2.2 Carotenoid
Biosynthetic Pathway
in Plants
The first committed step for carotenoid biosynthesis in plants is the
head-to-head condensation of two GGPP molecules by phytoene
synthase (PSY) to produce the first C40 carotenoid product 15-cisphytoene (Fig. 1). This reaction is generally considered as a major
rate-limiting step of carotenogenesis [1]. PSY typically comprises a
small family with up to three members in many crops although only
one in the model plant Arabidopsis [25]. Due to its crucial role in
governing carotenoid pool size, PSY has been subjected to intensive investigation and is a major target for carotenoid metabolic
engineering [5, 26]. After sequential desaturations and isomerizations, 15-cis-phytoene is converted to all-trans-lycopene (Fig. 1).
These reactions are catalyzed by phytoene desaturase (PDS) to
introduce two double bonds into 15-cis-phytoene via 9,15-di-cisphytofluene to form 9,15,9
0 -tri-cis-ζ-carotene. Then ζ-carotene
isomerase (Z-ISO), a last core enzyme discovered in the carotenoid
biosynthetic pathway, converts it into 9,9
0 -di-cis-ζ-carotene
[27]. The product is desaturated via ζ-carotene desaturase (ZDS)
to introduce two additional double bonds and form 7,9,9
0 ,7
0 -tetracis-lycopene (prolycopene). Carotenoid isomerase, CRTISO, catalyzes the tetra-cis-lycopene into all-trans-lycopene. The activities of
PDS and ZDS are associated with an electron transport mechanism,
which requires the operation of plastidial terminal oxidase (PTOX)
and plastoquinone [28]. Impaired plastoquinone biosynthesis in
plastids results in only low levels of phytoene desaturation
[29]. The proper function of isomerases also requires cofactors. A
heme cofactor is involved in Z-ISO function in redox-regulated
manner [30] and a FAD cofactor is used for CRTISO [31] in
addition for PDS and ZDS.
Cyclization of lycopene starts the branching point of carotenogenic pathway (Fig. 1) and is important for generating carotenoid
diversity. Two cyclases, lycopene ε-cyclase (LCYE) and lycopene
β-cyclase (LCYB), catalyze the reactions to form carotenoids with
ε- and/or β-ring in β,ε- and β,β-branch. LCYB cyclizes both ends of
lycopene to produce β-carotene with β,β-rings, whereas LCYE and
LCYB coordinately act to generate α-carotene with β,ε-rings.
Naturally, biased expression between LCYE and LCYB results in
disturbed accumulation of β,ε- and β,β-branch carotenoids. For
example, reduced expression of LCYE favors β-carotene accumulation, and absence of LCYB activity produces several unusual carotenes, including δ-carotene, ε-carotene and lactucaxanthin in
maize kernels [32, 33]. Thus, LCYE and LCYB activities play a
major role in branching the metabolic flux to β,ε- and β,β-branch.
Their direct products β-carotene and α-carotene are the predominant provitamin A carotenoids found in fruits and vegetables. The
production of β-carotene and α-carotene also represent the end of
carotene biosynthesis (Fig. 1).
6
Tianhu Sun et al.
Biosynthetic Pathway
in Plants
The first committed step for carotenoid biosynthesis in plants is the
head-to-head condensation of two GGPP molecules by phytoene
synthase (PSY) to produce the first C40 carotenoid product 15-cisphytoene (Fig. 1). This reaction is generally considered as a major
rate-limiting step of carotenogenesis [1]. PSY typically comprises a
small family with up to three members in many crops although only
one in the model plant Arabidopsis [25]. Due to its crucial role in
governing carotenoid pool size, PSY has been subjected to intensive investigation and is a major target for carotenoid metabolic
engineering [5, 26]. After sequential desaturations and isomerizations, 15-cis-phytoene is converted to all-trans-lycopene (Fig. 1).
These reactions are catalyzed by phytoene desaturase (PDS) to
introduce two double bonds into 15-cis-phytoene via 9,15-di-cisphytofluene to form 9,15,9
0 -tri-cis-ζ-carotene. Then ζ-carotene
isomerase (Z-ISO), a last core enzyme discovered in the carotenoid
biosynthetic pathway, converts it into 9,9
0 -di-cis-ζ-carotene
[27]. The product is desaturated via ζ-carotene desaturase (ZDS)
to introduce two additional double bonds and form 7,9,9
0 ,7
0 -tetracis-lycopene (prolycopene). Carotenoid isomerase, CRTISO, catalyzes the tetra-cis-lycopene into all-trans-lycopene. The activities of
PDS and ZDS are associated with an electron transport mechanism,
which requires the operation of plastidial terminal oxidase (PTOX)
and plastoquinone [28]. Impaired plastoquinone biosynthesis in
plastids results in only low levels of phytoene desaturation
[29]. The proper function of isomerases also requires cofactors. A
heme cofactor is involved in Z-ISO function in redox-regulated
manner [30] and a FAD cofactor is used for CRTISO [31] in
addition for PDS and ZDS.
Cyclization of lycopene starts the branching point of carotenogenic pathway (Fig. 1) and is important for generating carotenoid
diversity. Two cyclases, lycopene ε-cyclase (LCYE) and lycopene
β-cyclase (LCYB), catalyze the reactions to form carotenoids with
ε- and/or β-ring in β,ε- and β,β-branch. LCYB cyclizes both ends of
lycopene to produce β-carotene with β,β-rings, whereas LCYE and
LCYB coordinately act to generate α-carotene with β,ε-rings.
Naturally, biased expression between LCYE and LCYB results in
disturbed accumulation of β,ε- and β,β-branch carotenoids. For
example, reduced expression of LCYE favors β-carotene accumulation, and absence of LCYB activity produces several unusual carotenes, including δ-carotene, ε-carotene and lactucaxanthin in
maize kernels [32, 33]. Thus, LCYE and LCYB activities play a
major role in branching the metabolic flux to β,ε- and β,β-branch.
Their direct products β-carotene and α-carotene are the predominant provitamin A carotenoids found in fruits and vegetables. The
production of β-carotene and α-carotene also represent the end of
carotene biosynthesis (Fig. 1).
6
Tianhu Sun et al.