Biochemical and evolutionary studies of sesquiterpene lactone metabolism in the sunflower (Asteraceae) family
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2013-05-06
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Abstract
Plants have evolved the capacity to synthesize a myriad of specialized metabolites which enhance their fitness in specific living conditions. These compounds are also widely utilized for human purposes. Elucidating the enzymes in plant specialized metabolism has been one of the main forces driving plant biochemistry. The more intriguing question, however, is how these enzymes evolved to acquire their existent functions.
The Asteraceae, the largest flowering plant family, is well-known for its enormously diverse and lineage-characteristic contents of sesquiterpene lactones (STLs). Thousands of compounds in this subclass of specialized metabolites have been studied extensively for their structures and valuable bioactivities. However, the details of their metabolism are poorly understood. Studying STLs in the Asteraceae thus improves our knowledge of the biosynthesis of these compounds. Furthermore, the tight links between STLs and the Asteraceae family provide an excellent model to explore enzyme adaptive evolution.
My thesis aims to advance our understanding of STL metabolism by focusing on elucidating the enzyme that is responsible for the oxidation of sesquiterpene to sesquiterpene carboxylic acid in the general STL biosynthetic route of the Asteraceae. In lettuce, two cytochrome P450-dependent monooxygenase (P450) isoforms responsible for oxidizing the three consecutive oxidations of germacrene A to germacrene A carboxylic acid in the biosynthesis of costunolide were characterized. This was achieved using a combination of genomic and biochemical approaches, and the aid of a metabolically-engineered yeast system. Furthermore, this germacrene A oxidase (GAO) activity was demonstrated to be highly conserved throughout the Asteraceae, even in the phylogenetically basal subfamily Barnadesioideae, which split from the rest of the family at least 50 million years go.
Previous studies have characterized an Artemisia annua-specific sesquiterpene oxidase, amorphadiene oxidase (AMO), which is considered to have diverged from an ancestral GAO. The substrate specificity/promiscuity of AMO and GAOs towards each other’s natural substrates and seven other non-natural substrates was investigated to test the general hypothesis of enzyme evolution from ancestral promiscuity. The results from these combinatorial biochemistry studies and phylogenetic relations of AMO and GAOs provided deep insights into the evolution of these P450s in the context of the chemical diversity of the Asteraceae.
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Biological Sciences
Citation
Nguyen, T. D. (2013). Biochemical and evolutionary studies of sesquiterpene lactone metabolism in the sunflower (Asteraceae) family (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/25121