light in habitat. Since parietin absorbs light, it may help to protect the photosynthetic
apparatus of the photobiont against damage by high light levels [36, 84]. Solhaug
and Gauslaa [85] reported that UV-B radiation may trigger the resynthesis of this
cortical pigment parietin (= physcion) in the lichen Xanthoria parietina. Despite the
long-term study of parietin, we still cannot claim that this secondary metabolite
serves as UV-B or PAR screening pigment [86]. It is possible that parietin also acts as
an antioxidant [87].
4.1.2 Mevalonate Pathway
The terpenoids form a large and structurally diverse family of natural products
derived from C 5 isoprene units joined in a head-to-tail fashion. Isoprene is produced
naturally but is not involved in the formation of these compounds, and the biochemically active isoprene units were identified as the diphosphate esters – dimethylallyl
pyrophosphate and isopenthenyl pyrophosphate.
Two molecules of acetyl-coenzyme A combine initially in a Claisen condensation
to give acetoacetyl-CoA, and a third is incorporated via a stereospecific aldol
addition giving the branched-chain ester. The thioester is then reduced to primary
alcohol via hemithioacetal and aldehyde to give mevalonic acid. The six-carbon
mevalonic acid is then transformed into the five-carbon phosphorylated isoprene
units in a series of reactions, beginning with phosphorylation of the primary alcohol
group. Two different ATP-dependent enzymes are involved, resulting in mevalonic
acid diphosphate, and decarboxylation/dehydration then follows to give isopentenyl
pyrophosphate. Combination of two isoprene units head-to-tail forms monoterpenes.
Limonene is formed by cyclization reactions of geranyl pyrophosphate. Diterpenes
consist of four isoprene units. Geranyl PP reacts with first isoprene unit to give
farnesyl PP which reacts with second isoprene to give geranylgeranyl PP. Phytol is
one of the best known diterpenes. Triterpenes are derived from squalene, six
isoprene units containing compound. Steroids are then formed by cyclization of
squalene. Another well-known class of terpenoids – carotenes – are derivatives of
tetraterpene lycopene, which is formed by combination of two geranylgeranyl PP
units. Classes of lichen substances that are derived by mevalonate pathway are
terpenes, steroids, and carotenoids (Fig. 23).
Terpenes
Terpenes are large and diverse class of organic compounds. Terpenes are formed
biosynthetically from units of isopentenyl pyrophosphate, which is the product of
mevalonate pathway.
Limonene
In study of Kahriman et al. [88], the antimicrobial and antifungal activity of the
essential oil obtained by hydrodistillation from Evernia prunastri (L.) Ach. and
Evernia divaricata (L.) has been analyzed. The major substances in the essential oil
of Evernia prunastri and Evernia divaricata were β-pinene (6.3 and 8.0%), α-pinene
(6.6%, 7.2%), limonene (1.6%, 6.3%), α-phellandrene (3.3%, 4.4%), camphene
(3.0%, 3.1%), and p-cymene (1.5%, 1.8%), respectively. The antimicrobial and
194
M. Goga et al.
apparatus of the photobiont against damage by high light levels [36, 84]. Solhaug
and Gauslaa [85] reported that UV-B radiation may trigger the resynthesis of this
cortical pigment parietin (= physcion) in the lichen Xanthoria parietina. Despite the
long-term study of parietin, we still cannot claim that this secondary metabolite
serves as UV-B or PAR screening pigment [86]. It is possible that parietin also acts as
an antioxidant [87].
4.1.2 Mevalonate Pathway
The terpenoids form a large and structurally diverse family of natural products
derived from C 5 isoprene units joined in a head-to-tail fashion. Isoprene is produced
naturally but is not involved in the formation of these compounds, and the biochemically active isoprene units were identified as the diphosphate esters – dimethylallyl
pyrophosphate and isopenthenyl pyrophosphate.
Two molecules of acetyl-coenzyme A combine initially in a Claisen condensation
to give acetoacetyl-CoA, and a third is incorporated via a stereospecific aldol
addition giving the branched-chain ester. The thioester is then reduced to primary
alcohol via hemithioacetal and aldehyde to give mevalonic acid. The six-carbon
mevalonic acid is then transformed into the five-carbon phosphorylated isoprene
units in a series of reactions, beginning with phosphorylation of the primary alcohol
group. Two different ATP-dependent enzymes are involved, resulting in mevalonic
acid diphosphate, and decarboxylation/dehydration then follows to give isopentenyl
pyrophosphate. Combination of two isoprene units head-to-tail forms monoterpenes.
Limonene is formed by cyclization reactions of geranyl pyrophosphate. Diterpenes
consist of four isoprene units. Geranyl PP reacts with first isoprene unit to give
farnesyl PP which reacts with second isoprene to give geranylgeranyl PP. Phytol is
one of the best known diterpenes. Triterpenes are derived from squalene, six
isoprene units containing compound. Steroids are then formed by cyclization of
squalene. Another well-known class of terpenoids – carotenes – are derivatives of
tetraterpene lycopene, which is formed by combination of two geranylgeranyl PP
units. Classes of lichen substances that are derived by mevalonate pathway are
terpenes, steroids, and carotenoids (Fig. 23).
Terpenes
Terpenes are large and diverse class of organic compounds. Terpenes are formed
biosynthetically from units of isopentenyl pyrophosphate, which is the product of
mevalonate pathway.
Limonene
In study of Kahriman et al. [88], the antimicrobial and antifungal activity of the
essential oil obtained by hydrodistillation from Evernia prunastri (L.) Ach. and
Evernia divaricata (L.) has been analyzed. The major substances in the essential oil
of Evernia prunastri and Evernia divaricata were β-pinene (6.3 and 8.0%), α-pinene
(6.6%, 7.2%), limonene (1.6%, 6.3%), α-phellandrene (3.3%, 4.4%), camphene
(3.0%, 3.1%), and p-cymene (1.5%, 1.8%), respectively. The antimicrobial and
194
M. Goga et al.