ANOTHER REVIEW PAPER- ANTI FREEZE PROTEINS
Another review paper: Structure and function of antifreeze proteins. You can read the full article at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692999/pdf/12171656.pdf.
Have you ever wondered why some insects, mammals and fishes do not freeze and die.
Anti freeze proteins can be defined as proteins that have an affinity for ice. 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. Organisms, 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.
AFPs absorb to ice and restrict the growth of the ice front to the regions between the absorbed protein molecules. This region now grow with a curvature making it thermodynamically unfavourable for water molecules to add to the lattice. In other words, antifreeze proteins bind to ice crystals, blocking the surface and preventing growth of the crystal. For each of these structures, the ice-binding site of the AFP has been defined by site-directed mutagenesis, and ice etching has indicated that the ice surface is bound by the AFP.
In the crystal structure, the ice-binding surface of the protein is covered with strings of water molecules. The binding sites are also somewhat hydrophobic more so than that the portion of the protein is exposed to the solvent. Surface-surface complementarily appears to be the key to tight binding in which the contribution of hydrogen bonding seems to be secondary to van der Waals contacts. These water molecules are spaced similarly to the water molecules in ice crystals. Where AFPs are available, the ice crystals remain the same size for hours, even up to days at temperatures between the melting point and the lower non-equilibrium freezing point.
For extra reading on this rather “cool” topic: http://www.rcsb.org/pdb/education_discussion/molecule_of_the_month/download/Antifreeze-Prot.pdf
Nice Ice- Antifreeze proteins don’t stop the growth of ice crystals. They actually limit the growth to manageable sizes. Antifreeze proteins counteract the re-crystallization effect and binds to the surface of the small ice crystals and slow or prevent the growth into larger dangerous crystals.
Supercooling- Antifreeze proteins lower the freezing point of water by a few degrees without changing the melting point. The most effective antifreeze proteins are made by insects, which lower the freezing point by about 6 degrees.
Icy Ice Cream- Antifreeze proteins have been useful in industry. For instance, natural antifreeze proteins 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.
So next time you are eating your delicious, heavenly, luscious, mellow, heart satisfying ice cream remember the anti freeze proteins.