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ChemInform Abstract: Alkaloid Biosynthesis

Article in ChemInform · March 2010

DOI: 10.1002/chin.201012266

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Sarah E. O’Connor. In Comprehensive Natural Products II Chemistry and

Biology; Mander, L., Lui, H.-W., Eds.; Elsevier: Oxford, 2010;
volume 1, pp. 977–1007.
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1.25 Alkaloids
Sarah E. O’Connor, Massachusetts Institute of Technology, Cambridge, MA, USA
ª 2010 Elsevier Ltd. All rights reserved.

1.25.1 What Is an Alkaloid? 977

1.25.2 Classes of Alkaloids 977
1.25.3 Function and Diversity of Alkaloids 978
1.25.4 Strategies for Elucidating Alkaloid Biosynthesis 982
1.25.5 Benzylisoquinoline Alkaloid Biosynthesis 987
1.25.6 Monoterpene Indole Alkaloid Biosynthesis 992
1.25.7 Tropane Alkaloid Biosynthesis 999
1.25.8 Purine Alkaloid Biosynthesis 1001
1.25.9 Conclusions and Outlook 1001
References 1003

1.25.1 What Is an Alkaloid?

Alkaloids encompass an enormous class of approximately 12 000 natural products.1 The principal requirement
for classification as an alkaloid is the presence of a basic nitrogen atom at any position in the molecule, which
does not include nitrogen in an amide or peptide bond. As implied by this exceptionally broad definition, the
alkaloids form a group of structurally diverse and biogenically unrelated molecules. Most classes of natural
products are composed of similar chemical structures, in which the same starting materials are assembled in
related biosynthetic pathways. For example, all polyketides derive from acetate and propionate building blocks
that undergo a series of Claisen condensation reactions.2 However, no biochemical paradigm is centrally
applied throughout alkaloid biosynthesis. Instead, the biosynthetic pathways of alkaloids are as diverse as the
chemical structures found within this class of natural products.
Historical reasons account for this broad chemical definition of an alkaloid.3 Prior to the nineteenth century,
all compounds purified from plants – such as tartaric acid, oxalic acid, and tannins – exhibited acidic properties.
However, alkaline material called potash extracted from burnt wood was later found to contain basic
compounds of pharmacological interest. Meissner suggested in 1819 that these compounds be referred to as
alkaloids, meaning a plant-derived substance that displays alkaline properties.3
In 1806, Friedrich Wilhelm Serturner, for the first time, isolated a pure compound that exhibited the same
pharmacological sleep-inducing properties of the crude opium extract from which this compound was isolated.
The discovery of this alkaloid compound, subsequently named morphine, played a key role in the development
of what was to become the modern pharmaceutical industry, and the purification of many other pharmacolo-
gically important alkaloids such as strychnine, quinine, and caffeine rapidly followed suit (Figure 1).3 Notably,
accurate structural elucidation of the alkaloids was much more difficult. Many alkaloid structures remained
unknown until well into the twentieth century when X-ray spectroscopy became widely available, and after
organic chemistry had advanced to the point where these molecules could be produced synthetically.
Although plants were the first known source of alkaloid compounds, it is now known that fungi, bacteria, insects, and
animals also produce a wide array of alkaloids. Therefore, the definition of alkaloids has been expanded to state
‘‘alkaloids are nitrogen-containing organic substances of natural origin with a greater or lesser degree of basic character’’.3

1.25.2 Classes of Alkaloids

Alkaloids are most commonly constructed from amino acid starting materials, although some purine-derived
alkaloids are also known. The structural class of the alkaloid is typically defined by the substrate starting
material. For example, tyrosine is used for the production of tetrahydroisoquinoline alkaloids (Figure 2(a)).

977
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978 Alkaloids

Figure 1 Pharmacologically important alkaloids discovered early in the history of alkaloids. Morphine, strychnine, quinine,
and caffeine are shown.

Tryptophan is the starting material for indole-containing alkaloids, such as alkaloids containing a -carboline
moiety (Figure 2(b)). Additionally, the indole of tryptophan can be modified to form the quinoline alkaloids
(Figure 2(b)). Ornithine, a nonproteogenic amino acid derived from glutamate or arginine, is used to produce
the pyrrolizidine- and tropane-type alkaloids (Figure 2(c)). Nicotinic acid can be used in combination with
other amino acids to yield the nicotinic alkaloids (Figure 2(d)). Lysine, which contains one extra methylene
group compared to ornithine, produces the structurally analogous piperidine, quinolizidine, and indolizidine
alkaloids (Figure 2(e)). Known alkaloids derived from other amino acids are more rare. In addition to amino
acid building blocks, a number of other nitrogen-containing starting materials can serve as alkaloid precursors.
Anthranilic acid, a precursor to tryptophan, is used to produce quinazoline-, quinoline-, and acridine-type
alkaloids (Figure 2(f)). A number of purine-derived alkaloids have also been isolated (Figure 2(g)). Finally,
some alkaloids acquire the requisite basic nitrogen through transamination of an existing polyacetate or
terpenoid framework. These compounds are referred to as pseudoalkaloids.

