Biosynthesis of Linalool
Linalool is the chief component of European basil. The biosynthesis of this compound proceeds by way of mevalonic acid, and starts with three molecules of acetic acid (more precisely, acetyl coenzyme A). With the aid of the enzyme mevalonate-5-diphosphate carboxylase, and elimination of CO2, isopentenyl pyrophosphate [IPP, (6)] is produced. (Pyrophosphoric acid (HO)2-P(O)-O-P(O)(OH)2 is today referred to as diphosphoric acid, but the old abbreviation PP is still in use.)
Part of the IPP, in the presence of IPP-isomerase, is rearranged to the isomeric dimethylallyl pyrophosphate [DMAPP, (7)].
Figure 5. Biosynthesis of linalool.
The enzyme IPP-isomerase establishes an equilibrium between the two isomers, where DMAPP is the thermodynamically more stable. Only DMAPP can produce, through elimination of a diphosphate anion, a resonance-stablilized carbocation (8). This, despite its resonance stabilization, is highly reactive, reacting in uncontrolled fashion with both water and numerous other substrates present in aqueous solution. For this reason, the cation is formed only in a deep pocket in the prenyltransferase enzyme, where it is protected from undesired contact with water or other bases. Shielded within this enzyme pocket, the cation attacks in electrophilic fashion the double bond of an IPP molecule. Although attack of the cation is predisposed within the enzyme complex to occur at the terminal carbon as a consequence of steric conditions, this mode of reaction is also consistent with the familiar Markownikov rule. Two of the basic units are joined by a C–C bond to produce a C10 molecule (geranyl pyrophosphate).
Another C5 cation is able to attack geranyl pyrophosphate electrophilically, leading to the carbon skeleton of a sesquiterpene, consisting of 15 carbon atoms. In the plant world, a great many fragrant substances can be generated in a few simple reaction steps starting with geranyl pyrophosphate, including geraniol (geranium), citronellol (rose), and geranial (lemon).
The basil plant isomerizes geranyl diphosphate first to linaldoyl pyrophosphate, which then undergoes hydrolysis to linalool.