Monoterpene cyclases: Use of the noncyclizable substrate analog 6,7-dihydrogeranyl pyrophosphate to uncouple the isomerization step of the coupled isomerization-cyclization reaction

Abstract

Enzymes from Salvia officinalis capable of catalyzing the isomerization and subsequent cyclization of geranyl pyrophosphate to the monoterpenes (+)-α-pinene and (+)-bornyl pyrophosphate were examined with the noncyclizable substrate analog 6,7-dihydrogeranyl pyrophosphate in an attempt to dissect the cryptic isomerization step from the normally coupled reaction sequence. The analog inhibited the cyclization of geranyl pyrophosphate and was itself catalytically active, affording acyclic terpene olefins and alcohols as products. The enzymatic products generated from 6,7-dihydrogeranyl pyrophosphate qualitatively resembled the solvolysis products of 6,7-dihydrolinalyl pyrophosphate, yet they constituted a far higher proportion of olefins, suggesting that enzymatic product formation occurs in an environment relatively inaccessible to water. Since the normal cyclization of geranyl pyrophosphate is considered to proceed via preliminary isomerization to the bound tertiary intermediate (3 R)-linalyl pyrophosphate, the results suggest that the analog undergoes the normal pyrophosphate ionization-migration step, giving rise in this case to (3 R)-6,7-dihydrolinalyl pyrophosphate which is reionized, and because the subsequent cyclizations are precluded, the resulting cation is either deprotonated or captured by water. In divalent metal ion requirement, pH optimum, and other characteristics, the enzymatic transformation of the analog resembles the normal monoterpene cyclase reaction.