Subsequent sequential ring-specific hydroxylation of
α-carotene and β-carotene by two different types of hydroxylases
generates yellow xanthophylls of lutein and zeaxanthin (Fig. 1).
The first type of hydroxylases includes two nonheme β-ring hydroxylases (BCH1 and BCH2) that primarily hydroxylate β-rings of
β-carotene to form β-cryptoxanthin and then zeaxanthin. The second type of hydroxylases contains two heme-containing cytochrome P450 type hydroxylases (CYP97C and CYP97A) that
mainly hydroxylates β- and ε-ring of α-carotene to synthesize lutein
[34]. Lutein is the most abundant carotenoid pigment in photosynthetic tissues of plants. The synthesis of lutein represents the
classic final product of the β,ε-branch in the carotenoid biosynthesis
pathway.
In the β, β-branch, zeaxanthin is epoxidized by zeaxanthin
epoxidase (ZEP) to sequentially produce antheraxanthin and then
violaxanthin. Violaxanthin can be reversed back to zeaxanthin via
light induced de-epoxidation by violaxanthin de-epoxidase (VDE).
Interconversion of zeaxanthin and violaxanthin forms the xanthophyll cycle. The ubiquitous xanthophyll cycle along with the taxonomically restricted lutein epoxide cycle is an important mechanism
to protect plants against photodamage [35]. The conversion of
violaxanthin into neoxanthin by neoxanthin synthase (NXS) finalizes the core biosynthetic pathway [36].
Carotenoid biosynthetic enzymes implement carotenoid production. It is believed that carotenogenic enzymes are organized in
multienzyme complexes to facilitate biosynthesis and metabolic
channeling [7, 37]. Two large complexes containing PDS were
found in daffodil chromoplasts and in Arabidopsis chloroplasts,
and PDS in the small complex is enzymatic activity and membrane
bound, and that in the large complex is inactive in stroma
[38, 39]. Recently, GGPS was discovered to form carotenoid
enzyme complex via physical interaction with PSY [22]. CYP97A
and CYP97C also interact to facilitate the carotene hydroxylation of
α-carotene [40]. In addition, other proteins like chaperones interact directly with carotenogenic proteins to assist enzyme functions.
A J-protein J20 specifically interacts with inactive form of DXS and
aids DXS destination either for activity or degradation
[41, 42]. The protein homeostasis and activity of PSY are counterbalanced by ORANGE (OR) protein and Clp protease through
direct physical associations [43–46].
2.3 Biosynthesis
in Bacteria, Fungi,
and Algae
Microalgae, photosynthetic bacteria, and some nonphotosynthetic
bacteria and fungi also synthesize carotenoids [2, 3]. Unlike plants
that synthesize mainly C40 carotenoids, some archaea and nonphotosynthetic bacteria also produce C30, C45 and C50 carotenoids. While most bacteria utilize MEP pathway as photosynthetic
organisms to produce C5 IPP and DMAPP, fungi and some bacteria use mevalonate (MVA) pathway to provide the precursors for
Pathways for Carotenoid Biosynthesis, Degradation, and Storage
7
α-carotene and β-carotene by two different types of hydroxylases
generates yellow xanthophylls of lutein and zeaxanthin (Fig. 1).
The first type of hydroxylases includes two nonheme β-ring hydroxylases (BCH1 and BCH2) that primarily hydroxylate β-rings of
β-carotene to form β-cryptoxanthin and then zeaxanthin. The second type of hydroxylases contains two heme-containing cytochrome P450 type hydroxylases (CYP97C and CYP97A) that
mainly hydroxylates β- and ε-ring of α-carotene to synthesize lutein
[34]. Lutein is the most abundant carotenoid pigment in photosynthetic tissues of plants. The synthesis of lutein represents the
classic final product of the β,ε-branch in the carotenoid biosynthesis
pathway.
In the β, β-branch, zeaxanthin is epoxidized by zeaxanthin
epoxidase (ZEP) to sequentially produce antheraxanthin and then
violaxanthin. Violaxanthin can be reversed back to zeaxanthin via
light induced de-epoxidation by violaxanthin de-epoxidase (VDE).
Interconversion of zeaxanthin and violaxanthin forms the xanthophyll cycle. The ubiquitous xanthophyll cycle along with the taxonomically restricted lutein epoxide cycle is an important mechanism
to protect plants against photodamage [35]. The conversion of
violaxanthin into neoxanthin by neoxanthin synthase (NXS) finalizes the core biosynthetic pathway [36].
Carotenoid biosynthetic enzymes implement carotenoid production. It is believed that carotenogenic enzymes are organized in
multienzyme complexes to facilitate biosynthesis and metabolic
channeling [7, 37]. Two large complexes containing PDS were
found in daffodil chromoplasts and in Arabidopsis chloroplasts,
and PDS in the small complex is enzymatic activity and membrane
bound, and that in the large complex is inactive in stroma
[38, 39]. Recently, GGPS was discovered to form carotenoid
enzyme complex via physical interaction with PSY [22]. CYP97A
and CYP97C also interact to facilitate the carotene hydroxylation of
α-carotene [40]. In addition, other proteins like chaperones interact directly with carotenogenic proteins to assist enzyme functions.
A J-protein J20 specifically interacts with inactive form of DXS and
aids DXS destination either for activity or degradation
[41, 42]. The protein homeostasis and activity of PSY are counterbalanced by ORANGE (OR) protein and Clp protease through
direct physical associations [43–46].
2.3 Biosynthesis
in Bacteria, Fungi,
and Algae
Microalgae, photosynthetic bacteria, and some nonphotosynthetic
bacteria and fungi also synthesize carotenoids [2, 3]. Unlike plants
that synthesize mainly C40 carotenoids, some archaea and nonphotosynthetic bacteria also produce C30, C45 and C50 carotenoids. While most bacteria utilize MEP pathway as photosynthetic
organisms to produce C5 IPP and DMAPP, fungi and some bacteria use mevalonate (MVA) pathway to provide the precursors for
Pathways for Carotenoid Biosynthesis, Degradation, and Storage
7