1.25.3 Function and Diversity of Alkaloids

The function of alkaloids is still not entirely clear. Although the pharmacological uses of many alkaloids are
well defined, the specific roles that these compounds play in the producing organism are not well elucidated in
most cases. At one point it was speculated that alkaloids were simply waste products derived from the
degradation of primary metabolites. However, although the exact roles of many alkaloids remain poorly
understood, these compounds are now believed to play an important ecological role, enabling the producing
organism to defend itself and interact with its environment. In fact, although natural products are often termed
secondary metabolites, the vital role that many natural products play in signaling and development renders the
term ‘secondary’ a misnomer.
Most obviously, many alkaloids are toxic, so biosynthesis of these compounds provides a general defensive
mechanism for the producing organism. For example, caffeine has been shown to act as an insecticide. Caffeine
may act as a natural insecticide in plants. When the three N-methyltransferase genes involved in caffeine
biosynthesis were overexpressed in tobacco, the resulting increase in caffeine production improved the
tolerance of the plants to certain pests.4 Occasionally, alkaloids that are present in animals are acquired by
predation on a plant alkaloid producer. In one example, a moth species Tyria jacobaeae feeds on the plant Senecio
jacobaea, which produces a number of pyrrolizidine alkaloids. The moth detoxifies the molecule by oxidizing the
nitrogen to an N-oxide using a flavin-dependent monooxygenase (Figure 3). If the moth is ingested by a
predator, the N-oxide is reduced in the gut of the predator where it is degraded to highly toxic pyrrole moieties.
The regulation of natural products also suggests a defensive function for these compounds. Many natural
products, including alkaloids, are upregulated under stressful conditions. For example, sterilized preparations
of fungal cell wall extracts, called fungal elicitors, can be added to plant cell culture to increase the levels of
alkaloid production in many plant species.
Nevertheless, it remains unclear why such a diversity of vastly different defensive, toxic molecules is
required. This diversity is particularly apparent in the tetrahydroisoquinoline and monoterpene indole classes
of alkaloids, where thousands of alkaloid products are generated from a single, central biosynthetic
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Figure 2 (Continued)
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Figure 2 (Continued)
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Figure 2 Representative members of the major structural classes of alkaloids. (a) Tyrosine-derived tetrahydroisoquinoline alkaloids. (b) Tryptophan-derived monoterpene
indole alkaloids containing a -carboline or quinoline moiety. (c) Ornithine-derived pyrrolizidine- and tropane-type alkaloids. (d) Nicotinic acid-derived alkaloid. (e) Lysine-derived
piperdine, quinolizidine, and indolizidine alkaloids. (f) Anthranilic acid-derived quinazoline-, quinoline-, and acridine-type alkaloids. (g) Purine-derived alkaloid.
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982 Alkaloids

Figure 3 Oxidation of pyrrolizidine alkaloid for detoxification.

intermediate. A single medicinal plant known to produce these alkaloids typically generate 100 or more
members of the alkaloid family. A recent commentary highlights that ‘primary metabolic pathways are
target-oriented’, in other words, designed to produce a single, highly optimized molecule.5 Many secondary
or natural product pathways are, in contrast, ‘diversity oriented’. Firn and Jones6 hypothesized that since potent
biological activity is a rare property for any molecule to have, ‘‘an organism needs the ability to make multiple
molecules in order to hit upon the rare potent ones’’. In many species, natural products serve as an immune
system of sorts. If the producing organism continuously evolves its capacity to generate new structures, the
organism may be advantageously positioned for survival in its environment.

1.25.4 Strategies for Elucidating Alkaloid Biosynthesis

First and foremost, a mechanistic elucidation begins with the accurate structural characterization of the alkaloid
product. Natural product structure elucidation posed a formidable challenge in the early days of alkaloid
isolation. A variety of strategies have made this process much easier, namely, X-ray analysis and high-field 2D
nuclear magnetic resonance (NMR) techniques.7 Additionally, total synthesis of a reported complex alkaloid
structure is a critical strategy for confirming the structural features of the natural product. A mechanistic
elucidation of a natural product biosynthetic pathway typically begins by establishing which precursors are
involved. Isotopically labeled (potential) precursors are fed to the producing organism, and the desired alkaloid
product is then isolated. If the alkaloid contains the isotopic label, then it is clear that the precursor was used in
the biosynthesis. Older experiments relied on radiolabeled precursors, since detection of radioactivity in the
final product is sensitive and straightforward. However, the routine use of powerful, high-resolution mass
spectrometry techniques has made the detection of safe, inexpensive stable isotopes such as hydrogen-2
(deuterium), carbon-13, and nitrogen-15 much more practical. After establishing the identity of the correct
starting materials, strategic positioning of isotopic labels can be used to probe the mechanism of the transfor-
mations and structural rearrangements that occur along the biosynthetic route. These experiments, when
placed in the context of previously known biochemical transformations, enable a series of logical enzymatic
reactions to be proposed.
Identification of the enzymes, and the corresponding genes that encode them, constitutes the next level of
pathway elucidation. How these genes are identified, or cloned, is dependent on the identity of the producing
organism. Historically, alkaloid research has focused on plant-derived compounds. Plants are immobile and
interact with their environment largely via the release of complex small molecules, so it is not surprising that
this evolutionary pressure has resulted in the production of an extremely diverse array of natural products by
plants. However, elucidating the genes of a plant pathway poses significant challenges. In contrast to microbes,
the genes of plant pathways – with a few exceptions8,9 – are not clustered on the genome, so each gene of a plant
pathway must be discovered individually. Additionally, the genome sizes of medicinal plants are much larger
(>1000 Mbp) than the typical natural product-producing bacteria (8 Mbp), which makes finding and screen-
ing putative biosynthetic genes difficult. Although spectacular successes have been achieved in elucidating
plant pathways, the challenges of plant biology have hindered the study of plant secondary metabolism and
most plant-derived pathways remain incompletely elucidated at the genetic level. However, complex, higher
plants frequently produce compounds that are not found in any known bacteria or fungi.
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Alkaloids 983

