synthesized in young seedlings (Kushima et al. 1980 ). CBCA is derived from
CBGA, and it was proposed that this biosynthetic reaction proceeds
non-stereospecifically because CBCA is characterized by little optical rotation
(Mechoulam 1970 ).
In order to reveal the precise mechanism of CBCA biosynthesis, Morimoto et al.
( 1997 ) extracted and purified CBCA synthase from young seedlings ofC. sativa
(strain CBDA). The structural characterization of enzymatically synthesized CBCA
demonstrated that CBCA is biosynthesized as a mixture of enantiomers in a ratio of
5:1, although their absolute configuration was not determined (Fig.8.8). The
stereospecificity of the CBCA synthase reaction is relatively lower than THCA or
CBDA synthase, as the latter two enzymes specifically synthesize (-)-THCA and (-
)-CBDA, respectively (Taura et al. 1995 , 1996 ). Thus, the reaction intermediate
might be released in part from the active site of CBCA synthase before completion
of the reaction.
The general properties of CBCA synthase were quite similar to those of THCA
synthase and CBDA synthase (Morimoto et al. 1998 ). For example, CBCA syn-
thase is a soluble oxidoreductase, which does not require metal ions, cofactors, or
coenzymes for the oxidocyclization of CBGA. Therefore, CBCA synthase might
also be a member of theflavoprotein oxidase group, as is the case with THCA
synthase and CBDA synthase. To confirm this possibility, the gene encoding this
synthase should be cloned and characterized.
Chromene motifs, probably derived from the oxidocyclization of prenyl groups,
often occur in various natural products (Beaudry et al. 2005 ). However, CBCA
synthase is thefirst chromene-forming plant oxidoreductase to be identified and
purified. Meanwhile, some related enzymes have been found in the plant kingdom.
For example, glyceollin synthase transforms the prenylated pterocarpan into both
the chromene and isopropenyldihydrofuran-containing glyceollins (Welle and
Grisebach 1988 ). Unlike CBCA synthase, glyceollin synthase is a membrane bound
P-450 type enzyme. In contrast, deguelin cyclase fromTephrosia vogeliiis a sol-
uble enzyme that catalyzes prenyl to chromene oxidocyclization in the absence of
any cofactors (Crombie et al. 1992 ). Thus, deguelin cyclase might be structurally
related to cannabinoid synthases.
Fig. 8.8 Cannabichromenic acid (CBCA) biosynthesis driven by CBCA synthase. The enzyme
produces both enantiomers in a molar ratio of 5:1, but which is the major product is unknown.
CBGAcannabigerolic acid
8 Cannabinoids: Biosynthesis and Biotechnological Applications 193