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Carotenoid biosynthesis

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Carotenoids are a class of natural pigments synthesized by various organisms, including plants, algae, and photosynthetic bacteria. They are characterized by their vibrant yellow, orange, and red colors, which contribute significantly to the coloration of fruits and vegetables. Carotenoids play essential roles in photosynthesis and offer various health benefits, such as antioxidant properties and serving as precursors to vitamin A.[1]

Biosynthetic pathway

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Carotenoid biosynthesis occurs primarily in the plastids of plant cells, particularly within chloroplasts and chromoplasts. The biosynthetic pathway initiates with the condensation of two molecules of geranylgeranyl pyrophosphate (GGPP), a 20-carbon isoprenoid precursor. The key steps in this pathway are as follows:

  1. Formation of phytoene: The enzyme phytoene synthase (PSY) catalyzes the condensation of two GGPP molecules to produce phytoene, a colorless carotenoid.[2]
  2. Desaturation to lycopene: Phytoene undergoes a series of desaturation reactions facilitated by enzymes such as phytoene desaturase (PDS) and ζ-carotene isomerase (Z-ISO), resulting in the formation of lycopene, a red carotenoid.
  3. Cyclization to carotenoids: Lycopene is cyclized into various carotenoids, including α-carotene and β-carotene, through the action of lycopene cyclase (LCY), which catalyzes cyclization at the ends of the lycopene molecule.[3]
  4. Further modifications: Subsequent modifications, such as hydroxylation and oxidation, lead to the formation of xanthophylls (e.g., lutein and zeaxanthin) and other derivatives.

Key enzymes

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Several enzymes play critical roles in the carotenoid biosynthetic pathway:

  1. Phytoene synthase (PSY): Catalyzes the first committed step in carotenoid biosynthesis, converting GGPP into phytoene.[4]
  2. Phytoene desaturase (PDS): Introduces double bonds into phytoene, facilitating its conversion into lycopene.[5]
  3. Lycopene cyclase (LCY): Responsible for the cyclization of lycopene into α-carotene or β-carotene.[6]
  4. Carotenoid hydroxylases: Enzymes such as lutein epoxide cyclase (LUT) introduce hydroxyl groups into carotenoids, leading to the formation of xanthophylls.[7]

Regulation

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The regulation of carotenoid biosynthesis is influenced by various factors, including:

  • Gene Expression: Many carotenoid biosynthetic genes are upregulated by light, enhancing the expression of PSY and subsequently increasing carotenoid production.[8]
  • Hormonal Regulation: Phytohormones such as auxins and abscisic acid modulate carotenoid biosynthesis. Notably, abscisic acid enhances carotenoid accumulation under stress conditions.[9]
  • Environmental Factors: Stressors like drought or pathogen attack can trigger carotenoid accumulation as a protective response, thereby enhancing plant resilience.[10]

Significance

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In plants

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Carotenoids play roles in photosynthetic organisms by:

In human health

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Carotenoids, especially provitamin A carotenoids such as β-carotene, are essential for human health. Their benefits include:

References

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  1. ^ "Carotenoid | Definition, Description, Functions, Examples, & Facts | Britannica". www.britannica.com. Retrieved 2024-10-20.
  2. ^ van der Hart, Onno (December 2012). "The use of imagery in phase 1 treatment of clients with complex dissociative disorders". European Journal of Psychotraumatology. 3 (1). doi:10.3402/ejpt.v3i0.8458. PMC 3402145. PMID 22893843.
  3. ^ Fraser, Paul D; Bramley, Peter M (2004-05-01). "The biosynthesis and nutritional uses of carotenoids". Progress in Lipid Research. 43 (3): 228–265. doi:10.1016/j.plipres.2003.10.002. ISSN 0163-7827.
  4. ^ "National Center for Biotechnology Information". www.ncbi.nlm.nih.gov. Retrieved 2024-10-20.
  5. ^ Cunningham, F. X., & Gantt, E. (1998). Genes and enzymes of carotenoid biosynthesis in plants. Annual Review of Plant Physiology and Plant Molecular Biology, 49(1), 557-583. https://doi.org/10.1146/annurev.arplant.49.1.557
  6. ^ Cunningham, F. X., & Gantt, E. (2001). One ring or two? Determination of ring number in carotenoids by lycopene ε-cyclases. Proceedings of the National Academy of Sciences, 98(5), 2905-2910. https://doi.org/10.1073/pnas.051618398
  7. ^ Kim, J., Smith, J. J., Tian, L., & DellaPenna, D. (2009). The evolution and function of carotenoid hydroxylases in Arabidopsis. The Plant Cell, 21(11), 3850-3863. https://doi.org/10.1105/tpc.109.069757
  8. ^ Toledo-Ortiz, G., Huq, E., & Rodríguez-Concepción, M. (2010). Direct regulation of phytoene synthase gene expression and carotenoid biosynthesis by phytochrome-interacting factors. Proceedings of the National Academy of Sciences, 107(25), 11626-11631. https://doi.org/10.1073/pnas.0914428107
  9. ^ Jiang, Y., Liang, G., & Yu, D. (2012). Activated expression of WRKY57 confers drought tolerance in Arabidopsis. Molecular Plant, 5(6), 1375-1388. https://doi.org/10.1093/mp/sss080
  10. ^ Havaux, M. (2014). Carotenoid oxidation products as stress signals in plants. The Plant Journal, 79(4), 597-606. https://doi.org/10.1111/tpj.12386
  11. ^ Sommer, A., & Vyas, K. S. (2012). A global clinical view on vitamin A deficiency and its prevention. Nutrition, 28(10), 728-730. https://doi.org/10.1016/j.nut.2011.12.014
  12. ^ Chew, B. P., & Park, J. S. (2004). Carotenoid action on the immune response. The Journal of Nutrition, 134(1), 257S-261S. https://doi.org/10.1093/jn/134.1.257S
  13. ^ Stahl, W., & Sies, H. (2012). β-Carotene and other carotenoids in protection from sunlight. The American Journal of Clinical Nutrition, 96(5), 1179S-1184S. https://doi.org/10.3945/ajcn.112.034819
  14. ^ Rao, A. V., & Rao, L. G. (2007). Carotenoids and human health. Pharmacological Research, 55(3), 207-216. https://doi.org/10.1016/j.phrs.2007.01.012
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