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Photoprotection

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Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative stress caused by excess or fluctuating light conditions. Humans and other animals have also developed photoprotective mechanisms to avoid UV photodamage to the skin, prevent DNA damage, and minimize the downstream effects of oxidative stress.

In photosynthetic organisms

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In organisms that perform oxygenic photosynthesis, excess light may lead to photoinhibition, or photoinactivation of the reaction centers, a process that does not necessarily involve chemical damage. When photosynthetic antenna pigments such as chlorophyll are excited by light absorption, unproductive reactions may occur by charge transfer to molecules with unpaired electrons. Because oxygenic phototrophs generate O2 as a byproduct from the photocatalyzed splitting of water (H2O), photosynthetic organisms have a particular risk of forming reactive oxygen species.[citation needed]

Therefore, a diverse suite of mechanisms has developed in photosynthetic organisms to mitigate these potential threats, which become exacerbated under high irradiance, fluctuating light conditions, in adverse environmental conditions such as cold or drought, and while experiencing nutrient deficiencies which cause an imbalance between energetic sinks and sources.

In eukaryotic phototrophs, these mechanisms include non-photochemical quenching mechanisms such as the xanthophyll cycle, biochemical pathways which serve as "relief valves", structural rearrangements of the complexes in the photosynthetic apparatus, and use of antioxidant molecules. Higher plants sometimes employ strategies such as reorientation of leaf axes to minimize incident light striking the surface. Mechanisms may also act on a longer time-scale, such as up-regulation of stress response proteins or down-regulation of pigment biosynthesis, although these processes are better characterized as "photoacclimatization" processes.

Cyanobacteria possess some unique strategies for photoprotection which have not been identified in plants nor in algae.[1] For example, most cyanobacteria possess an Orange Carotenoid Protein (OCP), which serves as a novel form of non-photochemical quenching.[2] Another unique, albeit poorly-understood, cyanobacterial strategy involves the IsiA chlorophyll-binding protein, which can aggregate with carotenoids and form rings around the PSI reaction center complexes to aid in photoprotective energy dissipation.[3] Some other cyanobacterial strategies may involve state-transitions of the phycobilisome antenna complex[4] , photoreduction of water with the Flavodiiron proteins,[5] and futile cycling of CO2[6] .

In plants

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It is widely known that plants need light to survive, grow and reproduce. It is often assumed that more light is always beneficial; however, excess light can actually be harmful for some species of plants. Just as animals require a fine balance of resources, plants require a specific balance of light intensity and wavelength for optimal growth (this can vary from plant to plant). Optimizing the process of photosynthesis is essential for survival when environmental conditions are ideal and acclimation when environmental conditions are severe. When exposed to high light intensity, a plant reacts to mitigate the harmful effects of excess light.

To best protect themselves from excess light, plants employ a multitude of methods to minimize harm inflicted by excess light. A variety of photoreceptors are used by plants to detect light intensity, direction and duration. In response to excess light, some photoreceptors have the ability to shift chloroplasts within the cell farther from the light source thus decreasing the harm done by superfluous light.[7] Similarly, plants are able to produce enzymes that are essential to photoprotection such as Anthocyanin synthase. Plants deficient in photoprotection enzymes are much more sensitive to light damage than plants with functioning photoprotection enzymes.[8] Also, plants produce a variety of secondary metabolites beneficial for their survival and protection from excess light. These secondary metabolites that provide plants with protection are commonly used in human sunscreen and pharmaceutical drugs to supplement the inadequate light protection that is innate to human skin cells.[9] Various pigments and compounds can be employed by plants as a form of UV photoprotection as well.[10]

Pigmentation is one method employed by a variety of plants as a form of photoprotection. For example, in Antarctica, native mosses of green color can be found naturally shaded by rocks or other physical barriers while red colored mosses of the same species are likely to be found in wind and sun exposed locations. This variation in color is due to light intensity. Photoreceptors in mosses, phytochromes (red wavelengths) and phototropins (blue wavelengths), assist in the regulation of pigmentation. To better understand this phenomenon, Waterman et al. conducted an experiment to analyze the photoprotective qualities of UVACs (Ultraviolet Absorbing Compounds) and red pigmentation in antarctic mosses. Moss specimens of species Ceratodon purpureus, Bryum pseudotriquetrum and Schistidium antarctici were collected from an island region in East Antarctica. All specimens were then grown and observed in a lab setting under constant light and water conditions to assess photosynthesis, UVAC and pigmentation production. Moss gametophytes of red and green varieties were exposed to light and consistent watering for a period of two weeks. Following the growth observation, cell wall pigments were extracted from the moss specimens. These extracts were tested using UV–Vis spectrophotometry which uses light from the UV and visible spectrum to create an image depicting light absorbance. UVACs are typically found in the cytoplasm of the cell; however, when exposed to high-intensity light, UVACs are transported into the cell wall. It was found that mosses with higher concentrations of red pigments and UVACs located in the cell walls, rather than intracellularly, performed better in higher intensity light. Color change in the mosses was found not to be due to chloroplast movement within the cell. It was found that UVACs and red pigments function as long-term photoprotection in Antarctic mosses. Therefore, in response to high-intensity light stress, the production of UVACs and red pigmentation is up-regulated.[10]

