Chemistry Reference
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in yeast (131); however, efforts to express heterologously terpenoid indole alka-
loids currently are limited because the majority of the biosynthetic genes remain
uncloned.
Transcription factors that upregulate strictosidine synthase (132), as well as a
transcription factor that coordinately upregulates expression of several terpenoid
indole alkaloid biosynthetic genes, have been found (133). Several zinc finger
proteins that act as transcriptional repressors to tryptophan decarboxylase and
strictosidine synthase also have been identified (134). Manipulation of these tran-
scription factors may allow tight control of the regulation of terpenoid indole
alkaloid production. Interestingly, expression of a transcription factor from
Ara-
bidopsis thaliana
in
C. roseus
cell cultures results in an increase in alkaloid
production (135).
9.4 TROPANE ALKALOIDS
The tropane alkaloids hyoscyamine and scopolamine (Fig. 9.3a) function as
acetylcholine receptor antagonists and are used clinically as parasympatholyt-
ics. The illegal drug cocaine also is a tropane alkaloid. The tropane alkaloids
are biosynthesized primarily in plants of the family
Solonaceae
, which includes
Hyoscyamus
,
Duboisia
,
Atropa
,and
Scopolia
(136, 137). Nicotine, although per-
haps not apparent immediately from its structure, is related biosynthetically to
the tropane alkaloids (Fig. 9.3b).
Tropane alkaloid biosynthesis has been studied at the biochemical level, and
several enzymes from the biosynthetic pathway have been isolated and cloned,
although the pathway has not been elucidated completely at the genetic level
(Fig. 9.3b) (138). L-arginine is converted to the nonproteogenic amino acid
L-ornithine by the urease enzyme arginase. Ornithine decarboxylase then decar-
boxylates ornithine to yield the diamine putrescine. In
Hyoscyamus
,
Duboisia
,
and
Atropa
, putrescine serves as the common precursor for the tropane alkaloids.
Putrescine is N-methylated by a SAM-dependent methyl transferase that has
been cloned to yield N-methylputrescine (139, 140). Putrescine N-methyl trans-
ferase now has been cloned from a variety of plant species (141-143), and
site-directed mutagenesis and homology models have led to insights into the
structure function relationships of this enzyme (143). N-methylputrescine then
is oxidized by a diamine oxidase to form 4-methylaminobutanal, which then
spontaneously cyclizes to form the N-methyl-D-pyrrolinium ion (144, 145). This
enzyme, which has been cloned, seems to be a copper-dependent amine oxidase
(146, 147). Immunoprecipitation experiments suggest that this enzyme associates
with the enzyme S-adenosylhomocysteine hydrolase (148). The pyrrolinium ion
then is converted to the tropanone skeleton by as yet uncharacterized enzymes
(Fig. 9.3b). Although no enzymatic information is available, chemical labeling
studies have indicated that an acetate-derived moiety condenses with the pyri-
dollium ion; one possible mechanism is shown in Fig. 9.3b (136).
