Monoterpene Synthases of Loblolly Pine (Pinus taeda) Produce Pinene Isomers and Enantiomers

The turpentine fraction of conifer oleoresin is a complex mixture of monoterpene olefins and plays important roles in defense and in the mediation of chemical communication between conifer hosts and insect predators. The stereochemistry of the turpentine monoterpenes is critical in these interactions, influencing host recognition, toxicity, and potency of derived pheromones, and the stereochemical composition of these compounds lends insight into their biogenetic origin, with implications for the numbers and types of enzymes responsible and their corresponding genes. Analysis of the oleoresin from several tissues of loblolly pine (Pinus taeda) showed the derived turpentine to consist mainly of (+)-(3R:5R)-α-pinene and (−)-(3S:5S)-β-pinene. Cell-free extracts from xylem tissue yielded three monoterpene synthases which together account for the monoterpene isomer and enantiomer content of the turpentine of this tissue. The major products of these enzymes, produced from the universal precursor of monoterpenes, geranyl diphosphate, were shown to be (+)-α-pinene, (−)-α-pinene, and (−)-β-pinene, respectively. In most properties (molecular mass of ∼60 kDa, Km for geranyl diphosphate of 3 μM, requirement for monovalent and divalent cations), these enzymes resemble other monoterpene synthases from conifer species.