Chemistry Reference
In-Depth Information
9.1 BIOLOGIC BACKGROUND
Hundreds of alkaloid biosynthetic pathways have been studied by chemical strate-
gies, such as isotopic labeling experiments (2, 3). However, modern molecular
biology and genetic methodologies have facilitated the identification of alkaloid
biosynthetic enzymes. This chapter focuses on pathways for which a signifi-
cant amount of genetic and enzymatic information has been obtained. Although
alkaloid natural products are produced by insects, plants, fungi, and bacteria,
this chapter focuses on four major classes of plant alkaloids: the isoquinoline
alkaloids, the terpenoid indole alkaloids, the tropane alkaloids, and the purine
alkaloids.
In general, plant biosynthetic pathways are understood poorly when compared
with prokaryotic and fungal metabolic pathways. A major reason for this poor
understanding is that genes that express complete plant pathways typically are
not clustered together on the genome. Therefore, each plant enzyme often is iso-
lated individually and cloned independently. However, several enzymes involved
in plant alkaloid biosynthesis have been cloned successfully, and many more
enzymes have been purified from alkaloid-producing plants or cell lines (4-6).
Identification and study of the biosynthetic enzymes has a significant impact on
the understanding of the biochemistry of the pathway. Furthermore, genetic infor-
mation also can be used to understand the complicated localization patterns and
regulation of plant pathways. This chapter focuses on the biochemistry responsi-
ble for the construction of plant alkaloids and summarizes the biosynthetic genes
that have been identified to date. Some of these pathways have been the sub-
ject of metabolic engineering studies; the results of these studies are mentioned
here also. An excellent, more detailed review that covers the biochemistry and
genetics of plant alkaloid biosynthesis up until the late 1990s is available also (7).
9.2
ISOQUINOLINE ALKALOIDS
The isoquinoline alkaloids include the analgesics morphine and codeine as well
as the antibiotic berberine (Fig. 9.1a). Morphine and codeine are two of the most
important analgesics used in medicine, and plants remain the main commercial
source of the alkaloids (8). Development of plant cell cultures of
Eschscholzia
californica
,
Papaver somniferum
,and
Coptis japonica
has aided in the isola-
tion and cloning of many enzymes involved in the biosynthesis of isoquinoline
alkaloids (9).
9.2.1 Early Steps of Isoquinoline Biosynthesis
Isoquinoline biosynthesis begins with the substrates dopamine and
p
-
hydroxyphenylacetaldehyde (Fig. 9.1b). Dopamine is made from tyrosine by
hydroxylation and decarboxylation. Enzymes that catalyze the hydroxylation
and decarboxylation steps in either order exist in the plant, and the predominant
