synthase.”The catalytic property of olivetol synthase was mysterious since olivetol
is not a prenyl acceptor in the prenyltransferase reaction.
Although olivetol synthase did not synthesize OLA, this enzyme preferred
hexanoyl-CoA as the starter substrate, and was highly expressed in
cannabinoid-producing tissues in plants. We assumed that olivetol synthase would
participate in OLA biosynthesis despite a lack of evidence. As described in the next
section, the mystery of OLA biosynthesis solved by the frontier transcriptome-
based study conducted by Page’s group (Gagne et al. 2012 ).
8.2.6 Olivetolic Acid Cyclase, the First Plant Polyketide
Cyclase
Olivetol synthase could possibly synthesize the tetraketide intermediate, but could
not fold properly to produce OLA. This implied that an accessory plant protein
would modulate or catalyze the cyclization leading to OLA. To assess this possi-
bility, Gagne et al. ( 2012 ) prepared a protein extract from the glandular trichomes,
and assayed polyketide producing activity. As a result, a clear OLA producing
activity was detected, suggesting that the accessory protein is actually present in
glandular trichomes. They next explored the trichome transcriptome for the can-
didate genes encoding the accessory protein, which met the following criteria: the
gene was prominently represented in the trichome EST dataset and encoded a
protein with polyketide cyclase-like sequence or structure. Three candidate genes
were selected for subsequent biochemical evaluation, in which one of the genes was
successfully determined to encode olivetolic acid cyclase (OAC), thefirst plant
polyketide cyclase to be identified (Gagne et al. 2012 ). OAC is a dimerica+b
(a)(b)Fig. 8.10 The reaction catalyzed by olivetol synthase (a), and olivetolic acid (OLA) biosynthesis
catalyzed by olivetolic acid cyclase (OAC) (b)
196 S. Sirikantaramas and F. Taura