Knowing that plants are able to differentially respond to varying concentrations and intensities of light, it is essential to understand why these reactions are important. Due to a steady rise in global temperatures in recent years, many plants have become more susceptible to light damage. Many factors including soil nutrient richness, ambient temperature fluctuation and water availability all impact the photoprotection process in plants. Plants exposed to high light intensity coupled with water deficits displayed a significantly inhibited photoprotection response.[11] Although not yet fully understood, photoprotection is an essential function of plants.

In humans

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Photoprotection of the human skin is achieved by extremely efficient internal conversion of DNA, proteins and melanin. Internal conversion is a photochemical process that converts the energy of the UV photon into small, harmless amounts of heat. If the energy of the UV photon were not transformed into heat, then it would lead to the generation of free radicals or other harmful reactive chemical species (e.g. singlet oxygen, or hydroxyl radical).

In DNA this photoprotective mechanism evolved four billion years ago at the dawn of life.[12] The purpose of this extremely efficient photoprotective mechanism is to prevent direct DNA damage and indirect DNA damage. The ultrafast internal conversion of DNA reduces the excited state lifetime of DNA to only a few femtoseconds (10−15s)—this way the excited DNA does not have enough time to react with other molecules.

For melanin this mechanism has developed later in the course of evolution. Melanin is such an efficient photoprotective substance that it dissipates more than 99.9% of the absorbed UV radiation as heat. [13] This means that less than 0.1% of the excited melanin molecules will undergo harmful chemical reactions or produce free radicals.

Synthetic Melanocyte-stimulating hormone

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In the European Union and United States, afamelanotide is indicated for the prevention of phototoxicity in adults with erythropoietic protoporphyria.[14][15][16] Afamelanotide is also being investigated as a method of photoprotection from in the treatment of polymorphous light eruption, actinic keratosis and squamous cell carcinoma (a form of skin cancer).[17]

Artificial melanin

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The cosmetic industry claims that the UV filter acts as an "artificial melanin". But those artificial substances used in sunscreens do not efficiently dissipate the energy of the UV photon as heat. Instead these substances have a very long excited state lifetime.[18] In fact, the substances used in sunscreens are often used as photosensitizers in chemical reactions. (see Benzophenone).

Oxybenzone, titanium oxide and octyl methoxycinnamate are photoprotective agents used in many sunscreens, providing broad-spectrum UV coverage, including UVB and short-wave UVA rays.[19][20]

UV-absorber other names percentage of molecules that dissipate the photon energy (quantum yield: Φ ) [18]
molecules not dissipating the energy quickly
DNA > 99.9% < 0.1%
natural melanin > 99.9% < 0.1%
2-phenylbenzimidazole-5-sulfonic acid PBSA, Eusolex 232, Parsol HS,
2-ethylhexyl 4-dimethylaminobenzoate Padimate-O, oxtyldimethyl PABA, OD-PABA 0.1 = 10% 90%
4-Methylbenzylidene camphor (4-MBC), (MBC), Parsol 5000, Eusolex 6300 0.3 = 30% 70%
4-tert-butyl-4-methoxydibenzoyl-methane (BM-DBM), Avobenzone, Parsol 1789, Eusolex 9020
Menthyl Anthranilate (MA), Menthyl-2-aminobenzoate, meradimate 0.6 = 60% 40%
Ethylhexyl methoxycinnamate (2-EHMC), (EHMC), EMC, Octyl methoxycinnamate, OMC, Eusolex 2292, Parsol 0.81 = 81% 19%

