amines and several peptides [lectins, protease inhibitors, and antimicrobial peptides
(AMPs)] [1, 3–5, 7, 8, 11, 16, 17, 44].
Whereas alkaloids are often powerful neurotoxins, NPAAs are mimics of the 20
proteinogenic amino acids and can be incorporated into proteins. These proteins
show a wrong secondary and tertiary structure and thus are often functionless. Thus,
NPAAs function as metabolic disruptors [14, 16, 17, 21, 44]. When cyanogenic
glucosides are hydrolyzed, they release HCN, which is a powerful inhibitor of
the respiratory chain in mitochondria and thus inhibits the formation of ATP.
They are thus powerful metabolic poisons [2, 4, 21]. When glucosinolates
are cleaved, lipophilic mustard oils or isothiocyanates are liberated. They disturb
membrane fluidity, bind to proteins and DNA bases, and cause inflammation, a
pungent taste, and pain [14, 16, 17]. Lectins often interfere with ribosomal protein
biosynthesis, protease inhibitors inhibit proteases of the digestion process, and
AMPs influence membrane fluidity and stability in microbial and eukaryotic cells.
Alkaloids contain one or several nitrogen atoms, mostly in their ring structures.
They form a free base under alkaline conditions and are charged molecules below
pH 7 (i.e., in most plants and in target organisms, alkaloids are mostly present in a
protonated form). However, when we draw them, we usually show the free base.
Depending on the biosynthetic pathways and ring structures, alkaloids are divided
into the following groups: pyridine, pyrimidine, pyrrol, piperidine, pyrrolizidine,
quinolizidine, indolizidine, isoquinoline, quinoline, indol, monoterpene indole,
terpenoid, and steroidal alkaloids. The main amino acids, which serve as precursors,
are phenylalanine/tyrosine (isoquinoline alkaloids, including protoberberine and
morphinane alkaloids), tryptophan (indol alkaloids, monoterpene indole alkaloids,
and quinoline alkaloids), lysine (piperidine and quinolizidine alkaloids), ornithine/
arginine (tropane and pyrrolizidine alkaloids) [45–51].
As mentioned above, alkaloids often function as neurotoxins, others are
cytotoxic, as they interfere with biomembranes, microtubules, actin filaments,
enzymes, and DNA/RNA and corresponding enzymes [5, 6, 15, 44, 45, 47,
51–54]. In case of neurotoxic alkaloids, they often mimic the structure of
neurotransmitters, such as acetylcholine, noradrenaline, adrenaline, serotonin,
dopamine, or endorphins (Table 1). They can either block the neurotransmitter
receptor as antagonist or stimulate it as agonist. Some alkaloids inhibit the activity
of enzymes, which degrade neurotransmitters, such as acetylcholine esterase or
monoamine oxidase (MAO). Also, the uptake protein for neurotransmitters into
the presynapse or the neurovesicles can be inhibited. Several toxic alkaloids
inhibit or activate ion channels (Na
+ , K
+ , Ca
++ ) or Na
+ , K
+ -ATPase [14, 16, 17,
43, 44, 48, 53–55]. Some alkaloids have a single target, many others and the
majority of PSM are multitarget compounds, which are directed against several
targets in animals and/or microorganisms. In conclusion, most alkaloids are
known for their pronounced toxicological properties and some of them are lethal
poisons others are used in medicine to treat health conditions [14, 16, 17]. From a
plants point of view, alkaloids are mostly employed as defense compounds against
herbivores and many of the alkaloid-accumulating plants are avoided by
herbivores.
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
157
(AMPs)] [1, 3–5, 7, 8, 11, 16, 17, 44].
Whereas alkaloids are often powerful neurotoxins, NPAAs are mimics of the 20
proteinogenic amino acids and can be incorporated into proteins. These proteins
show a wrong secondary and tertiary structure and thus are often functionless. Thus,
NPAAs function as metabolic disruptors [14, 16, 17, 21, 44]. When cyanogenic
glucosides are hydrolyzed, they release HCN, which is a powerful inhibitor of
the respiratory chain in mitochondria and thus inhibits the formation of ATP.
They are thus powerful metabolic poisons [2, 4, 21]. When glucosinolates
are cleaved, lipophilic mustard oils or isothiocyanates are liberated. They disturb
membrane fluidity, bind to proteins and DNA bases, and cause inflammation, a
pungent taste, and pain [14, 16, 17]. Lectins often interfere with ribosomal protein
biosynthesis, protease inhibitors inhibit proteases of the digestion process, and
AMPs influence membrane fluidity and stability in microbial and eukaryotic cells.
Alkaloids contain one or several nitrogen atoms, mostly in their ring structures.
They form a free base under alkaline conditions and are charged molecules below
pH 7 (i.e., in most plants and in target organisms, alkaloids are mostly present in a
protonated form). However, when we draw them, we usually show the free base.
Depending on the biosynthetic pathways and ring structures, alkaloids are divided
into the following groups: pyridine, pyrimidine, pyrrol, piperidine, pyrrolizidine,
quinolizidine, indolizidine, isoquinoline, quinoline, indol, monoterpene indole,
terpenoid, and steroidal alkaloids. The main amino acids, which serve as precursors,
are phenylalanine/tyrosine (isoquinoline alkaloids, including protoberberine and
morphinane alkaloids), tryptophan (indol alkaloids, monoterpene indole alkaloids,
and quinoline alkaloids), lysine (piperidine and quinolizidine alkaloids), ornithine/
arginine (tropane and pyrrolizidine alkaloids) [45–51].
As mentioned above, alkaloids often function as neurotoxins, others are
cytotoxic, as they interfere with biomembranes, microtubules, actin filaments,
enzymes, and DNA/RNA and corresponding enzymes [5, 6, 15, 44, 45, 47,
51–54]. In case of neurotoxic alkaloids, they often mimic the structure of
neurotransmitters, such as acetylcholine, noradrenaline, adrenaline, serotonin,
dopamine, or endorphins (Table 1). They can either block the neurotransmitter
receptor as antagonist or stimulate it as agonist. Some alkaloids inhibit the activity
of enzymes, which degrade neurotransmitters, such as acetylcholine esterase or
monoamine oxidase (MAO). Also, the uptake protein for neurotransmitters into
the presynapse or the neurovesicles can be inhibited. Several toxic alkaloids
inhibit or activate ion channels (Na
+ , K
+ , Ca
++ ) or Na
+ , K
+ -ATPase [14, 16, 17,
43, 44, 48, 53–55]. Some alkaloids have a single target, many others and the
majority of PSM are multitarget compounds, which are directed against several
targets in animals and/or microorganisms. In conclusion, most alkaloids are
known for their pronounced toxicological properties and some of them are lethal
poisons others are used in medicine to treat health conditions [14, 16, 17]. From a
plants point of view, alkaloids are mostly employed as defense compounds against
herbivores and many of the alkaloid-accumulating plants are avoided by
herbivores.
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
157