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.

Contents

In photosynthetic organisms

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

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

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

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

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-absorberother namespercentage 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 acidPBSA, Eusolex 232, Parsol HS,
2-ethylhexyl 4-dimethylaminobenzoate Padimate-O, oxtyldimethyl PABA, OD-PABA0.1 = 10%90%
4-Methylbenzylidene camphor(4-MBC), (MBC), Parsol 5000, Eusolex 63000.3 = 30%70%
4-tert-butyl-4-methoxydibenzoyl-methane(BM-DBM), Avobenzone, Parsol 1789, Eusolex 9020
Menthyl Anthranilate(MA), Menthyl-2-aminobenzoate, meradimate0.6 = 60%40%
Ethylhexyl methoxycinnamate(2-EHMC), (EHMC), EMC, Octyl methoxycinnamate, OMC, Eusolex 2292, Parsol0.81 = 81%19%

See also

Related Research Articles

<span class="mw-page-title-main">Melanin</span> Group of natural pigments found in most organisms

Melanin consist of oligomers or polymers arranged in a manner which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. They have been described as "among the last remaining biological frontiers with the unknown".

<span class="mw-page-title-main">Melanocyte</span> Melanin-producing cells of the skin

Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer of the skin's epidermis, the middle layer of the eye, the inner ear, vaginal epithelium, meninges, bones, and heart. Melanin is a dark pigment primarily responsible for skin color. Once synthesized, melanin is contained in special organelles called melanosomes which can be transported to nearby keratinocytes to induce pigmentation. Thus darker skin tones have more melanosomes present than lighter skin tones. Functionally, melanin serves as protection against UV radiation. Melanocytes also have a role in the immune system.

Photobiology is the scientific study of the beneficial and harmful interactions of light in living organisms. The field includes the study of photophysics, photochemistry, photosynthesis, photomorphogenesis, visual processing, circadian rhythms, photomovement, bioluminescence, and ultraviolet radiation effects.

<span class="mw-page-title-main">Sunless tanning</span> Indoor tanning lotion

Sunless tanning, also known as UV filled tanning, self tanning, spray tanning, or fake tanning, refers to the effect of a suntan without exposure to the Sun. Sunless tanning involves the use of oral agents (carotenids), or creams, lotions or sprays applied to the skin. Skin-applied products may be skin-reactive agents or temporary bronzers (colorants).

<span class="mw-page-title-main">Action spectrum</span> Graph of the rate of biological effectiveness plotted against wavelength of light

An action spectrum is a graph of the rate of biological effectiveness plotted against wavelength of light. It is related to absorption spectrum in many systems. Mathematically, it describes the inverse quantity of light required to evoke a constant response. It is very rare for an action spectrum to describe the level of biological activity, since biological responses are often nonlinear with intensity.

A light-harvesting complex consists of a number of chromophores which are complex subunit proteins that may be part of a larger super complex of a photosystem, the functional unit in photosynthesis. It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. The light which is captured by the chromophores is capable of exciting molecules from their ground state to a higher energy state, known as the excited state. This excited state does not last very long and is known to be short-lived.

In biophysics, the Kautsky effect is a phenomenon consisting of a typical variation in the behavior of a plant fluorescence when exposed to light. It was discovered in 1931 by H. Kautsky and A. Hirsch.

<span class="mw-page-title-main">Biological pigment</span> Substances produced by living organisms

Biological pigments, also known simply as pigments or biochromes, are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments. Many biological structures, such as skin, eyes, feathers, fur and hair contain pigments such as melanin in specialized cells called chromatophores. In some species, pigments accrue over very long periods during an individual's lifespan.

<span class="mw-page-title-main">Photoinhibition</span>

Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II (PSII) is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII. In living organisms, photoinhibited PSII centres are continuously repaired via degradation and synthesis of the D1 protein of the photosynthetic reaction center of PSII. Photoinhibition is also used in a wider sense, as dynamic photoinhibition, to describe all reactions that decrease the efficiency of photosynthesis when plants are exposed to light.

