An oxidosqualene cyclase from Arabidopsis thaliana makes arabidiol, a tricyclic triterpene reported with indeterminate side-chain stereochemistry. We established the full structure of arabidiol through chemical degradation, NOE experiments, and molecular modeling. By examining the mechanistic constraints that govern water addition in triterpene synthesis, we further show how the stereochemistry of hydroxylation can generally be deduced a priori, why deprotonation is more common than hydroxylation, and why cyclases that perform hydroxylation also generate olefinic byproducts.
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