algae [53]. In the later cases, astaxanthin is formed from β-carotene
by 3-hydroxylation and 4-ketolation of β-rings catalyzed by BCH
and BKT (CrtW), respectively. Ketolation is usually not found in
plants [54, 55]. Since astaxanthin has strong antioxidant activity
and is widely used in aquaculture supplements, great efforts have
been done to metabolically engineer by introducing BKT with or
without BCH for astaxanthin production in crops [56–59].
3 Carotenoid Degradation Pathways
3.1 Nonspecific
Cleavage
of Carotenoids
The electron-rich polyene chain of carotenoids makes them a group
of unstable compounds. Carotenoids are susceptible to oxidative
breakdown by nonspecific mechanism such as photochemical
oxidation, and oxidation by nonspecific enzymes including lipoxygenases and peroxidases. The nonspecific oxidization of carotenoids leads to the generation of unspecific apocarotenoid products
by random cleavage. Environment factors such as oxygen level,
temperature, and light affect carotenoid oxidative degradation
[60]. Nonenzymatic oxidation of carotenoids by reactive oxygen
species oxidizes carotenoids to produce a variety of oxidized products, some of which such as β-cyclocitral act as stress signals in
plants [61] (Fig. 2). Lipoxygenases are a family of nonheme ironcontaining enzymes that mainly catalyze the oxidation of polyunsaturated fatty acids to yield hydroperoxides. In the presence of
polyunsaturated fatty acids, lipoxygenases co-oxidize carotenoids
by random attack of the carotenoid molecules to produce unspecific
cleavage products [62].
3.2 Enzymatic
Oxidative Cleavage
of Carotenoids
Carotenoids also undergo specific enzymatic oxidative breakdown
to produce apocarotenoids in living organisms [11, 63]. This process is catalyzed by a family of carotenoid cleavage dioxygenases
(CCDs) or sometime referred to as carotenoid cleavage oxygenases
(CCOs). CCDs are a group of nonheme iron-containing enzymes
that cleave double bonds in the polyene chain of carotenoids. They
are widespread not only in carotenogenic organisms of plants,
algae, bacteria and fungi, but also in animals. In plants, members
of the CCD family are typically named based on their sequence
similarities to the Arabidopsis CCD enzymes. They are generally
divided into two functionally different groups: 9-cis-epoxycarotenoid dioxygenases (NCEDs) that cleave 9-cis-violaxanthin and
9-cis-neoxanthin into xanthoxin for ABA biosynthesis [64, 65],
and CCDs (CCD1, 2, 4, 7 and 8) that have different substrate
specificities and catalyze different cleavage steps to produce a large
number of apocarotenoids.
The NCEDs are exclusively involved in the production of ABA,
an important hormone in regulating plant responses to various
stresses and seed maturation. NCEDs normally exist as family
Pathways for Carotenoid Biosynthesis, Degradation, and Storage
9
by 3-hydroxylation and 4-ketolation of β-rings catalyzed by BCH
and BKT (CrtW), respectively. Ketolation is usually not found in
plants [54, 55]. Since astaxanthin has strong antioxidant activity
and is widely used in aquaculture supplements, great efforts have
been done to metabolically engineer by introducing BKT with or
without BCH for astaxanthin production in crops [56–59].
3 Carotenoid Degradation Pathways
3.1 Nonspecific
Cleavage
of Carotenoids
The electron-rich polyene chain of carotenoids makes them a group
of unstable compounds. Carotenoids are susceptible to oxidative
breakdown by nonspecific mechanism such as photochemical
oxidation, and oxidation by nonspecific enzymes including lipoxygenases and peroxidases. The nonspecific oxidization of carotenoids leads to the generation of unspecific apocarotenoid products
by random cleavage. Environment factors such as oxygen level,
temperature, and light affect carotenoid oxidative degradation
[60]. Nonenzymatic oxidation of carotenoids by reactive oxygen
species oxidizes carotenoids to produce a variety of oxidized products, some of which such as β-cyclocitral act as stress signals in
plants [61] (Fig. 2). Lipoxygenases are a family of nonheme ironcontaining enzymes that mainly catalyze the oxidation of polyunsaturated fatty acids to yield hydroperoxides. In the presence of
polyunsaturated fatty acids, lipoxygenases co-oxidize carotenoids
by random attack of the carotenoid molecules to produce unspecific
cleavage products [62].
3.2 Enzymatic
Oxidative Cleavage
of Carotenoids
Carotenoids also undergo specific enzymatic oxidative breakdown
to produce apocarotenoids in living organisms [11, 63]. This process is catalyzed by a family of carotenoid cleavage dioxygenases
(CCDs) or sometime referred to as carotenoid cleavage oxygenases
(CCOs). CCDs are a group of nonheme iron-containing enzymes
that cleave double bonds in the polyene chain of carotenoids. They
are widespread not only in carotenogenic organisms of plants,
algae, bacteria and fungi, but also in animals. In plants, members
of the CCD family are typically named based on their sequence
similarities to the Arabidopsis CCD enzymes. They are generally
divided into two functionally different groups: 9-cis-epoxycarotenoid dioxygenases (NCEDs) that cleave 9-cis-violaxanthin and
9-cis-neoxanthin into xanthoxin for ABA biosynthesis [64, 65],
and CCDs (CCD1, 2, 4, 7 and 8) that have different substrate
specificities and catalyze different cleavage steps to produce a large
number of apocarotenoids.
The NCEDs are exclusively involved in the production of ABA,
an important hormone in regulating plant responses to various
stresses and seed maturation. NCEDs normally exist as family
Pathways for Carotenoid Biosynthesis, Degradation, and Storage
9