The majority of plant biosynthetic enzymes have been identified using a classical approach in which the
enzyme of interest is purified from a crude plant lysate by standard protein chromatography.10 Once a homo-
genous preparation of enzyme is prepared, partial protein sequence is obtained from the purified protein, which is
then used to identify the gene encoding the desired enzyme. In another approach, if a guess can be made as to the
type of enzyme involved in the desired transformation, a homology-based cloning strategy can be used.11
Enzymes within a given class contain highly conserved regions in the protein sequence. Oligonucleotide primers
complementary to these consensus sequences can be used to amplify gene candidates that can be screened for
function. In a third approach called subtractive hybridization, cDNA from two types of tissue can be ‘subtracted’
from one another.12 The genes in tissue that produces natural products at high levels can be compared with gene
expression levels in tissue that produce low levels of alkaloid in question. The genes unique to each tissue type are
readily obtained, and presumably, at least some of the genes that are unique to the alkaloid-producing tissue are
involved in alkaloid biosynthesis. Finally, suppression (by RNAi)13 or activation (by T-DNA tagging)14 of large
numbers of genes, followed by screening for changes in the levels of alkaloid production, can also be used to
identify biosynthetic enzymes, provided that a fast screen or selection is available to interpret the phenotype of the
transformed lines. Many plant alkaloid enzymes have been elucidated, but the plant alkaloids that have been
studied in the most detail at the genetic level are the isoquinoline alkaloids,1,15 the terpenoid indole alkaloids,16
the tropane alkaloids,17,18 and the purine alkaloids19 (Figure 4).
Bacteria, whether terrestial or marine in habitat, typically export an arsenal of natural products involved in
defense and signaling. Bacteria have much smaller genomes than plants, and the genes of bacterial natural
product pathways tend to be organized in clusters, which makes identification of an entire metabolic pathway
much more straightforward than in plants. Historically, bacteria have not been a rich source of alkaloids,
although several complex alkaloid pathways have been recently discovered.20,21 Undoubtedly, many more
alkaloids will be discovered as the technical and financial barriers to sequence whole bacterial genomes
continue to decrease, and more bacterial species are readily available for genetic analysis.
The study of microbe-derived natural products underwent a revolution in the 1980s as improved sequencing
technologies allowed the rapid discovery of the genes that encode natural product biosynthetic pathways.22
Since the genes of bacterial-derived pathways are relatively easy to identify, elucidation of these pathways is
less dependent on isotopic labeling studies. Bioinformatic analysis of the genes in the cluster can provide clues
as to the types of enzymatic transformations that take place. Although traditionally, the desired gene cluster is
targeted and isolated in a cosmid vector for sequence analysis, it can now be cost-effective to simply sequence
the entire genome of the producing strain and use bioinformatic analysis to search for candidate gene clusters in
the genome sequence. After identification of the cluster, heterologous expression of candidate enzymes
followed by in vitro biochemical assay of various combinations of these enzymes and substrates provide
important information. Finally, if the producing organism can be genetically manipulated, targeted gene
deletions can be made to definitively validate that the cluster in question is responsible for the production of
the compound. Most notably, the prodiginines23 and the indolocarbazoles24–26 such as rebeccamycin, staur-
osporin, and violecin are prokaryotic alkaloids that have been the subject of several recent investigations.
Benzodiazapines27 and saframycins28–30 have also been the subject of recent study (Figure 5).
Fungi, like bacteria, also produce a wealth of natural products. Eukaryotic fungal organisms generally have
larger genomes and more complex life cycles, so, while fungi are still much simpler than higher plants they are
considerably more complex than bacteria. It appears that the genes of many metabolic pathways are also clustered
in fungal genomes, thereby greatly simplifying the study of fungal biosynthetic pathways. Nevertheless, fungal
clusters are typically larger than prokaryotic clusters, and the genes often contain short introns. Several fungal
alkaloids have also been partially elucidated at the genetic level. The ergot alkaloids31 and the indole diterpenes32
are two major classes of fungal alkaloids that have been studied extensively (Figure 6).
In addition to plants, bacteria, and fungi, many other organisms produce extraordinary alkaloid structures.
Sponges as well as other multicelled marine organisms, insects, amphibians, and even mammals all produce
complex alkaloid natural products.33,34 However, in general, the biosynthetic pathways from these complex
organisms are much less well characterized than the pathways from microbial and plant kingdoms. In many of
these cases, the producing organisms are not viable in a laboratory environment, which complicates the
elucidation of the biosynthetic mechanism. Additionally, genetic manipulation of many of these organisms is
not yet developed.
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Figure 4 Representative plant-derived alkaloids from the tetrahydroisoquinoline, monoterpene indole, tropane, and purine classes.
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Figure 5 Representative prokaryote-derived alkaloids from the prodiginine, indolocarbazole, saframicin, and benzodiazapine classes.
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Figure 6 Representative fungal-derived alkaloids of the ergot and indole diterpene types.
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Alkaloids 987

A comprehensive discussion of all known alkaloid biosynthetic mechanisms extends well beyond the scope
of this chapter. Here, we focus on the small number of plant-derived alkaloids for which genetic information
regarding the biosynthetic pathway has been elucidated. Even within this limited subset of alkaloid structures,
the structural diversity that is observed among the alkaloids becomes apparent, and the chemistry involved in
these relatively few pathways is wide ranging.

1.25.5 Benzylisoquinoline Alkaloid Biosynthesis

The isoquinoline alkaloids include, most famously, the opiates morphine and codeine as well as the antibiotic
berberine (Figure 7). Morphine and codeine are two of the most important analgesics used in medicine, and
plants remain the main commercial source of the alkaloids.35 Notably, development of plant cell cultures of
Eschscholzia californica, Papaver somniferum, and Coptis japonica has aided in the isolation and cloning of many
enzymes involved in the biosynthesis of isoquinoline alkaloids.15
Isoquinoline biosynthesis begins with the substrates dopamine and p-hydroxyphenylacetaldehyde to yield
the central intermediate of this biosynthetic pathway, (S)-reticuline (Figure 8). Tyrosine is hydroxylated and
decarboxylated to yield dopamine. Enzymes that catalyze the hydroxylation and decarboxylation steps in either
order exist in the plant, and the predominant pathway for the formation of dopamine from tyrosine is not clear.
The second substrate, p-hydroxyphenylacetaldehyde, is synthesized by transamination and decarboxylation of
tyrosine.36 Dopamine and p-hydroxyphenylacetaldehyde are coupled by the enzyme norcoclaurine synthase to
form (S)-norcoclaurine. Two norcoclaurine synthases with completely unrelated sequences have been cloned
(Thalictrum flavum and C. japonica) and heterologously expressed in Escherichia coli.37,38 One shows homology to
iron-dependent dioxygenases, whereas the other is homologous to a pathogenesis-related protein. Recent
structural analysis of one of these enzymes has shed light onto the mechanism of this enzymatic transformation.
Undoubtedly, future experiments will explain how two such widely divergent sequences can catalyze the same
reaction.
One of the hydroxyl groups of (S)-norcoclaurine is methylated by an S-adenosyl methionine (SAM)-
dependent O-methyltransferase to yield (S)-coclaurine. This enzyme has been cloned, and the heterologously
expressed enzyme exhibited the expected activity.39,40 The next biosynthetic intermediate is N-methylated to
yield N-methylcoclaurine, an enzyme that has also been cloned.41,42 N-Methylcoclaurine is then hydroxylated
by a P-450-dependent enzyme (CYP80B), N-methylcoclaurine 39-hydroxylase, that has been cloned.43 One of
the hydroxyl groups is methylated by the enzyme 39-hydroxy-N-methylcoclaurine 49-O-methyltransferase
(49-OMT) to yield (S)-reticuline, the common biosynthetic intermediate for this pathway (Figure 8).44–46 The
biosynthetic pathway then diverges to yield the different structural classes of isoquinoline alkaloids.
In one major pathway, (S)-reticuline is converted to (S)-scoulerine by the action of a well-characterized
flavin-dependent enzyme, berberine bridge enzyme (Figure 9). This enzyme has been cloned from several
plant species,47–49 and the mechanism of this enzyme has been studied extensively.50,51 Notably, a structural
analysis of this enzyme has been recently reported.52 (S)-Scolerine is then methylated by scoulerine 9-O-
methyltransferase to yield (S)-tetrahydrocolumbamine. Heterologous expression yielded an enzyme that had
the expected substrate specificity.53 The substrate-specific cytochrome P-450 oxidase canadine synthase that
generates the methylene dioxy bridge of (S)-canadine has been cloned.54 The final step of berberine

Figure 7 Representative tetrahydroisoquinoline alkaloids morphine, codeine, and berberine.

