Coca plant (Erythroxylum coca) and the molecular structure of cocaine (grey: carbon, blue: nitrogen, red: oxygen, white: hydrogen). |
Cocaine
is one of the most commonly used (and abused) plant-derived drugs in
the world, but we have almost no modern information on how plants
produce this complex alkaloid. Researchers from the Max Planck Institute
for Chemical Ecology in Jena, Germany, have just discovered a key
reaction in cocaine formation in the coca plant from South America, and
identified the responsible enzyme. This enzyme was shown to belong to
the aldo-keto-reductase protein family revealing some exciting new
insights into the evolution of cocaine biosynthesis.
Alkaloids
constitute a very large group of natural nitrogen-containing compounds
with diverse effects on the human organism. A large variety of
plant-produced alkaloids have strong pharmacological effects, and are
used as toxins, stimulants, pharmaceuticals or recreational drugs,
including caffeine, nicotine, morphine, quinine, strychnine, atropine
and cocaine. Atropine, used to dilate the pupils of the eye, and the
addictive drug cocaine are both tropane alkaloids which possess two
distinctive, inter-connecting five- and seven-membered rings.
Plants
commonly produce tropane and other alkaloids for protection against
herbivores and other enemies. Species in seven plant families are known
to produce tropane alkaloids, including the Brassicaceae (mustard
family), Solanaceae (nightshade or potato family) and Erythroxylaceae
(coca family). These families are not closely related to each other. For
example, it is assumed that the last common ancestor of the
Erythroxylaceae and the Solanaceae lived about 120 million years ago.
But how similar are the tropane alkaloid biosynthetic pathways in these
families? Was there a single original tropane alkaloid pathway which was
lost in most other plant families during the course of evolution? Or,
did tropane alkaloid biosynthesis arise independently on several
different occasions?
John
D’Auria, project leader in the Department of Biochemistry at the Max
Planck Institute for Chemical Ecology, has been studying the coca plant,
from which the drug cocaine is derived. Native tribes in South America
have been cultivating coca and chewing its leaves for at least 8000
years for their stimulant and hunger-suppressing properties.
Although
the formation of cocaine has not been investigated in the last 40
years, the biosynthesis of the related tropane alkaloid, atropine, from
belladonna (Solanaceae) is well-established. In the penultimate step, a
ketone function is reduced to an alcohol residue. This key reaction is
catalysed by an enzyme of the short-chain dehydrogenase/reductase (SDR)
protein family in belladonna. Among this group of enzymes are also many
alcohol-degrading dehydrogenases in animals.
Immunolabeling (green areas) of MecgoR, the enzyme catalyzing the penultimate step of cocaine biosynthesis. The picture shows the strong accumulation of the enzyme in a cross section of a very young E. coca leaf, which is still curled around the growing stem tip. Bar: 0.1 mm. Image: MPI for Chemical Ecology/ D’Auria, Jirschitzka |
To
find the corresponding enzyme in cocaine biosynthesis, Jan Jirschitzka,
a PhD student in the group, searched the genome of the coca plant to
look for SDR-like proteins. However, all the SDR genes that he cloned
and expressed did not show any activity for the key reaction in cocaine
formation. So he used a more classical approach—identifying the
cocaine-synthesizing enzyme activity in extracts from coca leaves,
purifying the responsible protein, isolating the polypeptide, and—after
partial sequencing—cloning the corresponding gene.
“We
obtained two very interesting results,” says Jonathan Gershenzon,
director at the institute. “The enzyme reaction analogous to that in
atropine synthesis—the conversion of the keto group into an alcohol
residue—is catalysed by a completely different enzyme in coca plants as
compared to that in the Solanaceae, namely by an aldo-keto reductase
(AKR).”
The
enzyme was named methylecgonone reductase (MecgoR). AKR enzymes are
known in plants and also mammals, amphibians, yeast, protozoa, and
bacteria. They are involved in the formation of steroid hormones, for
example. The second result is that the MecgoR gene, as well as the
protein, is highly active in the very young leaves of coca plants, but
not in the roots. Atropine, on the other hand, is synthesized
exclusively in the roots of belladonna, from where it is transported
into the green organs of the plant. Based on these results, the Max
Planck researchers conclude that the tropane alkaloid pathways in coca
and belladonna evolved completely independently.
Elucidation
of the MecgoR-catalyzed step in cocaine biosynthesis represents a major
success, but the researchers are now continuing to investigate other
important steps in the cocaine pathway. Also of interest is to learn how
cocaine is stored in leaf tissue in such high amounts. This alkaloid
can account for up to 10% of the dry weight of the immature coca leaf, a
phenomenal amount for the accumulation of any one particular alkaloid.
Source: Max Planck Institute
