The evolutionary history and modern diversity of triterpenoid cyclases.

Cyclic terpenoids are a class of lipid compounds containing immense structural and functional diversity, with many cyclic triterpenoids acting as regulators of the physical properties and spatial organization of lipid membranes. Cyclic terpenoids are also readily preserved as terpane fossils, such as steranes and hopanes, forming a rich record of the evolution of life on Earth. Formation of the multiple ring structure of all cyclic terpenoids is catalyzed by terpenoid cyclase enzymes, among which are whole clades of proteins - many from environmental metagenomes and uncultured organisms - whose substrates and products are completely unknown. We investigate the function of these divergent cyclases through biochemical assays, and the evolutionary processes that produced them by testing and applying a variety of evolutionary models. We find deep divergence between the diterpenoid cyclases and triterpenoid cyclases, with other clades branching between the two, rooting the triterpenoid cyclase subtree, between squalene-hopene cyclases and sterol cyclases. Through a simple test of evolutionary rate shifts, we find an elevated evolutionary rate in the enzyme active site on the squalene-hopene cyclase stem, potentially indicative of positive selection. Finally, by testing the activity of divergent cyclases for a variety of substrates, we find a group of early-branching sterol cyclases from bacteria that synthesize arborinols, two of which produce the molecular precursor to a Permian "orphan biomarker." Together, our data present an evolutionary framework for triterpenoid cyclases that can inform both the biochemical potential of these proteins and their products' occurrence in the geological record.