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Figure 8 Biosynthesis of (S)-reticuline, the central intermediate of tetrahydroisoquinoline alkaloid biosynthesis.

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Alkaloids 989

Figure 9 Biosynthesis of berberine.

biosynthesis is catalyzed by a substrate-specific oxidase, tetrahydroprotoberberine oxidase, the sequence of

which has not been identified yet.55 Berberine can be overproduced in C. japonica cell suspension cultures with
reported productivity of berberine reaching 7 g l 1.56,57 This overproduction is one of the first demonstrations
of production of a benzylisoquinoline alkaloid in cell culture at levels necessary for economic production.
Additionally, this cell line has enabled the identification of many of the biosynthetic enzymes.
A second major pathway branch is the biosynthesis of the highly oxidized benzo(c)phenanthidine alkaloid
sanguinarine, which is produced in a variety of plants and competes with morphine production in opium poppy.
The pathway to sanguinarine has been elucidated at the enzymatic level (Figure 10).58 Sanguinarine biosynth-
esis starts from (S)-scoulerine, as in berberine biosynthesis. Methylenedioxy bridge formation is then catalyzed
by the P-450 cheilanthifoline synthase to yield cheilanthifoline.59 A second P-450 enzyme, stylopine synthase,
catalyzes the formation of the second methyenedioxy bridge of stylopine. Stylopine synthase from E. californica
has been cloned recently.60 Stylopine then is N-methylated by (S)-tetrahydroprotoberberine cis-N-
methyltransferase to yield (S)-cis-N-methylstylopine, an enzyme that has been cloned recently from opium
poppy.61 A third P-450 enzyme, (S)-cis-N-methylstylopine hydroxylase, then forms protopine. Protopine is
hydroxylated by a fourth P-450 enzyme, protopine 6-hydroxylase, to yield an intermediate that rearranges to
dihydrosanguinarine.62 The copper-dependent oxidase dihydrobenzophenanthridine oxidase, which has been
purified,63,64 then catalyzes the formation of sanguinarine from dihydrosanguinarine.
A third major branch leading to morphine biosynthesis has been investigated in P. somniferum cells and tissue.
Notably, in morphine biosynthesis, (S)-reticuline is converted to (R)-reticuline, epimerizing the stereocenter
generated by norcoclaurine synthase at the start of the pathway (Figure 11). This clearly exemplifies that the
shortest enzymatic route to the final product is not always utilized by nature in metabolic pathways. Instead,
nature will often use an existing biosynthetic intermediate for the biosynthesis. (S)-Reticuline is converted to
(R)-reticuline through a 1,2-dehydroreticuline intermediate. Dehydroreticuline synthase catalyzes the oxida-
tion of (S)-reticuline to 1,2-dehydroreticulinium ion.65 This enzyme has not been cloned but has been purified
partially and shown to be membrane associated. This intermediate then is reduced by dehydroreticuline
reductase, an NADPH-dependent enzyme that stereoselectively transfers a hydride to dehydroreticulinium
ion to yield (R)-reticuline. This enzyme has not been cloned yet but has been purified to homogeneity.66 The
key carbon–carbon bond of the morphinan alkaloids is formed by the cytochrome P-450 enzyme salutaridine
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Figure 10 Biosynthesis of sanguinarine.

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Figure 11 Biosynthesis of morphine and codeine.

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992 Alkaloids

synthase. Activity for this enzyme has been detected in microsomal preparations, and the sequence has been
recently deposited.67 The keto moiety of the resulting product, salutaridine, then is stereoselectively reduced
by the NADPH-dependent salutaridine reductase to form salutardinol. Recent transcript analysis profile of
P. sominiferum has resulted in the identification of this gene.68 Salutaridinol acetyltransferase, also cloned,
transfers an acyl group from acetyl-CoA to the newly formed hydroxyl group, which leads to the formation of
salutaridinol-7-O-acetate.69 The molecule can then undergo a spontaneous reaction in which the acetate acts as
a leaving group. The resulting product, thebaine, then is demethylated by an as yet uncharacterized enzyme to
yield neopinione, which exists in equilibrium with its tautomer codeinone. The NADPH-dependent codeinone
reductase catalyzes the reduction of codeinone to codeine and has been cloned.70,71 Finally, codeine is
demethylated by an uncharacterized enzyme to yield morphine.
A series of elegant labeling experiments with human cell culture has indicated that mammalian tissue is
capable of synthesizing morphine from dopamine precursors.72 Presumably, morphine may play a role as an
endogenous pain reliever in humans and other mammals. The mammalian enzymes involved in morphine
biosynthesis have not been extensively investigated.
The localization of morphine biosynthesis has been investigated at the cellular level in intact poppy plants
by using in situ RNA hybridization and immunofluorescence microscopy. The localization of 49-O-
methyltransferase (reticuline biosynthesis), berberine bridge enzyme (saguinarine biosynthesis), salutaridinol
acetyltransferase (morphine biosynthesis), and codeinone reductase (morphine biosynthesis) has been probed.
49-O-Methyltransferase and salutaridinol acetyltransferase are localized to parenchyma cells, whereas codei-
none reductase is localized to laticifer cells in sections of capsule (fruit) and stem from poppy plants. Berberine
bridge enzyme is found in parenchyma cells in roots. Therefore, this study suggests that two cell types are
involved in isoquinoline biosynthesis in poppy and that intercellular transport is required for isoquinoline
alkaloid biosynthesis.73 Another study, however, implicates a single cell type (sieve elements and their
companion cells) in isoquinoline alkaloid biosynthesis.74,75 Therefore, it is not clear whether transport of
pathway intermediates is required for alkaloid biosynthesis or whether the entire pathway can be performed in
one cell type. Undoubtedly, future studies will provide more insight into the trafficking involved in plant
secondary metabolism.
The extensive knowledge of the genes of isoquinoline biosynthesis has enabled a variety of metabolic
engineering work to be done. In attempts to accumulate thebaine and decrease production of morphine
(a precursor to heroin), codeinone reductase in opium poppy plant was downregulated by using RNAi.35,76
Silencing of codeinone reductase results in the accumulation of (S)-reticuline but not the substrate codeinone
or other compounds on the pathway from (S)-reticuline to codeine (Figure 11). The cytochrome P-450
responsible for the oxidation of (S)-N-methylcoclaurine to (S)-39-hydroxy-N-methylcocluarine has been
overexpressed in opium poppy plants, and morphinan alkaloid production in the latex is increased subse-
quently to 4.5 times the level in wild-type plants.77 Additionally, suppression of this enzyme resulted in a
decrease in morphinan alkaloids to 16% of the wild-type level. Notably, analysis of a variety of biosynthetic
gene transcript levels in these experiments supports the hypothesis that this P-450 enzyme plays a regulatory
role in the biosynthesis of benzylisoquinoline alkaloids. Collectively, these studies highlight that the complex
metabolic networks found in plants are not redirected easily or predictably in all cases. Notably, portions of this
pathway have been reconstituted in Saccharomyces cerevisiae, which is an organism that is much more amenable
to rational metabolic engineering efforts.78,79

