208
13
Olivetolic acid is also the starting point for biosynthesis of other compounds, for
instance, primin, which accumulates in the trichomes of Primula obconica (Primulaceae)
and causes dermatitis.
In hops (Humulus lupulus, Cannabaceae) specialized trichomes, the so-called peltate
glandular trichomes, can be found especially at the base of hop cones. They contain, in
addition to terpenes, many C5-prenylated polyketides, with the major compound being
xanthohumol, a prenylflavonoid. Further oxidized prenylated polyketides are the socalled bitter acids such as humulone. These polyketides provide flavour for beer, especially
through the bitter taste of humulene. The non-ketone part of xanthohumol originates in
phenylpropanoid metabolism (see 7 Chap. 11), namely, chalconaringenin. Transfer of
DMAPP to chalconaringenin allows prenylation and yields desmethylxanthohumol, which
is methylated to form xanthohumol. Xanthohumol was found in significant amounts in
male hop plants, presumably due to the presence of lupulin glands on anthers (Nagel et al.
2008). Humulone is derived from leucine to form isovaleryl CoA and furthermore, after
the addition of three malonyl CoA, to phlorisovalerophenone. Two additional aromatic
prenylation steps, where the prenyl group is added via DMAPP, and final oxidation lead
to humulone.
Dinoflagellates seem to have specialized in the production of secondary metabolites
of polyketide origin (Rein and Snyder 2006; Van Wagoner et al. 2014). There is a frequent occurrence of five-, six-, seven-, eight- and even nine-membered all trans-fused
ether rings as a backbone. These rings can be found in isolation (e.g. amphidinolide), in
spiroketal formation (e.g. okadaic acid) or in as many as fourteen ether rings continuously
fused (e.g. brevetoxin). For dinoflagellates it was observed that the C1 carbons of acetate
were frequently missing, and pendant methyl groups were derived from acetate methyl, in
addition to S-adenosyl methionine (SAM). Brevetoxins, especially known from Karenia
brevis, have 10–11 fused ether rings and an unusual terminal aldehyde group derived from
an acetate methyl carbon. Brevetoxins are neurotoxins that bind to voltage-gated sodium
channels in nerve cells, causing neurotoxic shellfish poisoning. Brevenal has four fused
ether rings and appears not to share the sodium channel activity of the brevetoxins but
seems to act as a brevetoxin antagonist (Bourdelais et al. 2004; Bourdelais et al. 2005; Gold
et al. 2013). Gambiertoxin is produced by dinoflagellates from the genus Gambierdiscus.
They can be transformed into ciguatoxin in predatory fish guts, with molluscs being the
vector. It is a neurotoxin that is not destroyed by the cooking process and constitutes a
polyether with ring structures. Okadaic acid found in Dinophysis and Prorocentrum is
known to cause diarrheic shellfish poisoning by inhibiting protein phosphatases 1 and 2A
(Nielsen et al. 2016).
13.3 Fatty Acid-Derived Metabolites
Fatty acids, polyacetylenes and polyketides share the biosynthetic scheme of condensing
multiple acetate units and forming poly-β-keto-chains but differ in the chain extension
mechanism. Fatty acid composition varies between plant families. Very long-chain fatty
acids, which include the polyunsaturated fatty acid arachidonic acid, are characteristic
of algae, mosses and ferns. Flowering plants produce long-chain fatty acids with a chain
length of C16 and C18 and zero to three cis double bounds, for instance, linoleic acid.
Respectively, the derivatives can also vary between different plants (. Fig. 13.2). Many of
the fatty-acid derived metabolites accumulate in trichomes.
Chapter 13 · Minor Groups of Secondary Metabolites
13
Olivetolic acid is also the starting point for biosynthesis of other compounds, for
instance, primin, which accumulates in the trichomes of Primula obconica (Primulaceae)
and causes dermatitis.
In hops (Humulus lupulus, Cannabaceae) specialized trichomes, the so-called peltate
glandular trichomes, can be found especially at the base of hop cones. They contain, in
addition to terpenes, many C5-prenylated polyketides, with the major compound being
xanthohumol, a prenylflavonoid. Further oxidized prenylated polyketides are the socalled bitter acids such as humulone. These polyketides provide flavour for beer, especially
through the bitter taste of humulene. The non-ketone part of xanthohumol originates in
phenylpropanoid metabolism (see 7 Chap. 11), namely, chalconaringenin. Transfer of
DMAPP to chalconaringenin allows prenylation and yields desmethylxanthohumol, which
is methylated to form xanthohumol. Xanthohumol was found in significant amounts in
male hop plants, presumably due to the presence of lupulin glands on anthers (Nagel et al.
2008). Humulone is derived from leucine to form isovaleryl CoA and furthermore, after
the addition of three malonyl CoA, to phlorisovalerophenone. Two additional aromatic
prenylation steps, where the prenyl group is added via DMAPP, and final oxidation lead
to humulone.
Dinoflagellates seem to have specialized in the production of secondary metabolites
of polyketide origin (Rein and Snyder 2006; Van Wagoner et al. 2014). There is a frequent occurrence of five-, six-, seven-, eight- and even nine-membered all trans-fused
ether rings as a backbone. These rings can be found in isolation (e.g. amphidinolide), in
spiroketal formation (e.g. okadaic acid) or in as many as fourteen ether rings continuously
fused (e.g. brevetoxin). For dinoflagellates it was observed that the C1 carbons of acetate
were frequently missing, and pendant methyl groups were derived from acetate methyl, in
addition to S-adenosyl methionine (SAM). Brevetoxins, especially known from Karenia
brevis, have 10–11 fused ether rings and an unusual terminal aldehyde group derived from
an acetate methyl carbon. Brevetoxins are neurotoxins that bind to voltage-gated sodium
channels in nerve cells, causing neurotoxic shellfish poisoning. Brevenal has four fused
ether rings and appears not to share the sodium channel activity of the brevetoxins but
seems to act as a brevetoxin antagonist (Bourdelais et al. 2004; Bourdelais et al. 2005; Gold
et al. 2013). Gambiertoxin is produced by dinoflagellates from the genus Gambierdiscus.
They can be transformed into ciguatoxin in predatory fish guts, with molluscs being the
vector. It is a neurotoxin that is not destroyed by the cooking process and constitutes a
polyether with ring structures. Okadaic acid found in Dinophysis and Prorocentrum is
known to cause diarrheic shellfish poisoning by inhibiting protein phosphatases 1 and 2A
(Nielsen et al. 2016).
13.3 Fatty Acid-Derived Metabolites
Fatty acids, polyacetylenes and polyketides share the biosynthetic scheme of condensing
multiple acetate units and forming poly-β-keto-chains but differ in the chain extension
mechanism. Fatty acid composition varies between plant families. Very long-chain fatty
acids, which include the polyunsaturated fatty acid arachidonic acid, are characteristic
of algae, mosses and ferns. Flowering plants produce long-chain fatty acids with a chain
length of C16 and C18 and zero to three cis double bounds, for instance, linoleic acid.
Respectively, the derivatives can also vary between different plants (. Fig. 13.2). Many of
the fatty-acid derived metabolites accumulate in trichomes.
Chapter 13 · Minor Groups of Secondary Metabolites