Molecule of the Month: Antifreeze Proteins

Ice is a big problem for organisms that live in cold climates. Once the temperature dips below freezing, ice crystals steadily grow and burst cells. This danger, however, has not limited the spread of life on Earth to temperate regions. Organisms of all types--plants, animals, fungi and bacteria--have developed ways to combat the deadly growth of ice crystals. In some cases, they pack their cells with small antifreeze compounds like sugars or glycerol. But in cases where extra help is needed, cells make specialized antifreeze proteins to protect themselves as the temperature drops.

Antifreeze proteins don't stop the growth of ice crystals, but they limit the growth to manageable sizes. For this reason, they are also known as ice-restructuring proteins. This is necessary because of an unusual property of ice called recrystallization. When water begins to freeze, many small crystals form, but then a few small crystals dominate and grow larger and larger, stealing water molecules from the surrounding small crystals. Antifreeze proteins counteract this recrystallization effect. They bind to the surface of the small ice crystals and slow or prevent the growth into larger dangerous crystals.

Antifreeze proteins lower the freezing point of water by a few degrees, but surprisingly, they don't change the melting point. This process of depressing the freezing point while not effecting the melting point is termed thermal hysteresis. The most effective antifreeze proteins are made by insects, which lower the freezing point by about 6 degrees. However, antifreeze proteins, even the ones from plants and bacteria that have smaller effects on freezing point, are useful in another way. They are placed outside cells where they control the size of ice crystals and prevent catastrophic ice crystal formation when the temperature drops below the (lowered) freezing point.

Antifreeze proteins have been useful in industry. For instance, natural antifreeze proteins purified from cold-water ocean pout (shown here from PDB entry 1kdf ) have been used as a preservative in ice cream. They coat the fine ice crystals that give ice cream its smooth texture, and prevent it from recrystallizing during storage and delivery into chunky, icy ice cream. Researchers are also experimenting with antifreeze proteins as a way to preserve tissues and organs that are stored at low temperatures, reducing the possible damage from ice crystals.

Antifreeze proteins are a perfect example of convergent evolution. Looking at the proteins used by different organisms, we see that many different proteins have been selected to serve this same function. Several examples are included here. All of these are small proteins with a flat surface that is rich in threonine (colored lighter blue here), which binds to the surface of ice crystals. These include two proteins from fish, the ocean pout (1kdf ) and the winter flounder (1wfb ), and three very active proteins from insects, the yellow mealworm beetle (1ezg ), the spruce budworm moth (1eww ), and the snow flea (2pne ).