1.25.6 Monoterpene Indole Alkaloid Biosynthesis

The terpene indole alkaloids are a diverse class of natural products, comprising over 2000 members. These
complex natural products possess a range of chemical structures and a wealth of biological activities
(Figure 12).80,81 The biosynthetic pathways of some classes of terpene indole alkaloids are well understood,
and in some branches, many of the enzymes that are responsible for biosynthesis have been actually cloned and
mechanistically studied in vitro. In other cases, the biosynthetic pathway is only proposed based on the results of
feeding studies with isotopically labeled substrates and from the structures of isolated biosynthetic intermedi-
ates. Although many biosynthetic genes from this pathway remain unidentified, recent studies have correlated
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Alkaloids 993

Figure 12 Representative monoterpene indole alkaloids ajmalicine, ajmaline, vinblastine, and vincristine.

terpenoid indole alkaloid production with the transcript profiles of Catharanthus roseus cell cultures.82 Although
the genome sequences of none of these alkaloid-producing plants is available, a number of expressed sequence
tag (EST) libraries for C. roseus have been reported.83,84
All terpenoid indole alkaloids are derived from tryptophan and the iridoid terpene secologanin (Figure 13).
The involvement of an iridoid monoterpene in these indole alkaloid pathways was first proposed after the
structures of several iridoid terpenes were elucidated, and secologanin was identified as the specific iridoid
precursor.85–87 Secologanin is itself a natural product, and the biosynthetic pathway for this molecule has not
been fully elucidated, although feeding studies with C. roseus cell suspension cultures and 13C-glucose strongly
suggest that secologanin is ultimately derived from the triose phosphate/pyruvate or ‘nonmevalonate’

Figure 13 Biosynthesis of deglycosylated strictosidine, the central biosynthetic intermediate in monoterpene indole
alkaloid biosynthesis.
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994 Alkaloids

pathway.88 Several enzymes involved in the biosynthesis of Isoprenylpyrophosphate-1-deoxy-D-xylulose

5-phosphate (IPP)–2-Methyl-D-erythritol-4-phosphate (DXP) synthase, DXP reductoisomerase, and MEP
synthase – have been cloned from C. roseus.89,90 Several other genes involved in the later steps of secologanin
biosynthesis, namely geraniol-10-hydroxylase,91,92 and secologanin synthase93,94 have been identified.
Tryptophan decarboxylase, a pyridoxal-dependent enzyme, converts tryptophan to tryptamine to yield the
amine-containing starting material for this family of alkaloids.95,96 The enzyme strictosidine synthase catalyzes
a stereoselective Pictet–Spengler condensation between tryptamine and secologanin to yield strictosidine.
Strictosidine synthase97 has been cloned from the plants C. roseus,98 Rauwolfia serpentina,99 and Ophiorrhiza
pumila.100 A crystal structure of strictosidine synthase from R. serpentina has been reported,101 and a number of
reports have indicated that the substrate specificity of the enzyme can be modulated.102–104
In most monoterpene indole alkaloids, strictosidine is deglycosylated by a dedicated -glucosidase, which
converts the substrate to a reactive hemiacetal intermediate.105–107 This hemiacetal opens to form a dialdehyde
intermediate, which then forms dehydrogeissoschizine. The enol form of dehydrogeissoschizine can undergo
1,4 conjugate addition to produce the heteroyohimbine cathenamine, as well as a variety of other isomers.107–111
The biosynthetic pathway for ajmaline in R. serpentina is one of the best-characterized branches of the
terpenoid indole alkaloid pathways (Figure 14). Much of this progress has been detailed in an extensive
review.112 Like all other terpenoid indole alkaloids, ajmaline, an antiarrhythmic drug with potent sodium
channel-blocking properties,113 is derived from deglycosylated strictosidine. At least eight enzymes are
predicted to catalyze the subsequent steps of ajmaline biosynthesis that occur after strictosidine deglycosyla-
tion. The sarpagan alkaloid, polyneuridine aldehyde, is a known early intermediate of the ajmaline pathway. A
mechanism in which the sarpagan bridge enzyme transforms an isomer of deglycosylated strictosidine to
polyneuridine aldehyde has been proposed.114 A membrane-protein fraction of an R. serpentina extract trans-
formed labeled strictosidine into sarpagan-type alkaloids. The enzyme activity was shown to be dependent on
NADPH and molecular oxygen, suggesting that sarpagan bridge enzyme may be a cytochrome P-450

Figure 14 Biosynthesis of ajmaline.

