Coping With the Cold
Ice presents a big problem for organisms that live in frigid climates. Once the temperature drops below freezing, ice crystals form within cells and eventually burst. However, to this day organisms are found living in these harsh conditions. So how do they do it? Organisms of all types including plants, animals, fungi and bacteria have developed ways to combat the threat of ice formation (Venkatesh,2008). One way organisms deal with these conditions is to produce antifreeze proteins (AFP’s). These are specialized proteins that aid in protecting the organism as the temperature drops. AFP’s do not stop the growth of ice crystals, but instead limit the growth to a point that doesn’t cause harm (Dalal, 2001). This is necessary because of ice’s ability to recrystallize. When water begins to freeze, many small crystals form, but then a few small crystals dominate and grow larger and larger, incorporating water molecules from the surrounding smaller crystals. Antifreeze proteins counteract this recrystallization effect and will bind to the surface of the small ice crystals and slow or prevent the growth into larger crystals (Griffith, 2004). This lowers the freezing point of water in the presence of ice while not affecting the melting point. This produces a difference between the freezing and melting points which is termed thermal hysteresis (Duman, 2004).
AFP’s are a prime example of convergent evolution, which is when unrelated organisms evolve similar traits due to their environment (Venkatesh, 2008). By examining the proteins used by different organisms, scientists have discovered many different proteins have been selected to perform the same function. All of these are small proteins with a flat surface that is rich in threonine which binds to the surface of ice crystals (Davies, 2002). Three examples that produce a variety of AFP include the Notothenioids, winter rye and the mealworm beetle.
The specific structure of AFP’s varies among fish, insects and plants. The binding sites on AFP’s are relatively flat and comprise a substantial proportion of the surface area in fish and insects (Davies, 2002). The protein’s quaternary structure allows for an overall tight surface-surface complementary fit. Binding to the ice is stabilized by van der Waals and hydrophobic interactions from strategically arranged amino acids that match up with the spacing in the crystal lattice (Griffith, 2004). This results in a non-colligative (not relying on proportion to concentration ratio), non-equilibrium lowering of the freezing point and restricting growth of the ice crystals (Davies, 2002). The most effective antifreeze proteins are made by insects, which lower the freezing point by about 6 degrees (Venkatesh, 2008).
|Notothenioid "Ice Fish"|
Many types of organisms utilize AFP’s. The most well-known are the Notothenioids because AFP’s were first discovered in them. Notothenioids are Antarctic ice fish that inhabits temperatures between -2°C and 4°C (Eastman,2000). The notothenioid AFP exists in several isoforms of different sizes, but all composed of a simple glycotripeptide repeat with the disaccharide galactose-N-acetylgalactosamine attached to each Threonine, and the dipeptide Alanine-Alanine at the N terminus (Liangbiao,1997). Besides these anti-freeze glycotripeptide proteins (AFGPs), there are three other structurally different types of AFP’s from various polar fishes suggesting that these unique proteins evolved independently at least four times (Liangbiao,1997). While the exact method of evolution is unknown the most likely method of evolution was identified throughcharacterization and analyses of notothenioid AFP and trypsinogen genes. Other organisms using AFPs include plants, bacteria, fungi, insects and frogs!
A great deal of research is being done to apply AFP’s to several applications including industrial, medical, and agricultural application in different fields, such as food technology, preservation of cell lines, organs, cryosurgery, and cold hardy transgenic plants and animals (Venkatesh, 2008). One area where AFP’s have been successfully implemented is the dairy industry. AFP’s purified from cold adapted ocean pout have been used as a preservative in ice cream (Regand, 2006). The proteins are added to the fine ice crystals to prevent it from recrystallizing during storage and delivery. Researchers are also experimenting with AFP’s as a way to preserve tissues and organs that are stored at low temperatures (Regand, 2006). Utilizing these unique proteins the possible damage from ice crystals could be successfully reduced and hopefully have a major impact on the medical field as research continues.