carotenoid biosynthesis [9]. In bacteria, the genes involved in
carotenoid biosynthesis are normally organized in operons. The
early biosynthetic pathway for C40 carotenoids is similar as in
plants, which comprises of GGPP synthase (CrtE), phytoene
synthase (CrtB), phytoene desaturase (CrtI), lycopene β-cyclase
(CrtY), and β-carotene hydroxylase (CrtZ) to produce GGPP,
phytoene, all-trans-lycopene, β-carotene, and zeaxanthin. A different biosynthetic pathway for C30 carotenoids is utilized in bacteria
[3]. In fungi, CrtE, CrtI and CrtYB, a bifunctional enzyme with
both phytoene synthase and lycopene cyclase activities, catalyze the
synthesis of γ-carotene or β-carotene. In contrast to plants that
utilize four enzymes to desaturate and isomerize phytoene into
all-trans-lycopene, a single CrtI catalyzes all the desaturation and
isomerization steps in bacteria and fungi. CrtZ and CrtW (carotene
ketolase) in marine bacteria convert β-carotene into astaxanthin,
whereas β-carotene ketolase (BKT) in green algae participates the
formation of astaxanthin, the red carotenoid with strong antioxidant activity.
2.4 Special
Carotenoid Products
in Plants
Plants produce a large number and diverse secondary metabolites
due to evolution for fitness and enzyme neofunctionalization.
More than 1000 chemical structures of carotenoids have been
documented in living organisms [47]. Besides the main carotenoid
products from the core biosynthetic pathway, some crop species
synthesize and accumulate high levels of unique carotenoid products, as exampled below.
Pepper (Capsicum annuum) is well-known for its unique carotenoid composition with 35% capsanthin, 6% capsorubin, and 4%
cryptocapsin of total carotenoids in red ripen fruit [48]. Capsanthin-capsorubin synthase (CCS) catalyzes antheraxanthin and
violaxanthin from the main pathway to generate capsanthin and
capsorubin, respectively, which are also the signature pigments of
tiger lily (Lilium lancifolium). CCS has high sequence similarity
with lycopene cyclases. It is also a most abundant protein in pepper
chromoplasts [49].
While most plants only have β,β-ring or β,ε-ring cyclization
products, green lettuce (Lactuca sativa) produces a rare type of
double ε-ring carotene lactucaxanthin by lettuce LCYE [50].
Several key amino acid residues of LCYE function as molecular
switches to determine single or double ε-ring cyclization
[51]. The large amount of lactucaxanthin in light-harvesting complexes of Photosystem II indicates its importance in lettuce [52].
A few Adonis species with blood-red flowers are the only known
land plants to produce ketocarotenoid astaxanthin. Two enzymes
carotenoid 4-hydroxy-β-ring 4-dehydrogenase (HBFD) and carotenoid β-ring 4-dehydrogenase (CBFD) catalyze three distinct reactions to convert β-carotene into astaxanthin in Adonis, utilizing a
strikingly different way unlike in bacteria, cyanobacteria, and green
8
Tianhu Sun et al.
carotenoid biosynthesis are normally organized in operons. The
early biosynthetic pathway for C40 carotenoids is similar as in
plants, which comprises of GGPP synthase (CrtE), phytoene
synthase (CrtB), phytoene desaturase (CrtI), lycopene β-cyclase
(CrtY), and β-carotene hydroxylase (CrtZ) to produce GGPP,
phytoene, all-trans-lycopene, β-carotene, and zeaxanthin. A different biosynthetic pathway for C30 carotenoids is utilized in bacteria
[3]. In fungi, CrtE, CrtI and CrtYB, a bifunctional enzyme with
both phytoene synthase and lycopene cyclase activities, catalyze the
synthesis of γ-carotene or β-carotene. In contrast to plants that
utilize four enzymes to desaturate and isomerize phytoene into
all-trans-lycopene, a single CrtI catalyzes all the desaturation and
isomerization steps in bacteria and fungi. CrtZ and CrtW (carotene
ketolase) in marine bacteria convert β-carotene into astaxanthin,
whereas β-carotene ketolase (BKT) in green algae participates the
formation of astaxanthin, the red carotenoid with strong antioxidant activity.
2.4 Special
Carotenoid Products
in Plants
Plants produce a large number and diverse secondary metabolites
due to evolution for fitness and enzyme neofunctionalization.
More than 1000 chemical structures of carotenoids have been
documented in living organisms [47]. Besides the main carotenoid
products from the core biosynthetic pathway, some crop species
synthesize and accumulate high levels of unique carotenoid products, as exampled below.
Pepper (Capsicum annuum) is well-known for its unique carotenoid composition with 35% capsanthin, 6% capsorubin, and 4%
cryptocapsin of total carotenoids in red ripen fruit [48]. Capsanthin-capsorubin synthase (CCS) catalyzes antheraxanthin and
violaxanthin from the main pathway to generate capsanthin and
capsorubin, respectively, which are also the signature pigments of
tiger lily (Lilium lancifolium). CCS has high sequence similarity
with lycopene cyclases. It is also a most abundant protein in pepper
chromoplasts [49].
While most plants only have β,β-ring or β,ε-ring cyclization
products, green lettuce (Lactuca sativa) produces a rare type of
double ε-ring carotene lactucaxanthin by lettuce LCYE [50].
Several key amino acid residues of LCYE function as molecular
switches to determine single or double ε-ring cyclization
[51]. The large amount of lactucaxanthin in light-harvesting complexes of Photosystem II indicates its importance in lettuce [52].
A few Adonis species with blood-red flowers are the only known
land plants to produce ketocarotenoid astaxanthin. Two enzymes
carotenoid 4-hydroxy-β-ring 4-dehydrogenase (HBFD) and carotenoid β-ring 4-dehydrogenase (CBFD) catalyze three distinct reactions to convert β-carotene into astaxanthin in Adonis, utilizing a
strikingly different way unlike in bacteria, cyanobacteria, and green
8
Tianhu Sun et al.