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Alkaloids 995

enzyme.115,116 Isolation of this enzyme will yield further insight into this key step that commits the deglyco-
sylated strictosidine intermediate to the sarpagan- and ajmalan-type alkaloid pathways.
Polyneuridine aldehyde esterase then hydrolyzes the polyneuridine aldehyde methyl ester, which generates
an acid that spontaneously decarboxylates to yield epi-vellosamine. This enzyme has been cloned from a
Rauwolfia cDNA library, heterologously expressed in E. coli, and subjected to detailed mechanistic studies.117–119
Polyneuridine aldehyde esterase appears to be a member of the / hydrolase super family and contains a Ser,
His, Asp catalytic triad.117–119 Site-directed mutagenesis indicates that each residue of the catalytic triad is
required for activity. Vinorine synthase transforms the sarpagan alkaloid epi-vellosamine to the ajmalan
alkaloid vinorine.120 Vinorine synthase also has been purified from Rauwolfia cell culture, subjected to protein
sequencing, and cloned from a cDNA library.121,122 The enzyme, which seems to be an acetyltransferase
homolog, has been crystallized and subjected to site-directed mutagenesis studies of this protein, leading to a
proposed mechanism.123 Vinorine hydroxylase hydroxylates vinorine to form vomilene.124 Vinorine hydro-
xylase seems to be a P-450 enzyme, and has not been cloned yet. The indolenine bond of vomilene is
reduced by an NADPH-dependent reductase to yield 1,2-dihydrovomilenene. A second enzyme, 1,2-
dihydrovomilenene reductase, then reduces this product to acetylnorajmaline. Partial protein sequences have
been obtained for both of the purified reductases. Although several putative clones that encode these proteins
have been isolated, the activity of these clones has not been verified yet.125,126 An acetylesterase then
hydrolyzes the acetyl link of acetylnorajmaline to yield norajmaline. This esterase has been purified from
R. serpentina cell suspension cultures, and a full-length clone has been isolated from a cDNA library. Expression
of the gene in tobacco leaves successfully yielded protein with the expected enzymatic activity.127 In the final
step of ajmaline biosynthesis, an N-methyltransferase introduces a methyl group at the indole nitrogen of
norajmaline. Although this enzymatic activity has been detected in crude cell extracts, the enzyme has not been
characterized additionally.128 In summary, the enzymatic activities for all steps of ajmaline biosynthesis have
been detected. Strictosidine synthase, strictosidine glucosidases, polyneuridine aldehyde esterase, vinorine
synthase, and 17-O-acetyl-ajmalanesterase have been cloned. Putative clones for vinorine hydroxylase, vomi-
lenine reductase, and 1,2-dihydrovomilenen reductase have been isolated. N-Methyltransferase activity and
sarpagan bridge enzyme activities have only been detected in crude cell extracts.
Ajmalicine (raubasine) affects smooth muscle function and is used to help prevent strokes,129 and tetrahydroal-
stonine exhibits antipsychotic properties (Figure 15).130 These compounds are found in a variety of plants, including
C. roseus and R. serpentina. A partially purified NADPH-dependent reductase isolated from a tetrahydroalstonine that
produces a C. roseus cell line was shown to catalyze the conversion of cathenamine, a spontaneous reaction product
that results after strictosidine deglycosylation, to tetrahydroalstonine in vitro (Figure 15).131 A second C. roseus cell
line contains an additional reductase that produces ajmalicine. Labeling studies performed with crude C. roseus cell
extracts in the presence of D2O or deuterated, reduced form of Nicotinamide adenine dinucleotide phosphate
(NADPD) support a mechanism in which the reductase acts on the iminium form of cathenamine.132
Vindoline, an aspidosperma-type alkaloid produced by C. roseus, is a key precursor for vinblastine, an
anticancer drug that is the most important pharmaceutical product of C. roseus (Figure 16). Vindoline, like
ajmalicine and ajmaline, is produced from deglycosylated strictosidine. Deglycosylated strictosidine is con-
verted to tabersonine, an aspidosperma-type alkaloid, through a series of biochemical steps for which no
enzymatic information exists. Studies by numerous groups in the 1960s and 1970s enabled detailed hypothetical
proposals of the biosynthesis of aspidosperma-type alkaloids in C. roseus.133–143 These proposed pathways are
based on feeding studies of isotopically labeled substrates to seedlings or shoots, isolation of discrete inter-
mediates from plant materials, and from biomimetic model reactions. More details are known about the
elaboration of tabersonine to vindoline.144 Tabersonine-16-hydroxylase, a cytochrome P-450, hydroxylates
tabersonine to 16-hydroxy-tabsersonine.145,146 This hydroxyl group is then methylated by a SAM-dependent
O-methyltransferase to yield 16-methoxy-tabersonine; this enzyme (16-hydroxytabersonine-16-O-
methyltransferase) has recently been cloned.147 In the next step, hydration of a double bond by an unchar-
acterized enzyme produces 16-methoxy-2,3-dihydro-3-hydroxytabersonine. Transfer of a methyl group to the
indole nitrogen by an N-methyltransferase yields desacetoxyvindoline. This methyltransferase activity has
been detected only in differentiated plants, not in plant cell cultures.148 The resulting intermediate, deace-
tylvindoline, is produced by the oxoglutarate-dependent dioxygenase enzyme desacetylvindoline
4-hydroxylase. This enzyme has been cloned and is also absent from plant cell cultures.149 In the last step,
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Figure 15 Biosynthesis of ajmalicine and tetrahydroalstonine.

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Figure 16 Biosynthesis of vindoline and vinblastine.

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998 Alkaloids

desacetylvindoline is acetylated by desacetylvindoline O-acetyltransferase. This enzyme, also absent from

