Carotenoids are essential natural pigments in plants, determining both the color of the plant and playing crucial roles in photosynthesis, antioxidant defense, and human health.
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Structure and Classification of Carotenoids
Carotenoids, categorized as tetraterpene pigments, display a spectrum of colors ranging from yellow, orange, and red to purple. Widely distributed in nature, they are found in photosynthetic bacteria, archaea, fungi, algae, plants, and animals. Typically comprising eight isoprene units with a 40-carbon skeleton, carotenoids exhibit a general structure featuring a polyene chain with nine conjugated double bonds and distinct end groups at both ends. Carotenoids fall into two groups: carotenes (hydrocarbons like α-carotene, β-carotene, β,ψ-carotene, and lycopene) and xanthophylls (oxygen-containing compounds like β-cryptoxanthin, lutein, zeaxanthin, astaxanthin, fucoxanthin, and peridinin). Xanthophylls, showing structural diversity, may exist as esters, glycosides, sulfates, or protein complexes. While most carotenoids have a 40-carbon skeleton (C40 carotenoid), some possess 45 or 50 carbons, termed higher carotenoids. Apocarotenoids, with fewer than 40 carbons, are found in plants and animals as degradation products of C40 carotenoids.
Fig. 1 Basic structures of carotenoids and end groups (Maoka T., 2020).
Carotenoid Synthesis Pathways
Carotenoid biosynthesis in higher plants is a complex and tightly regulated process primarily occurring in plastids. The pathway involves crucial enzymes and precursor molecules. Geranylgeranyl diphosphate (GGDP) serves as the direct precursor, formed by the condensation of four isopentenyl diphosphate (IPP) molecules catalyzed by IPP isomerase and GGDP synthase (GGDPS). GGDPS, a multifunctional enzyme, catalyzes the formation of GDP, FDP, and GGDP.
The synthesis pathway proceeds through the class of enzymes known as carotenogenic enzymes. Phytoene synthase (PSY) is a pivotal enzyme that converts GGDP to colorless phytoene, initiating carotenoid biosynthesis. Subsequent desaturation reactions, catalyzed by phytoene desaturase (PDS) and ζ-carotene desaturase (ZDS), extend the conjugated double bond system, leading to the formation of lycopene. The pathway then branches into cyclic and acyclic carotenoids, influenced by enzymes like lycopene cyclase (LCY), contributing to the diversity of carotenoid structures.
Factors Influencing Carotenoid Synthesis
Numerous factors impact carotenoid synthesis, including developmental stages, light, temperature, plant hormones, and mineral elements. The content and types of carotenoids vary during different developmental stages. Moderate light promotes carotenoid synthesis, and temperature exerts a noticeable influence. Plant hormones such as ethylene, gibberellins (GA), abscisic acid (ABA), and mineral elements like Mn2+ play pivotal roles in carotenoid synthesis regulation.
Functions of Carotenoids: Beyond Pigmentation
Carotenoids play multifaceted roles beyond imparting color to plants. In addition to their contribution to photosynthesis and protection against oxidative damage, carotenoids influence various physiological processes. One crucial function involves their role in enhancing the quality of fruits and vegetables. Carotenoids, such as β-carotene, serve as precursors to vitamin A, crucial for human health. They contribute to immune system support and have been associated with anti-cancer properties.
Moreover, carotenoids participate in the modulation of seed texture and quality. Studies have revealed their impact on maize seed texture by influencing the composition of starch granule membranes. The interaction between carotenoids and starch granules affects the texture of mature seeds, showcasing the broader implications of carotenoids in plant development and crop quality.
Understanding these intricate synthesis pathways and diverse functions of carotenoids provides valuable insights for agricultural practices, nutritional enhancements, and potential applications in biotechnology. The exploration of these pathways and functions continues to unveil the intricate roles carotenoids play in the dynamic interplay of plant metabolism and human nutrition.
References
- Sun T., et al. Plant carotenoids: Recent advances and future perspectives. Molecular Horticulture. 2022, 2(1): 3.
- Maoka T. Carotenoids as natural functional pigments. Journal of Natural Medicines. 2020, 74(1): 1-16.
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