The evolution and engineering of Asteraceae oxidosqualene cyclases

Su, Honghao (2025) The evolution and engineering of Asteraceae oxidosqualene cyclases. Doctoral thesis, University of East Anglia.

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Abstract

Plants produce a diverse range of secondary metabolites that, while not essential for normal growth and development, play vital roles in communication, defence, and reproduction. Triterpenoids are one of the largest and most structurally diverse classes of secondary metabolites of which many are of interest to medicine and industry. The first step of triterpenoid biosynthesis is the cyclisation of 2,3-oxidosqualene into triterpene scaffolds by oxidosqualene cyclases (OSCs). This thesis investigates the evolution, specificity, regulation and engineering of taraxasterol synthases (TXSSs), a class of OSCs involved in the biosynthesis of triterpenoids reported to have a range of bioactivities, including being responsible for the anti-inflammatory activity of Calendula officinalis (pot marigold). In Chapter 3, I provide evidence that TXSSs likely evolved via gene duplication and neofunctionalisation of a mixed amyrin synthase soon after the emergence of the Asteraceae (the daisy family). This work also revealed that TXSSs have been maintained across the Asteraceae, however, TXSSs found in different lineages produce varying ratios of two scaffolds: ψ-taraxasterol and taraxasterol. Evolutionary studies described in Chapter 4 revealed that the likely predominant product of ancestral TXSSs was ψ-taraxasterol and the change to preferential production of taraxasterol in one Asteraceae tribe likely conferred a selected advantage. I identify two residues in the active site likely to be under positive selection and, using structure guided studies and computational simulations, characterise residues important for product specificity. In Chapter 5, I describe the identification of an R2R3-MYB-family transcription factor that contributes to the floral-specific transcriptional regulation of TXSS in C. officinalis. Finally, with the goal of providing new routes for heterologous biosynthesis, I explore rational and deep learning guided protein design methods to engineer a membrane detached TXSS (Chapter 6). Together, this work provides insights into the evolution and function of OSCs as well as new opportunities for biosynthesis.

Item Type:	Thesis (Doctoral)
Faculty \ School:	Faculty of Science > School of Biological Sciences
Depositing User:	Chris White
Date Deposited:	26 Feb 2026 08:43
Last Modified:	26 Feb 2026 08:43
URI:	https://ueaeprints.uea.ac.uk/id/eprint/102071
DOI:

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