Evidence for the ionization steps in monoterpene cyclization reactions using 2-fluorogeranyl and 2-fluorolinalyl pyrophosphates as substrates

Abstract

Conversion of geranyl pyrophosphate to cyclic monoterpenes is considered to involve the preliminary isomerization of this acyclic precursor to enzyme-bound linalyl pyrophosphate and the cyclization of this tertiary intermediate. 2-Fluorogeranyl pyrophosphate and 2-fluorolinalyl pyrophosphate are effective competitive inhibitors of the cyclization of geranyl pyrophosphate by several different monoterpene cyclases, and the electron withdrawing α-fluorine substituent was shown to suppress the rate of cyclic product formation from both tritium-labeled analogs by at least two orders of magnitude. These results indicate that both steps of the coupled isomerization-cyclization sequence are initiated by ionization of an allylic pyrophosphate, and they confirm the electrophilic nature of this enzymatic reaction type and its similarity to the prenyltransferase reaction.