activity of PRISE across major lineages of plants. We observed trace activities toward either 8-oxogeranial or progesterone in all PRISEs, including those from nonseed plants and green algae. Phylogenetic analysis, coupled with enzymatic assays, show that these activities appear to have become specialized in specific angiosperm lineages. This broad analysis of the PRISE family provides insight into how these enzymes evolved in plants and also suggests that iridoid synthase activity is an ancestral trait in all land plants, which might have contributed to the rise of iridoid metabolites.
Highlights
Iridoids constitute a noncanonical group of monoterpenoids with a distinctive cyclopentanopyran skeleton
Members of the progesterone 5β-reductase/iridoid synthase (PRISE) family have been reported in iridoidand cardenolide-free species such as Arabidopsis.[13−17] Earlier reports showed that both PRISEs from Catharanthus roseus, an iridoid-producing species, and PRISEs from the Brassicaceae, including those that do not produce iridoids and/or cardenolides, had different reductase activities toward 8oxogeranial and progesterone.[16,18,19]
Promiscuous enzymatic activities are likely starting points for specialization under different conditions, as demonstrated by several examples in enzyme evolution.[20−24] To determine whether such a process within the PRISE family played a role in iridoid metabolism evolution, we identified PRISE homologues from an encompassing range of plant lineages
Summary
Iridoids constitute a noncanonical group of monoterpenoids with a distinctive cyclopentanopyran skeleton. In the absence of additional enzymes that guide the stereoselective cyclization of this intermediate, it is converted nonenzymatically in aqueous solution to a combination of iriodial and nepetalactol stereoisomers (see Figure 1, as well as Figure S2 in the Supporting Information).[6−9] The specific short-chain dehydrogenase family to which ISY belongs is known for its progesterone 5β-reductase activity, the stereoselective reduction of progesterone to 5β-pregnan3,20-dione that occurs in the biosynthesis of cardenolide variety of triterpenoids.[10,11] This enzyme family, named PRISE (for progesterone 5β-reductase/iridoid synthase activity), appears to play a critical ecophysiological role, since both iridoids and cardenolides are major groups of signaling molecules and semiochemicals.[12] Intriguingly, members of the PRISE family have been reported in iridoidand cardenolide-free species such as Arabidopsis.[13−17] Earlier reports showed that both PRISEs from Catharanthus roseus, an iridoid-producing species, and PRISEs from the Brassicaceae, including those that do not produce iridoids and/or cardenolides, had different reductase activities toward 8oxogeranial and progesterone.[16,18,19] Promiscuous enzymatic activities are likely starting points for specialization under different conditions, as demonstrated by several examples in enzyme evolution.[20−24] To determine whether such a process within the PRISE family played a role in iridoid metabolism evolution, we identified PRISE homologues from an encompassing range of plant lineages. We found that PRISEs appear to be ubiquitous in plants, and the substrate specificities of these enzymes for either 8-oxogeranial and progesterone, as measured within a phylogenetic framework, may provide clues regarding how iridoid pathways evolved
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