Photoprotection
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Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by
sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...
. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and
oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...
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

In organisms that perform
oxygenic photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metaboli ...
, 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 Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
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 In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...
. 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 Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
possess some unique strategies for photoprotection which have not been identified in plants nor in algae. For example, most cyanobacteria possess an Orange Carotenoid Protein (OCP), which serves as a novel form of non-photochemical quenching. 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. Some other cyanobacterial strategies may involve state-transitions of the phycobilisome antenna complex , photoreduction of water with the Flavodiiron proteins, and futile cycling of CO2 .


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 Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
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. 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. 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. Various pigments and compounds can be employed by plants as a form of UV photoprotection as well. 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,
phytochrome Phytochromes are a class of photoreceptor proteins found in plants, bacteria and fungi. They respond to light in the red and far-red regions of the visible spectrum and can be classed as either Type I, which are activated by far-red light, or ...
s (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. 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. Although not yet fully understood, photoprotection is an essential function of plants.


In humans

Photoprotection of the
human skin The human skin is the outer covering of the body and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue (biology), tissue guarding Skeletal muscle, muscles, bones, ligaments and organ (anato ...
is achieved by extremely efficient
internal conversion Internal conversion is an atomic decay process where an excited nucleus interacts electromagnetically with one of the orbital electrons of an atom. This causes the electron to be emitted (ejected) from the atom. Thus, in internal conversion (o ...
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 In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...
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. 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. 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 In medicine, an indication is a valid reason to use a certain test, medication, procedure, or surgery. There can be multiple indications to use a procedure or medication. An indication can commonly be confused with the term diagnosis. A diagnosis ...
for the prevention of phototoxicity in adults with erythropoietic protoporphyria. Afamelanotide is also being investigated as a method of photoprotection from in the treatment of polymorphous light eruption, actinic keratosis and
squamous cell carcinoma Squamous-cell carcinoma (SCC), also known as epidermoid carcinoma, comprises a number of different types of cancer that begin in squamous cells. These cells form on the surface of the skin, on the lining of hollow organs in the body, and on the ...
(a form of
skin cancer Skin cancers are cancers that arise from the Human skin, skin. They are due to the development of abnormal cells (biology), cells that have the ability to invade or metastasis, spread to other parts of the body. It occurs when skin cells grow ...
).Clinuvel FAQs


Artificial melanin

The cosmetic industry claims that the UV filter acts as an "artificial melanin". But those artificial substances used in
sunscreen Sunscreen, also known as sunblock, sun lotion or sun cream, is a photoprotection, photoprotective topical product for the Human skin, skin that helps protect against sunburn and prevent skin cancer. Sunscreens come as lotions, sprays, gels, fo ...
s do not efficiently dissipate the energy of the UV photon as heat. Instead these substances have a very long excited state lifetime. In fact, the substances used in sunscreens are often used as photosensitizers in chemical reactions. (see
Benzophenone Benzophenone is a naturally occurring organic compound with the formula (C6H5)2CO, generally abbreviated Ph2CO. Benzophenone has been found in some fungi, fruits and plants, including grapes. It is a white solid with a low melting point and ros ...
).
Oxybenzone Oxybenzone or benzophenone-3 or BP-3 (trade names Milestab 9, Eusolex 4360, Escalol 567, KAHSCREEN BZ-3) is an organic compound belonging to the class of aromatic ketones known as benzophenones. It takes the form of pale-yellow crystals that are ...
, titanium oxide and octyl methoxycinnamate are photoprotective agents used in many sunscreens, providing broad-spectrum UV coverage, including UVB and short-wave UVA rays.


See also

*
Sunscreen Sunscreen, also known as sunblock, sun lotion or sun cream, is a photoprotection, photoprotective topical product for the Human skin, skin that helps protect against sunburn and prevent skin cancer. Sunscreens come as lotions, sprays, gels, fo ...
* Photocarcinogen * Direct DNA damage * Indirect DNA damage


References

{{Authority control Biological defense mechanisms Photochemistry Skin physiology Sun tanning