Introduction

The tens of thousands of sesquiterpenes known are derived metabolically from some 300 distinct C₁₅- hydrocarbon skeleta, which in turn are produced from the single substrate farnesyl diphosphate (FPP) by the action of sesquiterpene synthases. It is known that these enzymic cyclisations proceed by ionisation of the diphosphate group of FPP to form an enzyme-bound allylic cation, which then undergoes isomerisation, before cyclisation to yield new cations. Subsequent rearrangements and final quenching of the carbocations by elimination or by reaction with water yields the various C₁₅ hydrocarbon and alcohol products that are typical of a sesquiterpene synthase.

Recently, our ability to study the enzymology of these reactions has increased dramatically as a result of the cloning and functional expression in E. coli, of a number of cDNAs encoding sesquiterpene synthases of microbial and plant origin. In addition, crystal structures of three recombinant sesquiterpene synthases have given insight into the active site components involved in the binding of FPP and catalysis of its cyclisation.