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Functional analysis of (4S)-limonene synthase mutants reveals determinants of catalytic outcome in a model monoterpene synthase

Significance Terpene synthases catalyze complex, chain length-specific, electrophilic cyclization reactions that constitute the first committed step in the biosynthesis of structurally diverse terpenoids. (4S)-limonene synthase [(4S)-LS] has emerged as a model enzyme for enhancing our comprehension of the reaction cycle of monoterpene (C10) synthases. While the stereochemistry of the cyclization of geranyl diphosphate to (−)-(4S)-limonene has been the subject of several mechanistic studies, the structural basis for the stabilization of carbocation intermediates and the termination of the reaction sequence have remained enigmatic. We present extensive experimental evidence that the aromatic amino acids W324 and H579 play critical roles in the stabilization of intermediate carbocations. A possible function of these residues as the terminal catalytic base is also discussed. Crystal structural data for (4S)-limonene synthase [(4S)-LS] of spearmint (Mentha spicata L.) were used to infer which amino acid residues are in close proximity to the substrate and carbocation intermediates of the enzymatic reaction. Alanine-scanning mutagenesis of 48 amino acids combined with enzyme fidelity analysis [percentage of (−)-limonene produced] indicated which residues are most likely to constitute the active site. Mutation of residues W324 and H579 caused a significant drop in enzyme activity and formation of products (myrcene, linalool, and terpineol) characteristic of a premature termination of the reaction. A double mutant (W324A/H579A) had no detectable enzyme activity, indicating that either substrate binding or the terminating reaction was impaired. Exchanges to other aromatic residues (W324H, W324F, W324Y, H579F, H579Y, and H579W) resulted in enzyme catalysts with significantly reduced activity. Sequence comparisons across the angiosperm lineage provided evidence that W324 is a conserved residue, whereas the position equivalent to H579 is occupied by aromatic residues (H, F, or Y). These results are consistent with a critical role of W324 and H579 in the stabilization of carbocation intermediates. The potential of these residues to serve as the catalytic base facilitating the terminal deprotonation reaction is discussed.