Farnesol diphosphate

Fig. 1.8 Catalytic activities of the isoprenyl diphosphate synthases PalDSl, PaIDS4, PaIDS5, and PaIDS6 after heterologous expression in colL Products were measured by radio-gas chromatography (plotted in Bequcrcl, upper four panels) and identified by co-injection of non-radioactive terpenc standards, detected via a thermal conductivity detector (plotted as detector response, bottom panel). The main products after acid hydrolysis are listed G, geraniol F, farnesol and GG, geranylgeraniol. Bacteria containing only the expression vector without an isoprenyl diphosphate synthase sequence show ed no enzyme activity (top panel).

Geranyl diphosphate is an allylic diphosphate and, like dimethylallyl diphosphate, can react with isopentenyl diphosphate. A 15-carbon carbocation is formed, which on deprotonation gives farnesyl diphosphate. Hydrolysis of farnesyl diphosphate gives the sesquiterpene farnesol. 

The enzyme-catalyzed reactions that lead to geraniol and farnesol (as their diphosphate esters) are mechanistically related to the acid-catalyzed dimerization of alkenes discussed in Section 6.21. The reaction of an ally lie diphosphate or a carbocation with a source of 7T electrons is a recurring theme in terpene biosynthesis and is invoked to explain the origin of more complicated structural types. Consider, for example, the formation of cyclic monoterpenes. Neryl diphosphate, formed by an enzyme-catalyzed isomerization of the E double bond in geranyl diphosphate, has the proper geometry to form a six-membered ring via intramolecular attack of the double bond on the allylic diphosphate unit. 

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