nondifferentiated plant material, has been cloned successfully.150
Vinblastine and the structurally related vincristine are highly effective anticancer agents currently used
clinically against leukemia and other cancers.151,152 Inspection of these bisindole alkaloids indicates that they
are derived from coupling of vindoline and catharanthine, which is believed to proceed via the formation of an
iminum intermediate with catharanthine (Figure 16). This iminium intermediate is reduced to form anhy-
drovinblastine, a naturally occurring compound in C. roseus plants.153
Peroxidase-containing fractions of plant extracts were found to catalyze the formation of the
bisindole dehydrovinblastine from catharanthine and vindoline.154,155 The peroxidase CRPRX1 ( -39,49-
anhydrovinblastine synthase), purified and cloned from C. roseus leaves, has been demonstrated to convert
vindoline and catharanthine to anhydrovinblastine.156,157 Catharanthine is most likely oxidized to an iminium
ion, which then reacts with the relatively nucleophilic vindoline.158 Although this peroxidase is not highly
substrate specific for catharanthine and vindoline, localization studies strongly suggest that CRPRX1 is the
dedicated peroxidase for bisindole formation.157,158 Finally, after formation of dehydrovinblastine, hydroxyla-
tion of the double bond yields vinblastine, and oxidation of the N-methyl group yields vincristine.
As in morphine biosynthesis, the knowledge of the enzyme sequences allows a more detailed understanding
of the localization of the enzymes.159 Strictosidine synthase seems to be localized to the vacuole,98 and
strictosidine glucosidase is believed to be associated with the cytosol and the membrane of the endoplasmic
reticulum.160 Tabersonine-16-hydroxylase is associated with the endoplasmic reticulum membrane;145
N-Methyltransferase activity is believed to be associated with the thylakoid, a structure located within the
chloroplast;148,161 and vindoline-4-hydroxylase and desacetylvindoline O-acetyltransferase are believed to be
localized to the cytosol.161,162 In addition to subcellular compartmentalization, specific cell types are required
for the biosynthesis of some terpenoid alkaloids, as is the case in morphine biosynthesis described above.
Several enzymes involved in the early stages of secologanin biosynthesis seem to be localized to the phloem
parenchyma, as evidenced by immunocytochemistry and in situ RNA hybridization studies.163 However,
additional studies have suggested that these genes also are observed in the epidermis and laticifers.164
Studies of the localization of vindoline biosynthetic enzymes by using immunocytochemistry and in situ
RNA hybridization strongly suggest that the midpart of the vindoline pathway (tryptophan decarboxylase,
strictosidine synthase, and tabersonine-16-hydroxylase) takes place in epidermal cells of leaves and stems.
However, the later steps catalyzed by desacetylvindoline 4-hydroxylase and desacetylvindoline
O-acetyltransferase take place in specialized cells, the laticifers, and idioblasts.165 As with isoquinoline alkaloid
biosynthesis, deconvolution of the enzyme localization patterns remains a challenging endeavor.
Again, the partially elucidated pathways of monoterpene indole alkaloid biosynthesis have allowed meta-
bolic engineering efforts. These studies have primarily taken place in C. roseus. Strictosidine synthase and
tryptophan decarboxylase have been overexpressed in C. roseus cell cultures.166,167 Generally, overexpression of
tryptophan decarboxylase does not seem to have a significant impact on alkaloid production, although over-
expression of strictosidine synthase does seem to improve alkaloid yields. Overexpression of tryptophan and
secologanin biosynthetic enzymes in C. roseus hairy root cultures resulted in modest increases in terpenoid
indole alkaloid production.168,169 Secologanin biosynthesis seems to be the rate-limiting factor in alkaloid
production.170 Using a combination of unnatural tryptamine analogs and a reengineered strictosidine synthase
enzyme, the biosynthetic pathway can be used to produce alkaloid derivatives.171,172 Although strictosidine
synthase and strictosidine glucosidase enzymes have been expressed heterologously in yeast,173 efforts to
express heterologously terpenoid indole alkaloids currently are limited because the majority of the biosynthetic
genes remain uncloned.
Transcription factors that upregulate strictosidine synthase,174 as well as a transcription factor that coordi-
nately upregulates expression of several terpenoid indole alkaloid biosynthetic genes, have been found.14
Several zinc finger proteins that act as transcriptional repressors to tryptophan decarboxylase and strictosidine
synthase also have been identified.175 Manipulation of these transcription factors may allow tight control of the
regulation of terpenoid indole alkaloid production. Interestingly, expression of a transcription factor from
Arabidopsis thaliana in C. roseus cell cultures results in an increase in alkaloid production.176 The dramatic
increases in alkaloid production that have been noted in morphine metabolic engineering efforts have for the
most part not been observed in the monoterpene indole alkaloids. Notably, more enzymes in the isoquinoline
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Alkaloids 999

biosynthetic pathway are known. Presumably, as more monoterpene indole alkaloid biosynthetic enzymes are
identified, one may be identified that has a significant impact on the alkaloid expression levels.

1.25.7 Tropane Alkaloid Biosynthesis

The tropane alkaloids hyoscyamine and scopolamine (Figure 17) function as acetylcholine receptor antago-
nists and are used clinically as parasympatholytics. The illegal drug cocaine is also a tropane alkaloid. The
tropane alkaloids are biosynthesized primarily in plants of the family Solonaceae, which includes Hyoscyamus,
Duboisia, Atropa, and Scopolia.17,18 Nicotine, although perhaps not apparent immediately from its structure, is
related biosynthetically to the tropane alkaloids.
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 (Figure 18).177 In plants, L-arginine is converted to the nonproteogenic amino acid
L-ornithine by the urease enzyme arginase. Ornithine decarboxylase then decarboxylates ornithine to yield
the diamine putrescine. In Hyoscyamus, Duboisia, and Atropa, putrescine (so-named because of its odor) 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.178,179 Putrescine N-methyltransferase now has
been cloned from a variety of plant species,180–182 and site-directed mutagenesis and homology models have led
to insights into the structure–function relationships of this enzyme.182 N-Methylputrescine is then oxidized by
a diamine oxidase to form 4-methylaminobutanal, which then cyclizes, most likely nonenzymatically, to form
the N-methyl-D-pyrrolinium ion.183–185 This enzyme, which recently has been cloned, seems to be a copper-
dependent amine oxidase.186 Immunoprecipitation experiments suggest that this enzyme associates with the
enzyme S-adenosylhomocysteine hydrolase.187 The pyrrolinium ion is then converted to the tropanone
skeleton by as yet uncharacterized enzymes. Although no enzymatic information is available, chemical labeling
studies have indicated that an acetate-derived moiety condenses with the pyrrolinium ion.17 Tropanone is then
reduced via an NADPH-dependent reductase to tropine that has been cloned from Hyoscyamus niger.188,189 All
tropane-producing plants seem to contain two tropinone reductases, which create a branch point in the
pathway. Tropinone reductase I yields the tropane skeleton, whereas tropinone reductase II yields the opposite
stereocenter, pseudotropine.190 Tropane is converted to scopolamine or hyoscyamine, while the tropinone
reductase II product pseudotropine leads to calystegines.191 These two tropinone reductases have been
crystallized, and site-directed mutagenesis studies indicate that the stereoselectivity of the enzymes can be
switched by rational protein engineering.192,193
The biosynthesis of scopolamine is the best characterized of the tropane alkaloids. After action by tropinone
reductase I, tropine is condensed with phenylacetate through the action of a P-450 enzyme to form littorine.13
The phenyllactate moiety is believed to derive from an intermediate involved in phenylalanine
metabolism.17 Littorine then undergoes rearrangement to form hyoscyamine. The enzyme that catalyzes this
rearrangement was originally believed to proceed via a radical mechanism using SAM as the source of an
adenosyl radical.194–197 However, a large-scale RNAi study performed in H. niger suggests that a P-450 enzyme,
followed by action of a reductase enzyme, is responsible for the rearrangement. Hyoscyamine 6 -hydroxylase

Figure 17 Representative type tropane and nicotinic alkaloids nicotine, cocaine, and scopolamine.
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Figure 18 Biosynthesis of scopolamine.

