Strictly Conserved Residues in Euphorbia tirucalli β-Amyrin Cyclase: Trp612 Stabilizes Transient Cation through Cation-π Interaction and CH-π Interaction of Tyr736 with Leu734 Confers Robust Local Protein Architecture.

The functions of Trp612, Leu734, and Tyr736 of Euphorbia tirucalli β-amyrin synthase were examined. The aliphatic variants (Ala, Val, Met) of Trp612 showed almost no activity, but the aromatic variants exhibited high activities: 12.5 % of the wild-type activity for the W612H variant, 43 % for W612F, and 63 % for W612Y. That is, the enzymatic activities of the variants increased in proportion to the increase in π-electron density. Thus, the major function of Trp612 is to stabilize transient cations through a cation-π interaction. The Phe and Tyr variants caused a distorted folding conformation, especially at the E-ring site, which generated the aberrantly cyclized products germanicol and lupeol. The L734G and L734A variants exhibited significantly decreased activities but yielded taraxerol in a high production ratio. The Val, Ile, and Met variants showed markedly high activities (56-78 % of wild-type activity); therefore, appropriate steric bulk is required at this position. The aliphatic variants of Tyr736 showed markedly decreased activities, but the Phe mutant exhibited high activity (67 %), which indicates that the π electrons are critical for catalysis. Homology modeling indicated that Tyr736 and Leu734 are perpendicular to the substrate and are situated face to face, which suggests that a CH-π interaction occurs between Tyr736 and Leu734, reinforcing the protein architecture, and that Tyr736 cannot stabilize cationic intermediates through a cation-π interaction.