Antifreeze proteins (AFPs) and Antifreeze glycoproteins (AFGPs) are produced as a specialised adaptation by certain fish, tortoises, insects, plants and bacteria. AFPs and AFGPs are capable of lowering the freezing point of a solution in a non-colligative manner by lowering the freezing point relative to the melting point. The measured temperature difference between the melting point and the freezing point is referred to as thermal hysteresis. AF(G)Ps are also capable of preventing the recrystallization of ice, which is a thermodynamically favoured process in ice which involves the growth of large ice crystals at the expense of smaller ice crystals.

The frigid polar seas of the Antarctic and the Arctic have water temperatures that may often be below the freezing points of fish body fluids. For example, in the Antarctic seas, temperatures between -1º and -2º Celsius are common, the water temperature varies very little from season to season, and the waters often contain ice crystals. Whereas the freezing point of standard seawater is approximately -1.86ºC, those of teleost fish fluids can be as much as 1ºC higher (warmer), since their fluids are hypoosmotic to (less salty than) seawater. Fish partially decrease their melting point by increasing the solute (salt and sugar) content of their fluids. However in these waters, fish would be unable to survive without specialised adaptations.

Fishes such as the Polar cod of the Arctic and the Antarctic toothfish and its relatives in the family Nototheniidae are adapted to these frigid seas. In their blood they have specialised proteins called antifreeze glycoproteins or AFGPs. The AFGPs are thought to adhere to growing ice crystals thus changing the thermodynamics of the crystal growth and preventing the crystals from growing to hazardous sizes, thereby preventing tissue damage and continuing to allow free flow of the blood.

The AFGPs in the Polar cods and the Antarctic cods and their relatives are very similar in their molecular (protein) structure. However, because the DNA sequences (genes) encoding the AFGP proteins differ in these two groups of fishes (Arctic vs. Antarctic), the AFGP is thought to be an example of convergent evolution. AFGPs may be made in the liver (although still unclear) and secreted into the blood circulatory system. AFGPs rely on specific glycosylations in order to operate as an antifreeze protein. They are present in all bodily fluids and are absent from the urine.

AFPs isolated from some insects have shown 'hyperactive' thermal hysteresis activity. Both the spruce budworm and the mealworm beetle AFPs consist of a beta-helix whose ice-binding face is a flat array of amino acids. Insect antifreeze proteins are capable of decreasing the freezing point of a solution by 4 to 6 ºC.

AFPs isolated from certain plants have shown reduced thermal hysteresis activity. However, the protein can still effectively prevent the recrystallization of ice at low concentrations, which is more beneficial to a plant exposed to extreme cold.

Antifreeze protein activity is usually measured by looking at the thermal hysteresis (antifreeze) activity using a nanolitre osmometer.

External links

For more on AFGP see: http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=9108061

• Cold, Hard Fact: Fish Antifreeze Produced in Pancreas
• Ice structuring proteins - a new name for antifreeze proteins Pubmed.gov

Proteins