Phylogenetic and Functional Analysis of Enzyme Families to Reveal Enzyme Evolution

Many studies, including genomic analysis, have supported the idea that plants devote significant energy into synthesizing secondary metabolites. Plant secondary metabolites are involved in specialized functions, such as plant defense; and they are much greater in number and structural diversity than primary metabolites. One major class of plant secondary metabolites is triterpene alcohols. These compounds are biosynthesized by cyclization of oxidosqualene, and this reaction is catalyzed by oxidosqualene cyclases (OSCs). Higher plant species encode a variety of OSCs, which catalyze various triterpene syntheses leading to diverse derivatives. Those different OSCs have evolved to produce different products while maintaining high substrate specificity. Because of that, an approach that combines genome mining and heterologous expression of the OSCs has led to the discovery of many new OSCs of different products.

The model plant Arabidopsis thaliana is the only plant with all of its OSCs characterized. Other plants with sequenced genomes present great opportunities to uncover new OSCs. While phylogenetic analysis of different OSCs from related plants can give insights into how enzymes evolve, our approach brings functional assessment of those enzymes into the analysis. Therefore, OSCs are a powerful system to investigate how enzymes evolve to give closely-related homologs that have diverse functions. Moreover, this study can give insights into the divergence of enzymatic capabilities of related plant species. Correspondingly, this study can be applied to explore the limitations of using model species in questions of secondary metabolism.