2 Carotenoids in Phototrophic Microalgae …
31
2.3.3 β-Carotene Derivatives and Their Synthesis
2.3.3.1 Cyanobacteria
Some cyanobacteria produce zeaxanthin, and some produce both zeaxanthin and
nostoxanthin (Fig. 2.3). First, the C-3 and C-3
hydroxyl groups of zeaxanthin
are introduced to β-carotene by β-carotene hydroxylase (CrtR, CrtR-b) via βcryptoxanthin. Then, the C-2 and C-2
hydroxyl groups of nostoxanthin are introduced by 2,2
-β-hydroxylase (CrtG) via caloxanthin (Table 2.2) (Masamoto et al.
1998, Mochimaru et al. 2008; Makino et al. 2008; Iwai et al. 2008). The same
enzymes, CrtR and CrtG, can also introduce hydroxyl groups to deoxymyxol and
myxol to produce myxol and 2-hydroxymyxol, respectively (Lagarde and Vermaas
1999; Takaichi and Mochimaru 2007; Iwai et al. 2008); consequently, the same
enzymes are used in two pathways, and they have no sequence homology (Takaichi
and Mochimaru 2007).
Cyanobacteria contain ketocarotenoids, namely, echinenone, canthaxanthin, and
4-ketomyxol, and two distinct β-carotene ketolases, CrtO and CrtW, are known
(Table 2.2) (Takaichi and Mochimaru 2007). CrtO catalyzes β-carotene to echinenone, and the final product is canthaxanthin (Fernández-González et al. 1997;
Lagarde et al. 2000; Steiger et al. 2005; Mochimaru et al. 2005; Makino et al. 2008).
CrtW can introduce a keto group into β-carotene, zeaxanthin, and myxol to produce
canthaxanthin, astaxanthin, and 4-ketomyxol, respectively (Fig. 2.3) (Steiger and
Sandmann 2004; Mochimaru et al. 2005; Steiger et al. 2005; Tsuchiya et al. 2005;
Makino et al. 2008); therefore, these ketolases are used in two pathways, β-carotene,
and myxol, depending on the species (Takaichi and Mochimaru 2007).
The pathway and the enzymes required to produce the right half of myxol 2
-
pentoside compound are, however, not clear (Fig. 2.3) (Takaichi and Mochimaru
2007); although, two enzymes, carotene 1
,2
-hydratase (CruF) and glycosyltransferase (CruG), have been functionally confirmed in Synechococcus sp. PPC 7002
(Graham and Bryant 2009).
2.3.3.2 Land Plants
Most of the carotenogenic pathways in land plants are known, and their enzymes
have been functionally confirmed (Fig. 2.2). Hydroxyl groups are introduced by
β-carotene hydroxylase (CrtR, CrtR-b, BCH) to β-carotene to produce zeaxanthin.
Epoxy groups are introduced into zeaxanthin by zeaxanthin epoxidase (Zep, NPQ) to
produce violaxanthin via antheraxanthin. Under high light conditions, violaxanthin
is converted into zeaxanthin by violaxanthin de-epoxidase (Vde) to disperse excess
energy from excited chlorophylls. One end group of violaxanthin is converted to an
allene group of neoxanthin by neoxanthin synthase (Nsy). Because all neoxanthin
in chloroplasts is in the 9
-cis form, an unknown 9
-isomerase that converts all-trans
neoxanthin to 9
-cis neoxanthin is likely to be present (Takaichi and Mimuro 1998).
31
2.3.3 β-Carotene Derivatives and Their Synthesis
2.3.3.1 Cyanobacteria
Some cyanobacteria produce zeaxanthin, and some produce both zeaxanthin and
nostoxanthin (Fig. 2.3). First, the C-3 and C-3
hydroxyl groups of zeaxanthin
are introduced to β-carotene by β-carotene hydroxylase (CrtR, CrtR-b) via βcryptoxanthin. Then, the C-2 and C-2
hydroxyl groups of nostoxanthin are introduced by 2,2
-β-hydroxylase (CrtG) via caloxanthin (Table 2.2) (Masamoto et al.
1998, Mochimaru et al. 2008; Makino et al. 2008; Iwai et al. 2008). The same
enzymes, CrtR and CrtG, can also introduce hydroxyl groups to deoxymyxol and
myxol to produce myxol and 2-hydroxymyxol, respectively (Lagarde and Vermaas
1999; Takaichi and Mochimaru 2007; Iwai et al. 2008); consequently, the same
enzymes are used in two pathways, and they have no sequence homology (Takaichi
and Mochimaru 2007).
Cyanobacteria contain ketocarotenoids, namely, echinenone, canthaxanthin, and
4-ketomyxol, and two distinct β-carotene ketolases, CrtO and CrtW, are known
(Table 2.2) (Takaichi and Mochimaru 2007). CrtO catalyzes β-carotene to echinenone, and the final product is canthaxanthin (Fernández-González et al. 1997;
Lagarde et al. 2000; Steiger et al. 2005; Mochimaru et al. 2005; Makino et al. 2008).
CrtW can introduce a keto group into β-carotene, zeaxanthin, and myxol to produce
canthaxanthin, astaxanthin, and 4-ketomyxol, respectively (Fig. 2.3) (Steiger and
Sandmann 2004; Mochimaru et al. 2005; Steiger et al. 2005; Tsuchiya et al. 2005;
Makino et al. 2008); therefore, these ketolases are used in two pathways, β-carotene,
and myxol, depending on the species (Takaichi and Mochimaru 2007).
The pathway and the enzymes required to produce the right half of myxol 2
-
pentoside compound are, however, not clear (Fig. 2.3) (Takaichi and Mochimaru
2007); although, two enzymes, carotene 1
,2
-hydratase (CruF) and glycosyltransferase (CruG), have been functionally confirmed in Synechococcus sp. PPC 7002
(Graham and Bryant 2009).
2.3.3.2 Land Plants
Most of the carotenogenic pathways in land plants are known, and their enzymes
have been functionally confirmed (Fig. 2.2). Hydroxyl groups are introduced by
β-carotene hydroxylase (CrtR, CrtR-b, BCH) to β-carotene to produce zeaxanthin.
Epoxy groups are introduced into zeaxanthin by zeaxanthin epoxidase (Zep, NPQ) to
produce violaxanthin via antheraxanthin. Under high light conditions, violaxanthin
is converted into zeaxanthin by violaxanthin de-epoxidase (Vde) to disperse excess
energy from excited chlorophylls. One end group of violaxanthin is converted to an
allene group of neoxanthin by neoxanthin synthase (Nsy). Because all neoxanthin
in chloroplasts is in the 9
-cis form, an unknown 9
-isomerase that converts all-trans
neoxanthin to 9
-cis neoxanthin is likely to be present (Takaichi and Mimuro 1998).