Melanin

What is melanin? The word melanin is used to describe a unique class of pigments found throughout the biosphere (Figure 1) with a wide variety of functions, structures, and presentations. The word derives from the Greek word ‘melanos,’ meaning black, although melanins have diverse colors ranging from black and brown to yellow and red. Melanin types may be classified based on their source (animal, plant, fungal, or bacterial) or chemical and physical features (eumelanin, pheomelanin, neuromelanin, allomelanin, and pyomelanin). They all share a combination of characteristics that include monotonic broadband optical absorption, resistance to strong acids, insolubility in most solvents, and the presence of a stable free-radical population. Why is melanin dark? Depending on the chemical composition, natural melanins like eumelanin and allomelanin range in color from black to brown. Their dark appearance is the result of melanin’s broad optical absorption. Unlike other pigments in nature, melanins can absorb all visible light frequencies and reflect almost none to the observer. Since the optical absorption spectra of melanins extend to the ultraviolet and infrared region, some consider melanins as darker than black. What does melanin do? In nature, melanin serves diverse functions that range from camouflage and protection to energy harvesting. As pigments, melanins affect visual perception by enhancing or decreasing visual communication (think peacock feathers and chameleons, respectively). In terms of protection, melanins are well-known to protect against ionizing ultraviolet radiation, but that is just one example. Melanization has also been associated with protection against diverse biotic factors, for example, host defense against pathogens, as well abiotic factors, including heat and cold, and osmotic stresses. The immune systems of some insects and nematodes is based on melanin, and some microbial pathogens use melanin to evade host immune defenses. Finally, many melanotic organisms use melanin to capture energy from radiation. Animals, plants, and fungi can use melanin to harvest heat from radiation, influencing their adaptation to different thermal environments (so called thermal melanism). Studies in fungi have also shown that melanin can harvest energy from electromagnetic radiation for metabolic use in a process that involves melanin’s electrical properties (referred to as radiosynthesis). How does melanin protect against ionizing radiation? Ionizing radiation includes high-energy waves or particles that remove electrons from matter. Melanins protect against ionizing radiation by absorbing the radiation energy and dissipating it in the form of heat as well as by scavenging and/or neutralizing the ionized molecules inside the cell (for example, reactive oxygen species). The latter relates to the strong antioxidant properties of melanin, a feature that is also used by many microbial pathogens to counteract host immune defenses. What is the structure of melanin? Melanin is recognized as an amorphous polymer of high molecular weight. Although it is known that melanin is formed by the polymerization of phenolic and indolic compounds, the detailed structure of melanin remains undefined. These polymers form graphite-like planar sheets that aggregate in a hierarchal fashion to form a colloidal particle. Depending on the source, these melanin particles can reach diameters of hundreds of nanometers. The structure of melanin is resistant to acid hydrolysis but susceptible to degradation by alkaline conditions or oxidative chemicals such as permanganate or hydrogen peroxide. How is melanin synthesized? Melanin polymer biosynthesis is catalyzed by laccases and phenoloxidases. The eumelanin found in human skin is synthesized from the amino acid L-tyrosine via a tyrosinase. Tyrosinase converts L-tyrosine into L-3,4-dihydroxyphenylalanine (L-DOPA), and L-DOPA into dopaquinone, which then undergoes a series of reduction and oxidation reactions resulting in the building blocks of eumelanin: 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid. Melanin synthesis takes place in vesicles known as melanosomes. Collectively, the fungi encode at least three pathways for the synthesis of melanin, with DOPA- and 1,8-dihydroxynaphthalene-derived melanins being the most common. Since fungal melanins play key roles during human and plant pathogenesis, drugs that can inhibit melanin biosynthesis in fungi are of potential interest. In the case of plant pathogens, compounds that inhibit fungal melanin biosynthesis (for example, tricyclazole and carpropamid) already serve as important fungicides in agriculture. What can melanin be used for? Melanins exhibit a unique set of physicochemical properties, such as electrical conductivity, optical absorption, and affinity to a variety of organic and inorganic compounds. These properties have stimulated the research and development of novel technologies. For example, the electrical properties of melanin have been exploited in the creation of ingestible batteries. Other melanin applications include photo-protection and bioremediation of toxic metals and organics. Radames J.B. Cordero and Arturo Casadevall are co-founders of MelaTech, LLC.