See also

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References

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  1. ^ Bailey S, Grossman A (2008). "Photoprotection in cyanobacteria: regulation of light harvesting". Photochemistry and Photobiology. 84 (6): 1410–20. doi:10.1111/j.1751-1097.2008.00453.x. PMID 19067963. S2CID 8432700.
  2. ^ Kirilovsky D, Kerfeld CA (July 2013). "The Orange Carotenoid Protein: a blue-green light photoactive protein". Photochemical & Photobiological Sciences. 12 (7): 1135–43. doi:10.1039/C3PP25406B. PMID 23396391.
  3. ^ Berera R, van Stokkum IH, d'Haene S, Kennis JT, van Grondelle R, Dekker JP (March 2009). "A mechanism of energy dissipation in cyanobacteria". Biophysical Journal. 96 (6): 2261–7. Bibcode:2009BpJ....96.2261B. doi:10.1016/j.bpj.2008.12.3905. PMC 2717300. PMID 19289052.
  4. ^ Dong C, Tang A, Zhao J, Mullineaux CW, Shen G, Bryant DA (September 2009). "ApcD is necessary for efficient energy transfer from phycobilisomes to photosystem I and helps to prevent photoinhibition in the cyanobacterium Synechococcus sp. PCC 7002". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1787 (9): 1122–8. doi:10.1016/j.bbabio.2009.04.007. PMID 19397890.
  5. ^ Allahverdiyeva Y, Mustila H, Ermakova M, Bersanini L, Richaud P, Ajlani G, Battchikova N, Cournac L, Aro EM (March 2013). "Flavodiiron proteins Flv1 and Flv3 enable cyanobacterial growth and photosynthesis under fluctuating light". Proceedings of the National Academy of Sciences of the United States of America. 110 (10): 4111–6. Bibcode:2013PNAS..110.4111A. doi:10.1073/pnas.1221194110. PMC 3593875. PMID 23431195.
  6. ^ Tchernov D, Silverman J, Luz B, Reinhold L, Kaplan A (2003). "Massive light-dependent cycling of inorganic carbon between oxygenic photosynthetic microorganisms and their surroundings". Photosynthesis Research. 77 (2–3): 95–103. doi:10.1023/A:1025869600935. PMID 16228368. S2CID 21353640.
  7. ^ Galvão VC, Fankhauser C (October 2015). "Sensing the light environment in plants: photoreceptors and early signaling steps" (PDF). Current Opinion in Neurobiology. 34: 46–53. doi:10.1016/j.conb.2015.01.013. PMID 25638281. S2CID 12390801.
  8. ^ Zheng XT, Chen YL, Zhang XH, Cai ML, Yu ZC, Peng CL (April 2019). "ANS-deficient Arabidopsis is sensitive to high light due to impaired anthocyanin photoprotection". Functional Plant Biology. 46 (8): 756–765. doi:10.1071/FP19042. PMID 31023420.
  9. ^ Takshak S, Agrawal SB (April 2019). "Defense potential of secondary metabolites in medicinal plants under UV-B stress". Journal of Photochemistry and Photobiology B: Biology. 193: 51–88. doi:10.1016/j.jphotobiol.2019.02.002. PMID 30818154.
  10. ^ a b Waterman MJ, Bramley-Alves J, Miller RE, Keller PA, Robinson SA (November 2018). "Photoprotection enhanced by red cell wall pigments in three East Antarctic mosses". Biological Research. 51 (1): 49. doi:10.1186/s40659-018-0196-1. PMC 6247747. PMID 30463628.
  11. ^ Munné-Bosch S, Cela J (December 2006). "Effects of water deficit on photosystem II photochemistry and photoprotection during acclimation of lyreleaf sage (Salvia lyrata L.) plants to high light". Journal of Photochemistry and Photobiology B: Biology. 85 (3): 191–7. doi:10.1016/j.jphotobiol.2006.07.007. PMID 16962788.
  12. ^ Kohler B. "Ultrafast internal conversion of DNA". Department of Chemistry, The Ohio State University. Archived from the original on 20 July 2011. Retrieved 2008-02-13.
  13. ^ Meredith P, Riesz J (February 2004). "Radiative relaxation quantum yields for synthetic eumelanin". Photochemistry and Photobiology. 79 (2): 211–6. arXiv:cond-mat/0312277. doi:10.1111/j.1751-1097.2004.tb00012.x. PMID 15068035. S2CID 222101966.
  14. ^ Commissioner, Office of the (March 24, 2020). "FDA approves first treatment to increase pain-free light exposure in patients with a rare disorder". FDA. Retrieved 2024-04-24.
  15. ^ "Scenesse: Summary of Product Characteristics" (PDF). European Medicines Agency (EMA). 27 January 2016. Archived (PDF) from the original on 6 April 2017. Retrieved 6 April 2017.
  16. ^ "Scenesse EPAR". European Medicines Agency (EMA). 17 September 2018. Archived from the original on 19 November 2019. Retrieved 18 November 2019.
  17. ^ Clinuvel FAQs Archived 2008-04-11 at the Wayback Machine
  18. ^ a b Cantrell A, McGarvey DJ, Truscott TG (2001). "Chapter 26: Photochemical and Photophysical Properties of Sunscreens". In Giacomoni PU (ed.). Comprehensive Series in Photosciences. Vol. 3. pp. 497–519. doi:10.1016/S1568-461X(01)80061-2. ISBN 9780444508393. CAN 137:43484.
  19. ^ Burnett ME, Wang SQ (April 2011). "Current sunscreen controversies: a critical review". Photodermatology, Photoimmunology & Photomedicine. 27 (2): 58–67. doi:10.1111/j.1600-0781.2011.00557.x. PMID 21392107.
  20. ^ Serpone N, Salinaro A, Emeline AV, Horikoshi S, Hidaka H, Zhao J (December 2002). "An in vitro systematic spectroscopic examination of the photostabilities of a random set of commercial sunscreen lotions and their chemical UVB/UVA active agents". Photochemical & Photobiological Sciences. 1 (12): 970–81. doi:10.1039/b206338g. PMID 12661594.
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