5 Carotenoid Overproduction in Microalgae: Biochemical …
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the astaxanthin source: commercial synthetic astaxanthin is a mixture of the enantiomers (3R,3’R and 3S,3’S) and the meso compound (3R,3’S) in the ratio of 1:1:2,
respectively (Visioli and Artaria 2017; Higuera-Ciapara et al. 2006) whereas astaxanthin extracted from H. pluvialis is only (3S,3’S) and mostly esterified (Visioli and
Artaria 2017). Interestingly, the isomer conformation is not modified during astaxanthin metabolism in animal cells and therefore the conformation can be used for tracing
the biological origin of astaxanthin found in fish for instance (Turujman et al. 1997).
Keto-carotenoids derive from β-carotene. The capacity of keto-carotenoid biosynthesis has been described in one land plant petals, some yeasts and a few microalgal
taxa (Chromochloris zofingiensis: Huang et al. (2006); Tetradesmus: Pirastru et al.
(2011), Chlorosarcinopsis sp: Cherdchukeattisak et al. (2018), screening: Kopecky
et al. (2000)), Haematococcus pluvialis being the most famous (Chen et al. 2020)
(see Table 5.3 for the synonymous taxa).
Several pathways have been proposed for astaxanthin biosynthesis denoted as
zeaxanthin and canthaxanthin pathways, respectively. They all involve β-carotene
ketolase (β-carotene 4,4’-oxygenase) (BKT-microalgae: Lotan and Hirschberg
(1995), Fraser et al. (1998); CRTW-cyanobacteria: Kajiwara et al. (1995), Choi et al.
(2007)), the enzyme adding sequentially the keto groups in C4 and C4’ positions on
the β-ionone rings of β-carotene, yielding echinenone (4-monoketo-β-carotene) and
canthaxanthin (4,4’-diketo-β-carotene), respectively (Breitenbach et al. 1996). BKT
enzymes are characterized by conserved histidine motifs postulated as binding nonhemic iron required for enzymatic activity (Fraser et al. 1997) on the β-ionone rings
of β-carotene. Interestingly, the S251W mutation reduced the capacity of BKT1
to catalyse β-carotene transformation, to canthaxanthin that instead catalyses the
ketolutein production, suggesting that this region of the enzyme could be related to
the α-ionone and β-ionone-ring selectivity (Ye and Huang 2019). This mutation is
located at the C-terminus of the enzyme amino acid sequence.
Canthaxanthin is then sequentially hydroxylased by β-carotene hydroxylase
(BHY) in C3 and C3’ positions, resulting in the formation of astaxanthin (Varela
et al. 2015; Sun et al. 1996; Schoefs et al. 2001). The production of keto-carotenoids
can be restricted to part of the algal life cycle such as the zygospores (Chlamydomonas: Lohr et al. (2005)). Astaxanthin accumulates in the cytoplasm in lipid
globules (Lemoine and Schoefs 2010).
A few microalgae such as Chlorosarcinopsis PY02 accumulate canthaxanthin
rather than astaxanthin. The biosynthetic pathway is similar to that found in Haematococcus pluvialis i.e. β-carotene is first hydroxylated twice to yield echinenone
and canthaxanthin, respectively by BKT enzymes. To explain the preferential accumulation of 3-OH-echinenone and canthaxanthin, Cherdchukeattisak et al. (2018)
hypothesize that both hydroxylase and ketolase compete for β-carotene and suggest
that the hydroxylase (BHY) is less active than the ketolase (BKT).
Two genes code for BKT enzymes, namely BKT1 and BKT2. BKT1, in addition
to catalyse the hydroxylation of β-carotene to canthaxanthin can also convert zeaxanthin to astaxanthin (Huang et al. 2018; Wang and Chen 2008). A single nucleotide
insertion (+G395) or substitution in highly conserved regions (H168R, 552K, P284L
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the astaxanthin source: commercial synthetic astaxanthin is a mixture of the enantiomers (3R,3’R and 3S,3’S) and the meso compound (3R,3’S) in the ratio of 1:1:2,
respectively (Visioli and Artaria 2017; Higuera-Ciapara et al. 2006) whereas astaxanthin extracted from H. pluvialis is only (3S,3’S) and mostly esterified (Visioli and
Artaria 2017). Interestingly, the isomer conformation is not modified during astaxanthin metabolism in animal cells and therefore the conformation can be used for tracing
the biological origin of astaxanthin found in fish for instance (Turujman et al. 1997).
Keto-carotenoids derive from β-carotene. The capacity of keto-carotenoid biosynthesis has been described in one land plant petals, some yeasts and a few microalgal
taxa (Chromochloris zofingiensis: Huang et al. (2006); Tetradesmus: Pirastru et al.
(2011), Chlorosarcinopsis sp: Cherdchukeattisak et al. (2018), screening: Kopecky
et al. (2000)), Haematococcus pluvialis being the most famous (Chen et al. 2020)
(see Table 5.3 for the synonymous taxa).
Several pathways have been proposed for astaxanthin biosynthesis denoted as
zeaxanthin and canthaxanthin pathways, respectively. They all involve β-carotene
ketolase (β-carotene 4,4’-oxygenase) (BKT-microalgae: Lotan and Hirschberg
(1995), Fraser et al. (1998); CRTW-cyanobacteria: Kajiwara et al. (1995), Choi et al.
(2007)), the enzyme adding sequentially the keto groups in C4 and C4’ positions on
the β-ionone rings of β-carotene, yielding echinenone (4-monoketo-β-carotene) and
canthaxanthin (4,4’-diketo-β-carotene), respectively (Breitenbach et al. 1996). BKT
enzymes are characterized by conserved histidine motifs postulated as binding nonhemic iron required for enzymatic activity (Fraser et al. 1997) on the β-ionone rings
of β-carotene. Interestingly, the S251W mutation reduced the capacity of BKT1
to catalyse β-carotene transformation, to canthaxanthin that instead catalyses the
ketolutein production, suggesting that this region of the enzyme could be related to
the α-ionone and β-ionone-ring selectivity (Ye and Huang 2019). This mutation is
located at the C-terminus of the enzyme amino acid sequence.
Canthaxanthin is then sequentially hydroxylased by β-carotene hydroxylase
(BHY) in C3 and C3’ positions, resulting in the formation of astaxanthin (Varela
et al. 2015; Sun et al. 1996; Schoefs et al. 2001). The production of keto-carotenoids
can be restricted to part of the algal life cycle such as the zygospores (Chlamydomonas: Lohr et al. (2005)). Astaxanthin accumulates in the cytoplasm in lipid
globules (Lemoine and Schoefs 2010).
A few microalgae such as Chlorosarcinopsis PY02 accumulate canthaxanthin
rather than astaxanthin. The biosynthetic pathway is similar to that found in Haematococcus pluvialis i.e. β-carotene is first hydroxylated twice to yield echinenone
and canthaxanthin, respectively by BKT enzymes. To explain the preferential accumulation of 3-OH-echinenone and canthaxanthin, Cherdchukeattisak et al. (2018)
hypothesize that both hydroxylase and ketolase compete for β-carotene and suggest
that the hydroxylase (BHY) is less active than the ketolase (BKT).
Two genes code for BKT enzymes, namely BKT1 and BKT2. BKT1, in addition
to catalyse the hydroxylation of β-carotene to canthaxanthin can also convert zeaxanthin to astaxanthin (Huang et al. 2018; Wang and Chen 2008). A single nucleotide
insertion (+G395) or substitution in highly conserved regions (H168R, 552K, P284L