Ordered polycyclization catalyzed by oxidosqualene synthases (OSCs) morph a common linear precursor into structurally complex and diverse triterpene scaffolds with varied bioactivities. We identified three OSCs from Iris tectorum. ItOSC2 is a rare multifunctional α-amyrin synthase. Sequence comparisons, site-directed mutagenesis, and multiscale simulations revealed that three spatially clustered residues, Y531/L256/L258, form an unusual Y-LL triad at the active site, replacing the highly conserved W-xY triad occurring in other amyrin synthases. The discovery of this active site architecture in ItOSC2 underscores the plasticity of terpene cyclase catalytic mechanisms and opens avenues for protein engineering toward custom designed OSCs.

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