96
M. Scarsini et al.
produced in the cytoplasm as pre-proteins before being imported into the chloroplast. Carotenoid biosynthetic pathway has been the topic of several recent review
papers (Cherdchukeattisak et al. 2018; Gateau et al. 2017). In most microalgae, the
carotenoid biosynthetic pathway is highly conserved thought that some species are
able to accumulate unusual carotenoids via biosynthetic appendices such as the one
resulting in the formation of keto-carotenoids and allelic carotenoids.
Carotenoids are synthesized from a C5 building block, the isopentenyl pyrophosphate (IPP) and its isomer, the dimethylallyl pyrophosphate (DMAPP). The isomerization step is catalyzed by IPP isomerase, coded by the gene ipi. Two pathways resulting in the production of these compounds have been described. The
first one, denoted the mevalonate pathway occurs in animals, fungi, archaebacteria and certain bacteria. In eukaryotic cells, it is localized in the cytoplasm. The
second pathway, the 2-C-methyl-D-erythriol 4-phosphate (MEP) pathway, is active in
many eubacteria, including Escherichia coli, apicomplexa, algae and land plants. In
some taxa, both pathways can be active (Table 5.6). In eukaryotic organisms, MEP
pathway is located in plastids. This pathway involves several enzymes namely 1deoxy-D-xylulose-5-phosphate synthase (DXS), 1-deoxy-D-xylulose-5-phosphate
reductosiomerase (DXR), 2-C-methyl-D-erythritol 4 phosphate (ISPD), 2-methyl-Derythritol 2,4-cyclodiphosphate synthase (ISPF), E-4-hydroxy-3-methylbut-2-enyl
diphosphate synthase (HDS), 4-hydroxy-3-methylbut-2-en-1-yl diphosphate reductase (HDR), isopentenyl diphosphate delta isomerase (IDI). The IPP building blocks
are then assembled to yield geranyl-geranyl pyrophosphate. The condensation of two
molecules of geranyl geranyl pyrophosphate (GGPP), catalyzed by the phytoene
synthase (cyanobacteria: CrtB, microalgae: PSY) yields phytoene, a colourless
compounds. CrtB/PSY enzyme is coded by a single gene or gene family (MeléndezMartínez et al. 2015). Sequential desaturations catalysed by phytoene desaturase
(cyanobacteria: CrtP, microalgae: PDS) and ζ-carotene desaturase (cyanobacteria:
CrtQ, microalgae: ZDS) resulting in the formation of pro-lycopene. Inhibition of
the PDS enzymatic activity by herbicides such as norflurazon (5-amino-4-chloro-2[3-(trifluoromethyl)phenyl]pyridazin-3-one) and fluridone (1-methyl-3-phenyl-5-[3(trifluoromethyl)phenyl]pyridin-4-one) (Ben-Amotz et al. 1988) results in the accumulation of phytoene (Laje et al. 2019; Zhekisheva et al. 2005) and the absence of
further carotenoids. Therefore, when applied in dividing microalgae, the carotenoid
cellular quota decreases by dilution within the daughter cells (Laje et al. 2019).
Pro-lycopene is then isomerized by a carotenoid isomerase (CRTISO) into alltrans-lycopene that serves as a precursor for the cyclases. Lycopene can be either
cyclized at both ends by lycopene β-cyclase (LCYB), yielding β-carotene with two
β–ionone end groups or to the combined actions of LCYB and lycopene ε-cyclases
(LCYEs) resulting in the formation of α-carotene. The amount of each carotenoid
type is determined by the absolute activities of LCYE and LCYB. α-carotene and
β-carotene are modified allowing the structural diversification of carotenoids.
M. Scarsini et al.
produced in the cytoplasm as pre-proteins before being imported into the chloroplast. Carotenoid biosynthetic pathway has been the topic of several recent review
papers (Cherdchukeattisak et al. 2018; Gateau et al. 2017). In most microalgae, the
carotenoid biosynthetic pathway is highly conserved thought that some species are
able to accumulate unusual carotenoids via biosynthetic appendices such as the one
resulting in the formation of keto-carotenoids and allelic carotenoids.
Carotenoids are synthesized from a C5 building block, the isopentenyl pyrophosphate (IPP) and its isomer, the dimethylallyl pyrophosphate (DMAPP). The isomerization step is catalyzed by IPP isomerase, coded by the gene ipi. Two pathways resulting in the production of these compounds have been described. The
first one, denoted the mevalonate pathway occurs in animals, fungi, archaebacteria and certain bacteria. In eukaryotic cells, it is localized in the cytoplasm. The
second pathway, the 2-C-methyl-D-erythriol 4-phosphate (MEP) pathway, is active in
many eubacteria, including Escherichia coli, apicomplexa, algae and land plants. In
some taxa, both pathways can be active (Table 5.6). In eukaryotic organisms, MEP
pathway is located in plastids. This pathway involves several enzymes namely 1deoxy-D-xylulose-5-phosphate synthase (DXS), 1-deoxy-D-xylulose-5-phosphate
reductosiomerase (DXR), 2-C-methyl-D-erythritol 4 phosphate (ISPD), 2-methyl-Derythritol 2,4-cyclodiphosphate synthase (ISPF), E-4-hydroxy-3-methylbut-2-enyl
diphosphate synthase (HDS), 4-hydroxy-3-methylbut-2-en-1-yl diphosphate reductase (HDR), isopentenyl diphosphate delta isomerase (IDI). The IPP building blocks
are then assembled to yield geranyl-geranyl pyrophosphate. The condensation of two
molecules of geranyl geranyl pyrophosphate (GGPP), catalyzed by the phytoene
synthase (cyanobacteria: CrtB, microalgae: PSY) yields phytoene, a colourless
compounds. CrtB/PSY enzyme is coded by a single gene or gene family (MeléndezMartínez et al. 2015). Sequential desaturations catalysed by phytoene desaturase
(cyanobacteria: CrtP, microalgae: PDS) and ζ-carotene desaturase (cyanobacteria:
CrtQ, microalgae: ZDS) resulting in the formation of pro-lycopene. Inhibition of
the PDS enzymatic activity by herbicides such as norflurazon (5-amino-4-chloro-2[3-(trifluoromethyl)phenyl]pyridazin-3-one) and fluridone (1-methyl-3-phenyl-5-[3(trifluoromethyl)phenyl]pyridin-4-one) (Ben-Amotz et al. 1988) results in the accumulation of phytoene (Laje et al. 2019; Zhekisheva et al. 2005) and the absence of
further carotenoids. Therefore, when applied in dividing microalgae, the carotenoid
cellular quota decreases by dilution within the daughter cells (Laje et al. 2019).
Pro-lycopene is then isomerized by a carotenoid isomerase (CRTISO) into alltrans-lycopene that serves as a precursor for the cyclases. Lycopene can be either
cyclized at both ends by lycopene β-cyclase (LCYB), yielding β-carotene with two
β–ionone end groups or to the combined actions of LCYB and lycopene ε-cyclases
(LCYEs) resulting in the formation of α-carotene. The amount of each carotenoid
type is determined by the absolute activities of LCYE and LCYB. α-carotene and
β-carotene are modified allowing the structural diversification of carotenoids.