Determinants of Enantiospecificity in Limonene Synthases

Monoterpene synthases catalyze the first committed step in the biosynthesis of monoterpenes and are in part responsible for the enormous structural diversity among this class of metabolites. Here, we explore the structure-function relationships underlying the formation of limonene enantiomers in limonene synthases that bind geranyl diphosphate as a common substrate. On the basis of analyses that consider both crystal structure data and amino acid sequence divergence, we identified candidate active site residues with potential roles in catalyzing reactions that involve accommodating reaction intermediates of opposite enantiomeric series. We demonstrate that spearmint (-)-limonene synthase [which generates >99% (-)-limonene over (+)-limonene] can be converted into a mutant enzyme, by exchanging four residues (C321S, N345I, I453V, and M458V), which produces (+)-limonene with reversed enantiospecificity [80% (+)-limonene and 3% (-)-limonene; the remainder are mostly bicyclic monoterpenes]. This study provides the foundation for a more in-depth understanding of the formation of enantiomeric series of monoterpenes, which can have vastly different olfactory properties.