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Alkaloids 1001

catalyzes the hydroxylation of hyoscyamine to 6 -hydroxyhyoscyamine as well as the epoxidation to scopo-

lamine.198,199 Hyoscyamine 6 -hydroxylase, which has been cloned and expressed heterologously,200 is a
nonheme, iron-dependent, oxoglutarate-dependent protein. The epoxidation reaction appears to occur more
slowly than the hydroxylation reaction. The tropane alkaloids seem to be formed in the roots and then
transported to the aerial parts of the plant.201
Atropa belladonna plants have been transformed with hyoscyamine 6 -hydroxylase from H. niger. A. belladonna
normally produces high levels of hyoscyamine, the precursor for the more pharmaceutically valuable alkaloid
scopolamine. However, in one of the earliest demonstrations of plant natural product metabolic engineering,
after transformation with hyoscyamine 6 -hydroxylase, transgenic A. belladonna plants were shown to accumu-
late scopolamine almost exclusively.202 Additionally, the levels of tropane alkaloid production in a variety of
hairy root cultures were altered by overexpression of methyltransferase putrescine-N-methyltransferase and
hyoscyamine 6 -hydroxylase. Overexpression of both of these enzymes in a hairy root cell culture resulted in
significant increases in scopolamine production.202,203 Fluorinated phenyllactic acid substrates could be
incorporated into the pathway,204 and several substrates derived from putrescine analogs were turned over
by the enzymes of several Solonaceae species.205

1.25.8 Purine Alkaloid Biosynthesis

Caffeine, a purine alkaloid, is one of the most widely ingested of all natural products. Caffeine is a natural
component of coffee, tea, and cocoa, and the impact of caffeine on human health has been studied extensively.
The biosynthetic pathway of caffeine has been elucidated recently on the genetic level (Figure 19), and most
work has focused on the plant species Coffea (coffee) and Camellia (tea).19,206 Xanthosine, which is derived from
purine metabolites, is the first committed intermediate in caffeine biosynthesis. Xanthosine can be formed from
de novo purine biosynthesis, SAM cofactor, the adenylate pool, and the guanylate pool.19 De novo purine
biosynthesis and the adenosine from SAM are believed to be the most important sources of xanthosine.19,207
Xanthosine is methylated to yield N-methylxanthosine by the enzyme xanthosine N-methyltransferase (XMT)
(also called 7-methylxanthosine synthase).208–210 N-Methylxanthosine is converted to N-methylxanthine
by methylxanthine nucleosidase, an enzyme that has not been cloned, but is present in many noncaffeine-
producing organisms.211 N-Methylxanthine is converted to theobromine by 7-methylxanthine-N-
methyltransferase (MXMT) (also called theobromine synthase), a second N-methyltransferase.209,212
Theobromine is converted to caffeine by a final N-methyltransferase, dimethylxanthine-N-methyltransferase
(DXMT) (also called caffeine synthase).209
Coffee and tea plants seem to contain a variety of N-methyltransferase enzymes that have varying substrate
specificity.19,206 For example, a caffeine synthase enzyme isolated from tea leaves catalyzes both the
N-methylation of N-methylxanthine and theobromine.213 The substrate specificity of the methyltransferases
can be changed by site-directed mutagenesis.214
Coffee beans with low caffeine levels could be valuable commercially, given the demand for decaffeinated
coffee. Because of the discovery of these N-methyltransferase genes, genetically engineered coffee plants with
reduced caffeine content now can be constructed.212,215 For example, a 70% reduction in caffeine content in
Coffea was obtained by downregulating MXMT (theobromine synthase) using RNAi.216 Additionally, the
promoter of one of the N-methyltransferases has been discovered recently, which may allow transcriptional
gene silencing.217

1.25.9 Conclusions and Outlook

Alkaloids constitute a structurally diverse array of natural products, and these compounds have a wide range of
biological activities. Many have important pharmaceutical uses. Plants are regarded as the oldest source of
alkaloids, and some of the most widely recognized alkaloids, such as morphine, quinine, strychnine, and
cocaine, are derived from plants. However, rapid advances in molecular biology and sequencing of bacterial
and fungal genomes have fostered the discovery of new alkaloids in these simpler microbial organisms, and a
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Figure 19 Biosynthesis of caffeine.

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Alkaloids 1003

wealth of biosynthetic information for these compounds has been obtained in a number of recent mechanistic
studies. Undoubtedly, many more microbe-derived alkaloids remain to be discovered. Given the medicinal
importance of many alkaloids, metabolic engineering efforts serve as an important application of biosynthetic
studies. Metabolic engineering has been used to increase the production levels of alkaloids, reconstitute alkaloid
biosynthesis in simpler host organisms, and rationally modify the structure of alkaloids by engineering of the
biosynthetic enzymes. The past several years have resulted in major advances in alkaloid discovery, biosyn-
thetic pathway elucidation, and metabolic engineering.

Glossary
acridine-type alkaloids Anthranilic acid-derived natural products.
alkaloid Nitrogen-containing organic substances of natural origin with basic character.
indolizidine alkaloids Lysine-derived natural products.
monoterpene indole alkaloids Tryptophan- and secologanin-derived natural products.
piperdine alkaloids Lysine-derived natural products.
pyrrolizidine alkaloids Ornithine-derived natural products.
quinazoline alkaloids Anthranilic acid-derived natural products.
quinolizidine alkaloids Lysine-derived natural products.
tetrahydroisoquinoline alkaloids Tyrosine-derived natural products.
tropane alkaloids Ornithine-derived natural products.

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Biographical Sketch

Sarah E. O’Connor received a bachelor of science degree in chemistry from the University of
Chicago. She received her Ph.D. in chemistry under the direction of Barbara Imperiali at
Caltech and MIT, where her thesis work focused on the synthesis and structural analysis of
N-linked glycopeptides. She did her postdoctoral research with Chris Walsh at Harvard
Medical School where she studied the biosynthesis of epothilone and several other polyke-
tide and peptide natural products. She began her independent research program at MIT
where her group is investigating the biosynthesis of plant-derived alkaloid natural products.

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