<span class="mw-page-title-main">Pyrimidine dimer</span> Type of damage to DNA

Pyrimidine dimers represent molecular lesions originating from thymine or cytosine bases within DNA, resulting from photochemical reactions. These lesions, commonly linked to direct DNA damage, are induced by ultraviolet light (UV), particularly UVC, result in the formation of covalent bonds between adjacent nitrogenous bases along the nucleotide chain near their carbon–carbon double bonds, the photo-coupled dimers are fluorescent. Such dimerization, which can also occur in double-stranded RNA (dsRNA) involving uracil or cytosine, leads to the creation of cyclobutane pyrimidine dimers (CPDs) and 6–4 photoproducts. These pre-mutagenic lesions modify the DNA helix structure, resulting in abnormal non-canonical base pairing and, consequently, adjacent thymines or cytosines in DNA will form a cyclobutane ring when joined together and cause a distortion in the DNA. This distortion prevents DNA replication and transcription mechanisms beyond the dimerization site.

<span class="mw-page-title-main">Indirect DNA damage</span>

Indirect DNA damage occurs when a UV-photon is absorbed in the human skin by a chromophore that does not have the ability to convert the energy into harmless heat very quickly. Molecules that do not have this ability have a long-lived excited state. This long lifetime leads to a high probability for reactions with other molecules—so-called bimolecular reactions. Melanin and DNA have extremely short excited state lifetimes in the range of a few femtoseconds (10−15s). The excited state lifetime of compounds used in sunscreens such as menthyl anthranilate, avobenzone or padimate O is 1,000 to 1,000,000 times longer than that of melanin, and therefore they may cause damage to living cells that come in contact with them.

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Non-photochemical quenching (NPQ) is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity. It involves the quenching of singlet excited state chlorophylls (Chl) via enhanced internal conversion to the ground state, thus harmlessly dissipating excess excitation energy as heat through molecular vibrations. NPQ occurs in almost all photosynthetic eukaryotes, and helps to regulate and protect photosynthesis in environments where light energy absorption exceeds the capacity for light utilization in photosynthesis.

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<span class="mw-page-title-main">Chlorophyll fluorescence</span> Light re-emitted by chlorophyll molecules during return from excited to non-excited states

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<span class="mw-page-title-main">Angelicin</span> Chemical compound

Angelicin is the parent compound in a family of naturally occurring organic compounds known as the angular furanocoumarins. Structurally, it can be considered as benzapyra-2-one fused with a furan moiety in the 7,8-position. Angelicin is commonly found in certain Apiaceae and Fabaceae plant species such as Bituminaria bituminosa. It has a skin permeability coefficient (LogKp) of -2.46. The maximum absorption is observed at 300 nm. The 1HNMR spectrum is available; the infrared and mass spectra of angelicin can be found in this database. The sublimation of angelicin occurs at 120 °C and the pressure of 0.13 Pa. Angelicin is a coumarin.

Antheraxanthin is a bright yellow accessory pigment found in many organisms that perform photosynthesis. It is a xanthophyll cycle pigment, an oil-soluble alcohol within the xanthophyll subgroup of carotenoids. Antheraxanthin is both a component in and product of the cellular photoprotection mechanisms in photosynthetic green algae, red algae, euglenoids, and plants.

<span class="mw-page-title-main">Dark skin</span> Human skin color

Dark skin is a type of human skin color that is rich in melanin pigments. People with dark skin are often referred to as "black people", although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.

<span class="mw-page-title-main">Orange carotenoid protein</span>

Orange carotenoid protein (OCP) is a water-soluble protein which plays a role in photoprotection in diverse cyanobacteria. It is the only photoactive protein known to use a carotenoid as the photoresponsive chromophore. The protein consists of two domains, with a single keto-carotenoid molecule non-covalently bound between the two domains. It is a very efficient quencher of excitation energy absorbed by the primary light-harvesting antenna complexes of cyanobacteria, the phycobilisomes. The quenching is induced by blue-green light. It is also capable of preventing oxidative damage by directly scavenging singlet oxygen (1O2).

References

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  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.
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  10. 1 2 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.
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  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.
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  17. ↑ Clinuvel FAQs Archived 2008-04-11 at the Wayback Machine
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